灾害天气

灾害天气研究进展

Advances in Research on Severe Weather

1 灾害天气监测

1 Severe weather monitoring technology

1.1 A multiscale analysis of a nocturnal extreme rainfall event of 14 July 2017 in Northeast China

A multiscale observational analysis of a nocturnal extreme rainfall event that occurred at Changtu in Northeast China on 14 July 2017 is performed using global analysis, automated surface observations, Doppler radar, rawinsonde, and disdrometer data. Results show that the large-scale environment was characterized by high convective available potential energy and precipitable water, moderate convective inhibition, and a southwesterly low-level jet (LLJ) capped by an inversion layer. The first and subsequent convective cells developed along a quasi-stationary surface convergence zone in a convection-void region of a previously dissipated meso-a-scale convective line. Continuous convective initiation through backbuilding at the western end and the subsequent merging of eastward-moving convective cells led to the formation of a near-zonally oriented meso-β-scale rainband, with reflectivity exceeding 45 dBz (i.e., convective core intensity). This quasistationary rainband was maintained along the convergence zone by the LLJ of warm moist air, aided by local topographical lifting and convectively generated outflows. A maximum hourly rainfall amount of 96 mm occurred during 02:00−03:00 Beijing standard time as individual convective cores with a melting layer of > 55 dBz reflectivity moved across Changtu with little intermittency. The extreme-rain-producing stage was characterized with near-saturated vertical columns, and rapid number concentration increases of all raindrop sizes. It is concluded that the formation of the meso-β-scale rainband with continuous convective backbuilding, and the subsequent echo-training of convective cores with growing intensity and width as well as significant fallouts of frozen particles accounted for the generation of this extreme rainfall event. This extreme event was enhanced by local topography and the formation of a mesovortex of 20−30 km in diameter. (Wang Gaili, Zhang Da-Lin, Sun Jisong)

1.2 Analysis of the vertical air motions and raindrop size distribution retrievals of a squall line based on cloud radar Doppler spectral density data

A squall line is a type of strongly organized mesoscale convective system that can cause severe weather disasters. Thus, it is crucial to explore the dynamic structure and hydrometeor distributions in squall lines.This study analyzed a squall line over Guangdong Province on 6 May 2016 that was observed using a Kaband millimeter-wave cloud radar (CR) and an S-band dual-polarization radar (PR). Doppler spectral density data obtained by the CR were used to retrieve the vertical air motions and raindrop size distribution (DSD).The results showed the following: First, the CR detected detailed vertical profiles and their evolution before and during the squall line passage. In the convection time segment (segment B), heavy rain existed with a reflectivity factor exceeding 35 dBz and a velocity spectrum width exceeding 1.3 m s−1. In the PR detection,the differential reflectivity factor (Zdr) was 1−2 dB, and the large specific differential phase (Kdp) also represented large liquid water content. In the transition and stratiform cloud time segments (segments B and C), the rain stabilized gradually, with decreasing cloud tops, stable precipitation, and a 0 ℃ layer bright band.Smaller Kdpvalues (less than 0.9) were distributed around the 0 ℃ layer, which may have been caused by the melting of ice crystal particles. Second, from the CR-retrieved vertical air velocity, before squall line passage,downdrafts dominated in local convection and weak updrafts existed in higher-altitude altostratus clouds. In segment B, the updraft air velocity reached more than 8 m s−1below the 0 ℃ layer. From segments C to D, the updrafts changed gradually into weak and wide-ranging downdrafts. Third, in the comparison of DSD values retrieved at 1.5 km and DSD values on the ground, the retrieved DSD line was lower than the disdrometer, the overall magnitude of the DSD retrieved was smaller, and the difference decreased from segments C to D. The standardized intercept parameter (Nw) and shape parameter (µ) of the DSD retrieved at 1.8 km showed good agreement with the disdrometer results, and the mass-weighted mean diameter (Dm) was smaller than that on the ground, but very close to the PR-retrieved Dmresult at 2 km. Therefore, comparing with the DSD retrieved at around 2 km, the overall number concentration remained unchanged and Dmgot larger on the ground,possibly reflecting the process of raindrop coalescence. Lastly, the average vertical profiles of several quantities in all segments showed that, first of all, the decrease of Nwand Dmwith height in segments C and D was similar, reflecting the collision effect of falling raindrops. The trends were opposite in segment B, indicating that raindrops underwent intense mixing and rapid collision and growth in this segment. Then, PR-retrieved Dmprofiles can verify the rationality of the CR-retrieved Dm. Finally, a vertical velocity profile peak generated a larger Dmespecially in segments C and D. (Ma Ningkun, Liu Liping, Chen Yichen)

1.3 Cloud vertical structure measurements from a ground-based cloud radar over the southeastern Tibetan Plateau

The southeastern Tibetan Plateau (TP) is the critical region of water vapor transport over the TP. Groundbased Ka-band cloud radar (KaCR) measurements collected at the Motuo National Climate Observatory in 2019 were used to analyze the physical properties of clouds at the front of the water vapor channel over the TP. The cloud top heights (CTHs) measured by the KaCR were verified through a comparison with observations from the Fengyun-4A (FY-4A) satellite at the same location. The KaCR-obtained CTHs were generally consistent with the FY-4A-obtained CTHs, although the KaCR slightly overestimated the CTHs of nonprecipitation clouds and underestimated the CTHs of precipitation clouds. Negative deviations between the KaCR- and FY-4A-obtained CTHs increased with rainfall rate. Cloud cover frequently occurred over Motuo,with an annual mean occurrence frequency of 65.3%. Clouds tended to form frequently at night and dissipate gradually in the daytime. Two peaks in the cloud base height (CBH) distribution were observed in Motuo,located at 0−1 km and 2−3 km, corresponding to precipitation and nonprecipitation clouds, respectively.The CTHs reached a peak at 6−7 km, which might be related to precipitation clouds. Moreover, a maximum CTH probability was observed in the midnight-early morning hours during the rainy season, while the CTH frequency peaked in the evening during the dry season. In addition, low and middle clouds were prevalent in Motuo for the period of observation in 2019. Understanding the characteristics of clouds over the TP will provide key validations of satellite measurements and promote research on cloud forces over the TP. (Zhou Renran, Wang Gaili, Zhaxi Suolang)

1.4 Deep learning for polarimetric radar quantitative precipitation estimation during landfalling typhoons in South China

Heavy rain associated with landfalling typhoons often leads to disasters in South China, which can be reduced by improving the accuracy of radar quantitative precipitation estimation (QPE). At present, raindrop size distribution (DSD) -based nonlinear fitting (QPE of DSD) and traditional neural networks are the main radar QPE algorithms. The former is not sufficient to represent the spatiotemporal variability of DSDs through the generalized Z-R or polarimetric radar rainfall relations that are established using statistical methods since such parametric methods do not consider the spatial distribution of radar observables, and the latter is limited by the number of network layers and availability of data for training the model. In this paper, we propose an alternative approach to dual-polarization radar QPE based on deep learning (QPENet). Three datasets of dualpolarization radar observations-surface rainfall (DPO-SR) were constructed using radar observations and corresponding measurements from automatic weather stations (AWS) and used for QPENet (V1), QPENet (V2),and QPENet (V3). In particular, 13×13, 25×25, and 41×41 radar range bins surrounding each AWS location were used in constructing the datasets for QPENet (V1), QPENet (V2), and QPENet (V3), respectively. For training the QPENet models, the radar data and AWS measurements from eleven landfalling typhoons in South China during 2017−2019 were used. For demonstration, an independent typhoon event was randomly selected(i.e., Merbok) to implement the three trained models to produce rainfall estimates. The evaluation results and comparison with traditional QPE (DSD) algorithms show that the QPENet model has a better performance than the traditional parametric relations. Only when the hourly rainfall intensity is less than 5 mm (R < 5 mm h−1), the QPE (DSD) model shows a comparable performance to QPENet. Comparing the three versions of the QPENet model, QPENet (V2) has the best overall performance. Only when the hourly rainfall intensity is less than 5 mm (R < 5 mm h−1), QPENet (V3) performs the best. (Zhang Yonghua, Bi Shuoben, Liu Liping)

1.5 Estimating raindrop size distributions and vertical air motions with spectral difference using vertically pointing radar

Doppler spectra measured by vertically pointing radars are inherently linked to raindrop size distributions(DSDs). But accurate estimation of DSDs remains challenging because raindrop spectra are broadened by atmospheric turbulence and shifted by vertical air motions. This paper presents a novel method to estimate vertical air motions in which there is no need to assume a model for DSD at each range gate. The theory of the new method is that the spectral difference between the adjacent range gates is contributed by vertical air motions and the variability of DSDs. The contribution of the change of DSDs is estimated by looking up the prepared tables of raindrop velocity difference and shape function difference. Then the vertical air motions can be estimated by minimizing the cost function of the two spectra between the adjacent range gates. The retrieval algorithm is applied to three cases including a stratiform case and two convective cases observed by a C-band vertically pointing radar in Longmen, Guangdong Province, China, in June 2016. Before that, the spectrum broadening effect is removed by the traditional deconvolution method with a wind profiler. The vertical profiles of precipitation parameters are also retrieved to investigate the microphysical process. The precipitation parameters retrieved near the surface are compared with the ground data collected by a two-dimensional video disdrometer (2DVD), and the results show good agreement. (Pang Suzhou, Ruan Zheng, Yang Ling)

1.6 Raindrop size distribution measurements on the southeast Tibetan Plateau during the STEP project

As part of the Second Tibetan Plateau Scientific Expedition and Research (STEP) field campaign, raindrop size distribution (DSD) measurements were conducted with a laser optical particle size velocity disdrometer in Motuo on the Tibetan Plateau (TP). The DSD characteristics of five different rain rate classes and convective and stratiform precipitation types were studied using the DSD data from July to September 2019, comprising a total of 47774 1-min raindrop spectra. The average raindrop spectral width and number concentration of large drops increased with the rainfall intensity. Furthermore, the combination of smalland midsize drops dominated the precipitation in Motuo, and their contributions exceeded 99% of the number concentration. The convective precipitation in Motuo was identified as maritime-like precipitation and was characterized by a large normalized intercept parameter lgNw= 4.0 − 4.4 and small mass-weighted mean diameter Dm= 1.1 − 1.4 mm.Empirical relations among the three parameters (intercept N0, shape μ, and slope λ) of the gamma distribution model, Nwand Dmof the normalized gamma distribution were derived. We also obtained power-law relationships of Z = ARbfor the stratiform and convective precipitation in Motuo, and the empirical relation of Z = 300R1.4significantly underestimated the convective precipitation in Motuo. When the precipitation type transitioned from stratiform into convective precipitation, coefficient A decreased and exponent b increased.Finally, empirical relations between Dmand the radar equivalent reflectivity factor at the Ku and Ka bands were obtained to improve the rainfall retrieval algorithms of the dual-frequency precipitation radar on the southeast TP. (Wang Gaili, Zhou Renran, Zhaxi Suolang)

1.7 Study on radar echo-filling in an occlusion area by a deep learning algorithm

Radar beam blockage is an important error source that affects the quality of weather radar data. An echofilling network (EFnet) is proposed based on a deep learning algorithm to correct the echo intensity under the occlusion area in the Nanjing S-band new-generation weather radar (CINRAD/SA). The training dataset is constructed by the labels, which are the echo intensity at the 0.5° elevation in the unblocked area, and by the input features, which are the intensity in the cube including multiple elevations and gates corresponding to the location of bottom labels. Two loss functions are applied to compile the network: one is the common mean square error (MSE), and the other is a self-defined loss function that increases the weight of strong echoes.Considering that the radar beam broadens with distance and height, the 0.5° elevation scan is divided into six range bands every 25 km to train different models. The models are evaluated by three indicators: explained variance (EVar), mean absolute error (MAE), and correlation coefficient (CC). Two cases are demonstrated to compare the effect of the echo-filling model by different loss functions. The results suggest that EFnet can effectively correct the echo reflectivity and improve the data quality in the occlusion area, and there are better results for strong echoes when the self-defined loss function is used. (Yin Xiaoyan, Hu Zhiqun, Zheng Jiafeng)

1.8 Supercooled liquid water and secondary ice production in Kelvin-Helmholtz instability as revealed by radar Doppler spectra observations

Mixed-phase clouds are globally omnipresent and play a major role in the Earth’s radiation budget and precipitation formation. The existence of liquid droplets in the presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life cycle of such clouds.This study presents observations that reveal the onset of drizzle inside a similar to 600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectral analysis, we show how large supercooled liquid droplets are generated in Kelvin-Helmholtz (K-H) instability despite ice particles falling from upper cloud layers. The spectral width of the supercooled liquid water mode in the radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets,characterized by reflectivity values larger than 20 dBz, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of columnar ice crystals in the K-H billows. The modeling study estimates that the concentration of these ice crystals is 3−8 L−1, which is at least 1 order of magnitude higher than that of primary ice-nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is, therefore, hypothesized that K-H in stability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles. (Li Haoran, Korolev Alexei, Moisseev Dmitri)

1.9 Two-year statistics of columnar-ice production in stratiform clouds over Hyytiala, Finland:Environmental conditions and the relevance to secondary ice production

Formation of ice particles in clouds at temperatures of 10 or warmer was documented by using groundbased radar observations. At these temperatures, the number concentration of ice-nucleating particles (INPs)is not only expected to be small, but this number is also highly uncertain. In addition, there are a number of studies reporting that the observed number concentration of ice particles exceeds expected INP concentrations,indicating that other ice generation mechanisms, such as secondary ice production (SIP), may play an important role in such clouds. To identify formation of ice crystals and report conditions in which they are generated, W-band cloud radar Doppler spectra observations collected at the Hyytiala station for more than 2 years were used. Given that at these temperatures ice crystals grow mainly as columns, which have distinct linear depolarization ratio (LDR) values, the spectral LDR was utilized to identify newly formed ice particles.It is found that in 5% to 13% of clouds, where cloud top temperatures are 12 or warmer, production of columnar ice is detected. For colder clouds, this percentage can be as high as 33%, 40% to 50% of columnarice-producing events last less than 1 h, while 5% to 15% can persist for more than 6 h. By comparing clouds where columnar crystals are produced and to the ones where these crystals are absent, the columnar-iceproducing clouds tend to have larger values of liquid water path and precipitation intensity. The columnariceproducing clouds were subdivided into three categories, using the temperature difference, ΔT, between the altitudes where columns are first detected and cloud top. The cases where ΔT is less than 2 K are typically singlelayer shallow clouds where needles are produced at the cloud top. In multilayered clouds where ΔT is larger tahan 2 K, columns are produced in a layer that is seeded by ice particles falling from above. This classification allows us to study potential impacts of various SIP mechanisms, such as the Hallet-Mossop process or freezing breakup, on columnar-ice production. To answer the question whether the observed ice particles are generated by SIP in the observed single-layer shallow clouds, ice particle number concentrations were retrieved and compared to several INP parameterizations. It was found that the ice number concentrations tend to be 1−3 orders of magnitude higher than the expected INP concentrations. (Li Haoran, Mohler Ottmar, Petaja Tuukka)

1.10 Vertical during snow events in middle latitudes of China observed by a C-band vertically pointing radar

This study applied the C-band vertically pointing radar with frequency-modulation continuous-wave technology to obtain the continuous observation data of four shallow and two deep snow events during the winter of 2015−2016 in the midlatitudes of China. Generating cells (GCs) were found near the echo tops in every event. The ice particle number concentration (N), ice water content (IWC), and median mass diameter (Dm)retrieved from radar Doppler spectra were used to analyze the microphysical properties in the snow clouds.The clouds were divided into upper GC and lower stratiform (St) regions according to their vertical structure.The fall streaks (FSs) associated with GCs were embedded in the St regions. In the GC regions, the N values in shallow events were smaller compared with those in deep events, and Dmand IWC were larger. In the St regions, N decreased compared with that in the GC regions, and Dmand IWC increased, implying the existence of aggregation and deposition growth. The growth of particle size and mass mainly occurred in the St regions.The increases of N were usually observed near −5 accompanied by bimodal Doppler spectra, which might be caused by ice multiplication. The average ratios of the median N, Dm, and IWC inside GCs to those outside GCs were 2, 1.3, and 2.5, respectively, for shallow events, with 1.7, 1.2, and 2.3, respectively, for deep events.These values were basically the same as those for the FSs, implying the importance of GCs to the enhanced ice growth subsequently found in FSs. The larger values of N, Dm, and IWC inside GCs could be related to the upward air motions inside GCs. The first Ze-IWC relationship suitable for snow clouds in the midlatitudes of China was also established. (Cui Ye, Ruan Zheng, Wei Ming)

1.11 Ka/Ku双波段毫米波雷达功率谱数据反演液态水含量方法研究

单波段雷达用Z-LWC关系反演液态水因不同降水类型反演的液态水含量相差较大。利用双波段雷达回波强度偏差反演液态水含量，直接将双波段雷达基数据回波强度相减的差值不仅包括了衰减的偏差也包括散射不同带来的偏差。因此，利用双波段雷达功率谱数据经过小粒子选择计算受Mie散射影响小的粒子，用最小二乘法计算DWR（Dual Wavelength Ratio）的变化率反演液态水含量和液态水积分总量，将双波段雷达基数据和功率谱数据反演结果与微波辐射计测量结果进行对比。结果表明：（1）经过小粒子选择，去掉了双波段雷达的回波强度因散射不同带来的偏差。（2）在高度较低时双波段雷达基数据和功率谱数据经过小粒子选择反演的液态水含量有着较大区别。随着高度的增加，反演的液态水含量比较接近。（3）双波段雷达基数据和功率谱数据反演的液态水含量与微波辐射计进行定性分析，变化趋势能够达到较为一致，微波辐射计液态水含量较强的区域，两者反演的液态水含量也相应较大。（4）双波段雷达基数据和功率谱数据反演的液态水积分总量和微波辐射计的液态水积分总量随着时间的变化趋势较为一致，但双波段雷达功率谱数据总体程度小于基数据反演的液态水积分，双波段雷达基数据与功率谱数据反演的液态水积分总量总体小于微波辐射计的液态水积分总量值。（邹琳，刘黎平）

1.12 Ka/Ku双波段云雷达反演空气垂直运动速度和雨滴谱方法研究及初步应用

在Ka波段云雷达上升级改造建成的Ka/Ku（Ka和Ku波段波长分别为8.9 mm和2.2 cm）双波段云雷达2019年用于华南云降水垂直结构观测，以改进云内动力和微物理参数探测能力。为了利用该双波段云雷达研究华南降水微物理和动力结构，本文提出了基于双波段云雷达回波强度谱密度（SZ）数据和最优估计技术的云内空气垂直运动速度（Vair）、雨滴谱（DSD）、液态水含量（LWC）、雨强（R）的反演方法（DWSZ），以及雨区衰减的订正方法。利用2019年在广东龙门观测的一次降水过程数据，对比分析了云雷达反演的微物理参数与雨滴谱直接观测量，并检验了云雷达反演的低层空气垂直运动速度，利用反演结果分析了一次混合云过程的Vair与这些微物理参数的垂直结构和相互关系。结果表明：Ka/Ku双波段云雷达合理反演了微降水微物理和动力参数及其垂直分布，经过衰减订正的Ka和Ku波段回波强度偏差明显减小。该双波段云雷达数据可以用于分析0～30 dBz回波强度的云降水垂直结构。本次过程为混合云降水，对流单体前部存在明显的上升气流，后部存在下沉气流；从平均垂直结构来看，Vair和粒子平均直径（Dm）在2 km高度层到达最大，粒子数密度（Nw）、LWC和R在2 km以下明显增强，粒子直径却减小，水汽凝结过程、雨滴碰并云滴是本次过程的主要机制。这一工作验证了Ka和Ku波段组合的双波段云雷达的可行性，为Ka/Ku波段云雷达技术的推广，单波段云雷达反演算法进一步改进，云降水精细结构分析等提供了基础。（刘黎平，张扬，丁晗）

1.13 X波段双偏振雷达相态识别与拼图的关键技术

基于统计的隶属函数参数改进方法和基于衰减程度的拼图融合方法，通过对比改进后可有效提升水凝物相态识别结果的可靠性和多雷达拼图结果的合理性。在2016年汛期北京典型个例中，融合后的X波段雷达网与当地S波段业务雷达相比能够提供更精细的回波结构和水凝物相态分布，有效缓解S波段雷达在近处探测能力降低的问题，识别的降雹区与地面观测相符。（吴翀，刘黎平，仰美霖）

1.14 毫米波云雷达观测和反演云降水微物理及动力参数方法研究进展

云雷达是探测和反演云降水微物理及动力参数精细结构的重要手段。回顾了世界和我国云雷达观测模式、数据质量控制和数据融合方法，特别是脉冲压缩、相干积累和非相干积累技术在提高云雷达灵敏度的应用，分析了基于回波强度与粒子下落速度关系、单波段云雷达小粒子跟踪方法、双波段云雷达回波强度谱密度比值方法等空气垂直运动速度反演和雨滴谱反演方法，并讨论了这些方法的特色，为今后云雷达观测方法设定和数据分析提供参考。（刘黎平）

1.15 降水条件下的云雷达与微波辐射计反演液态水含量对比分析

为了发展云雷达与微波辐射计联合反演液态水含量的方法，利用2019年4—9月中国气象科学研究院在广东龙门开展的综合观测试验中的双波段云雷达和微波辐射计数据，首先检验了在降水条件下微波辐射计天顶观测和斜路径观测两种探测模式反演温度（T）、相对湿度（RH）、液态水含量（LWC）和液态水路径（LWP）的合理性，然后分析了两种探测设备反演LWC和LWP的差别。得到以下结论：（1）微波辐射计在斜路径观测模式下反演的产品受降水影响较小，其反演结果明显优于天顶观测模式。（2）两种探测设备反演的LWP相关性较好且随时间变化较为一致，但云雷达反演LWP与平均回波强度有明显相关，随着雷达回波强度的增大，云雷达与微波辐射计反演的LWP之比越大。（3）两种探测设备反演的LWC相关性较差且存在明显偏差，在不考虑融化层的情况下单波段云雷达反演LWC与微波辐射计随高度变化趋势相近，双波段云雷达反演LWC与微波辐射计反演结果在1 km及其以上区间存在明显差异。（邹明龙，刘黎平，郑佳锋）

1.16 利用贝叶斯方法改进华南地区冰雹识别效果

使用2019年广东S波段双偏振雷达观测的冰雹和非冰雹数据，统计得到冰雹和非冰雹的雷达反射率Z、差分反射率ZDR和相关系数CC先验概率密度分布，采用贝叶斯方法，根据雷达参量在冰雹和非冰雹条件下的概率以及冰雹和非冰雹的先验概率来确定某一距离库上所测到的（Z、ZDR、CC）所代表冰雹和非冰雹的概率，并用两个个例，比较分析了WSR-88D冰雹识别算法和贝叶斯方法对冰雹识别的效果，分析表明，两种方法都能较准确地识别出冰雹云，但是贝叶斯方法识别范围较大，这可能与华南地区多为雨夹雹有关。（李博勇，胡志群，郑佳锋）

1.17 青藏高原墨脱地区云降水综合观测及初步统计特征分析

为加深对雅鲁藏布大峡谷水汽通道入口处云和降水三维结构及微物理特征的认识，在第二次青藏高原综合科学考察研究专题和国家重点研发计划项目的支持下，中国气象科学研究院于2019年在西藏墨脱地区建立了野外观测试验基地，开展了水汽、云和降水的综合观测，先后布设了先进的Ka波段云雷达、微波辐射计、X波段双偏振相控阵雷达、降水现象仪、K波段微雨雷达等设备，获取了高时、空分辨率的云和降水的宏、微观数据。文中简单介绍了此次观测的情况，并利用云雷达2019年的观测数据和降水现象仪2019年6月至2020年6月的观测数据对云的宏观特征及雨滴谱特征进行了统计分析。从云的宏观特征来看，该地区云的发生率较高，云廓线占2019年云雷达观测廓线的67%，降水云廓线占总云廓线的45%。旱季和雨季云底高度的频率分布在垂直方向均有两个高值区，分别为0～1 km和2～3 km，且超过40%的云底高度低于1 km，这可能是墨脱降水云较多造成的。接近60%的云顶高度在4～7 km。总的来说，墨脱地区以中云和低云为主，云通常在下午到晚上形成，早上到中午慢慢消散；从雨滴谱分布特征来看，该地区平均的雨滴谱谱宽和大雨滴的浓度随雨强的增大而增大，降水以中、小粒子为主，中、小粒子浓度超过粒子数浓度的99%。对流云降水的特点是粒子直径较小，而数浓度较高。粒子质量加权平均直径（Dm）的范围在1.0～1.6 mm（平均1.38 mm），标准化截距参数（lgNw）的范围在3.6～4.5（平均4.01），表现出海洋性对流降水的特征。此外，该地区降水的lgNw呈现双峰特征，分别对应于对流云和层状云降水。（王改利，周任然，扎西索郎）

1.18 双偏振相控阵雷达与业务雷达的定量对比及观测精度研究

相控阵天气雷达突破了全机械驱动天线天气雷达的时空分辨率瓶颈，能够提供更加快速、精细的观测资料。但阵列天线存在性能参数随扫描角偏离法向而恶化的情况，使相控阵雷达定量测量存在困难。本文针对中国华南地区最新布网的双偏振相控阵天气雷达，通过与当地S波段业务雷达在相同区域内的定量对比，评估了反射率因子差分反射率因子的误差量级及其随扫描角、观测时间的变化趋势。研究结果表明，相控阵雷达反射率因子的误差不大，约0.82 dB，而差分反射率因子的误差则高达1.04 dB，并且不同仰角、不同时刻之间也存在一定的波动。为此本文提出了基于S波段雷达实时数据的订正方案，能够较好地解决双偏振相控阵天气雷达的定标问题，为相控阵雷达的业务应用提供了保障。（张蔚然，吴翀，刘黎平）

1.19 星载雷达DPR与地基雷达CR的匹配对比及系统偏差初探

将GPM卫星搭载的双频降水雷达DPR和广东龙门观测站的地基Ka/Ku双波段雷达CR在2019年4—9月的观测数据进行时空匹配，结合微雨雷达MRR 的雨滴谱数据对CR衰减订正处理，比较DPR与CR探测回波强度差异，最后对两者的系统偏差进行分析。结果表明：（1） DPR和CR探测到的回波强度随高度变化的趋势大体一致，零度层亮带高度基本一致，同样高度下DPR探测到的回波强度明显大于CR。（2）统计弱的非降水云算得CR双频系统偏差约为3.1 dBz。（3）雨强越强，CR天线罩水膜厚度越大，回波衰减越大。（4）逐库订正过程中利用雨滴谱数据计算衰减系数，CR在Ka与Ku波段的反射率因子订正量不超过4 dBz。（5）综合所有适配数据，CR与DPR的反射率因子在Ka波段内订正后平均系统偏差为15.7 dBz，在Ku波段内订正后平均系统偏差为14.4 dBz。虽然经订正处理后两部雷达间的系统偏差偏大，但对改善地基Ka/Ku双波段雷达的数据质量有明显的改善。（曾震瑜，刘黎平，郑佳锋）

1.20 一次强雹云过程冰雹增长机制分析及防雹探讨

2018年9月29日大连市出现了强雹云天气过程。初始雹云在渤海上空生成，经3 h东移和发展演变，在大连北部沿海登陆并发展为成熟的强雹云。当地作业点及时开展人工防雹与联防作业，并对作业前后雹云雷达特征参数变化及成熟强雹云空间结构特征等进行分析，深入研究和探讨了成熟强雹云有效人工防雹两道防线技术方法，即在重要作物保护区上游空旷地带，或低值作物区增设人工防雹高炮，当成熟强雹云进入该高炮射程，立即实施高炮作业（且仰角≥55°，即打在雹云强回波区），促使冰雹提前和充分降落，从而保护下游的重要作物，将雹灾损失降到最低，达到人工防雹的目的。（李红斌，胡志群，张靖萱）

1.21 风廓线雷达组网观测新型应用研究进展

风廓线雷达具有高的时间和空间分辨率，已经被广泛应用于大气探测、环境气象以及灾害性雷暴天气监测预警等领域。但风廓线雷达组网二次开发产品缺乏，一定程度限制了其在天气气候领域的深度应用。简要介绍中国风廓线雷达网站点分布及其水平风廓线和谱宽等数据产品；阐述基于风廓线雷达观测产生的高时间分辨率边界层高度、垂直风切变、湍流强度和散度等新产品特点，并给出了相应的应用场景或案例。最后，展望了风廓线雷达组网观测的不足及其未来应用前景。（郭建平，刘博铭，郭晓冉）

2 青藏高原天气研究

2 Research on weather over the Tibetan Plateau

2.1 2021年度第二次青藏高原综合科学考察研究进展

中国工程院院士徐祥德承担“第二次青藏高原综合科学考察研究”十大科考任务的第一任务，并负责第5专题。项目科考及研究重点目标是针对亚洲水塔水资源、生态变化的影响因素，研究气候变化及其西风与季风协同作用，拓展青藏高原科考计划实施的多圈层综合观测系统的应用领域；提升科考团队研究成果在青藏高原地球系统前沿领域中的重要地位，着力解决气候变化对青藏高原亚洲水塔水资源与生态环境的影响理论及其应用问题，并提出有助于西藏社会经济高质量发展的科学合理气候应对战略决策；助力深入理解气候变化对青藏高原水资源和生态环境的影响机理，筑牢气象防灾减灾第一道防线。青藏高原科学考察面向国家战略需求，在复杂地形和水循环条件下，构建青藏高原天—地—空一体化的综合观测体系，揭示青藏高原气候和生态变化机理，着力解决青藏高原生态环境、灾害风险、国家重大工程建设安全等方面的问题，推动青藏高原可持续发展，推进国家生态文明建设。

项目除在雅鲁藏布沿江建立微波辐射计、自动气象站与边界层站外，还在水汽输送关键入口区墨脱、珠峰北侧等设立雷达超级站，在墨脱及雅鲁藏布江沿岸实施天—地—空一体化综合观测试验，填补了复杂河谷地形多尺度水汽输送及云降水结构特征综合探测的空白；开展了云、降水粒子微物理特征分析研究，拓展了青藏高原天—地—空一体化的综合观测体系。为获取中国东部地区灾害天气上游“强信号”区青藏高原高时空分辨率的大气温度、湿度和水汽的垂直结构信息。项目还设计和研制了基于车载平台的气象与环境三维结构动态探测系统，首次实施了青藏高原东南部气象与环境三维结构动态探测试验，获取了可靠的高时空分辨率温湿度、水汽、气溶胶等三维结构动态探测数据集。在历史与现今气象观测站网数据、多类再分析与卫星遥感产品大数据助力下，项目提出了亚洲水塔核心区热力驱动下高原区域与全球能量、水分循环“窗口效应”综合相关模型，揭示了青藏高原低云活动、深对流对全球，尤其是南北极水汽输送的重要作用；通过地面观测站与和高分辨率卫星（TRMM）观测相结合的方式，得到高原地区高分辨率降水资料，揭示了青藏高原大气降水“补给”差异对高原南、北冰川消融速率影响。项目研究高原气候变暖背景下冻土退化对生态及国家重大工程建设带来的影响及其风险，并提交气候变暖对青藏高原铁路、公路路基稳定性风险研究成果。此次科学考察拓展了多圈层多元信息观测分析数据集，构建了雅鲁藏布江水循环三维结构数据集、卫星遥感冰川面积数据集、墨脱雷达综合观测数据集、珠峰雷达综合观测数据集、高原湖泊数据集等9个数据集。数据及研究结果应用到数值天气预领域，发展了川藏铁路敏感区数值模式追踪技术，建立了川藏铁路工程区天气雷达资料同化预报系统；搭建了夹卷混合机制的参数化方案，改进了中尺度天气预报模式（WRF）中Mirrison微物理方案，为有效解决模式预报高原降水高估问题提供技术支撑；开展了高原地区卫星协同多元观测的融合技术研究，并实施了我国风云4号气象卫星AGRI和LMI资料协同地面、雷达数据的三维大气和三维云融合技术研究。（徐祥德，张胜军）

2.2 A new inverse modeling approach for emission sources based on the DDM-3D and 3DVAR techniques: An application to air quality forecasts in the Beijing-Tianjin-Hebei region

We develop a new inversion method which is suitable for linear and nonlinear emission source (ES)modeling, based on the three-dimensional decoupled direct (DDM-3D) sensitivity analysis module in the community multiscale air quality (CMAQ) model and the three-dimensional variational (3DVAR) data assimilation technique. We established the explicit observation operator matrix between the ES and receptor concentrations and the background error covariance (BEC) matrix of the ES, which can reflect the impacts of uncertainties of the ES on assimilation. Then we constructed the inversion model of the ES by combining the sensitivity analysis with 3DVAR techniques. We performed the simulation experiment using the inversion model for a heavy haze case study in the Beijing-Tianjin-Hebei (BTH) region during 27−30 December 2016.Results show that the spatial distribution of sensitivities of SO2and NOxESs to their concentrations, as well as the BEC matrix of ES, is reasonable. Using a posteriori inversed ES, underestimations of SO2and NO2during the heavy haze period are remarkably improved, especially for NO2. Spatial distributions of SO2and NO2concentrations simulated by the constrained ES were more accurate compared with an apriori ES in the BTH region. The temporal variations in regionally averaged SO2, NO2, and O3modeled concentrations using a posteriori inversed ES are consistent with in situ observations at 45 stations over the BTH region, and simulation errors decrease significantly. These results are of great significance for studies on the formation mechanism of heavy haze, the reduction of uncertainties of the ES and its dynamic updating, and the provision of accurate virtual emission inventories for airquality forecasts and decision-making services for optimization control of air pollution. (Cheng Xinghong, Hao Zilong, Zang Zengliang)

2.3 Development of three-dimensional variational data assimilation method of aerosol for the CMAQ model: An application for PM2.5 and PM10 forecasts in the Sichuan Basin

A three-dimensional variational (3DVAR) data assimilation method for the aerosol variables of the community multiscale air quality (CMAQ) model was developed. This 3DVAR system uses PM2.5and PM2.5-10(the difference between PM10and PM2.5) as control variables and uses the AERO6 aerosol chemical mechanism in the CMAQ model. Two parallel experiments, one with and one without data assimilation (DA), were performed to evaluate the assimilating effects of surface PM2.5and PM10during a heavy haze episode from 13 to 16 January, 2018 in the Sichuan Basin (SCB) region. The results show that simulations without DA clearly underestimated PM2.5and PM10concentrations, and the analysis field with aerosol DA is skillful at fitting the observations and effectively improving subsequent forecasts of PM2.5and PM10. For the analysis fields of PM2.5and PM10after DA comparing with those without DA, the correlation coefficient (CORR) of PM2.5and PM10increased by 0.59 and 0.65, the bias (BIAS) increased by 82.29 and 125.41 µg m−3, and the root mean square error (RMSE) declined by 73.69 and 116.30 µg m−3, respectively. Improvement of subsequent 24-h forecasts of PM2.5and PM10with DA is also significant. Statistical results of forecasting improvement with DA indicated that the CORR, BIAS, and RMSE for PM2.5and PM10at 78% and 89% of stations in the SCB region are improved, respectively. From the perspective of assimilation duration time, the improvement of PM2.5and PM10can be maintained for similar to 24 h. (Zhang Zhendong, Zang Zengliang, Cheng Xinghong)

2.4 How were the eastward-moving heavy rainfall events from the Tibetan Plateau to the lower reaches of the Yangtze River enhanced?

This study investigates eastward-moving summer heavy rainfall events in the lower reaches of the Yangtze River (LRYR), which are associated with the Tibetan Plateau (TP) vortices. On the basis of rainfall data from gauges and additional atmospheric data from ERA-Interim, the dynamic and thermodynamic effects of moisture transport and diabatic heating are estimated to determine the physical mechanisms that support the eastward-moving heavy rainfall events. As the rainband moves eastward, it is accompanied by anomalous cyclonic circulation in the upper and middle troposphere and enhanced vertical motion throughout the troposphere. In particular, the rainfall region is located in the fore of the upper-level trough, which is ideal for baroclinic organization of the convective system and further development of the eastward-moving vortex. The large atmospheric apparent heat source also contributes to lifting the lower-level air into the upper atmosphere and to enhancing the low-level convective motion and convergence during the heavy rainfall process. Piecewise potential vorticity inversion further verifies the crucial role that the diabatic heating played in developing the anomalous geopotential height favorable for the enhanced rainfall. The combined action of the dynamic and thermodynamic processes, as well as the rich moisture supply from the seas, synergistically sustained and enhanced the eastward-moving rainfall. (Zhao Yang, Chen Deliang, Deng Yi)

2.5 Intensified moisture sources of heavy precipitation events contributed to interannual trend in precipitation over the Three-Rivers-Headwater region in China

Evidence has indicated an overall wetting trend over the Three-Rivers Headwater region (TRHR) in the recent decades, whereas the possible mechanisms for this change remain unclear. Detecting the main moisture source regions of the water vapor and its increasing trend over this region could help understand the longterm precipitation change. Based on the gauge-based precipitation observation analysis, we find that the heavy precipitation events act as the main contributor to the interannual increasing trend of summer precipitation over the TRHR. A Lagrangian moisture tracking methodology is then utilized to identify the main moisture source of water vapor over the target region for the boreal summer period of 1980−2017, with focus particularly on exploring its change associated with the interannual trend of precipitation. On an average, the moisture sources for the target regions cover vast regions, including the west and northwest of the Tibetan Plateau by the westerlies, the southwest by the Indian summer monsoon, and the adjacent regions associated with the local recycling. However, the increased interannual precipitation trend over the TRHR could be largely attributed to the enhanced moisture sources from the neighboring northeastern areas of the targeted region, particularly associated with the heavy precipitation events. The increased water vapor transport from the neighboring areas of the TRHR potentially related to the enhanced local hydrological recycling over these regions plays a first leading role in the recent precipitation increase over the TRHR. (Zhao Ruiyu, Chen Bin, Xu Xiangde)

2.6 The key supply source of long-distance moisture transport for the extreme rainfall event on July 21, 2012 in Beijing

In this study, the weather research and forecasting (WRF) model and meteorological observation data were used to research the long-distance moisture transport supply source of the extreme rainfall event that occurred on July 21, 2012 in Beijing. Recording a maximum rainfall amount of 460 mm in 24 h, this rainstorm event had two dominant moisture transport channels. In the early stage of the rainstorm, the first channel comprised southwesterly monsoonal moisture from the Bay of Bengal (BOB) that was directly transported to North China along the eastern edge of Tibetan Plateau (TP) by orographic uplift. During the rainstorm,the southwesterly moisture transport was weakened by the transfer of Typhoon Vicente. Moreover, the southeasterly moisture transport between the typhoon and the western Pacific subtropical high (WPSH) became another dominant moisture transport channel. The moisture in the lower troposphere was mainly associated with the southeasterly moisture transport from the South China Sea and the East China Sea, and the moisture in the middle troposphere was mainly transported from the BOB and Indian Ocean. The control experiment well reproduced the distribution and intensity of rainfall and moisture transport. By comparing the control and three sensitivity experiments, we found that the moisture transported from Typhoon Vicente and a tropical cyclone in the BOB both significantly affected this extreme rainfall event. After Typhoon Vicente was removed in a sensitivity experiment, the maximum 24-h accumulated rainfall in North China was reduced by approximately 50% compared with that of the control experiment, while the rainfall after removing the tropical cyclone was reduced by 30%. When both the typhoon and tropical cyclone were removed, the southwesterly moisture transport was enhanced. Moreover, the sensitivity experiment of removing Typhoon Vicente also weakened the tropical cyclone in the BOB. Thus, the moisture pump driven by Typhoon Vicente played an important role in maintaining and strengthening the tropical cyclone in the BOB through its westerly airflow. Typhoon Vicente was not only the moisture transfer source for the southwesterly monsoonal moisture but also affected the tropical cyclone in the BOB, which was a key supply source of long-distance moisture transport for the extreme rainfall event on July 21, 2012 in Beijing. (Li Juan, Xu Xiang de, Li Yue qing)

2.7 Warm cover: Precursory strong signals for haze pollution hidden in the middle troposphere

Eastern China (EC), located in the downstream region of the Tibetan Plateau (TP), is a large area with frequent haze pollution. In addition to air pollutant emissions, meteorological conditions are a key inducement for air pollution episodes. Based on the study of the Great Smog of London in 1952 and haze pollution in EC over recent decades, it is found that the abnormal warm cover (air-temperature anomalies) in the middle troposphere, as a precursory strong signal, could be connected to severe air pollution events. The convection and vertical diffusion in the atmospheric boundary layer (ABL) were suppressed by a relatively stable structure of warm cover in the middle troposphere leading to ABL height decreases, which were favorable for the accumulation of air pollutants in the ambient atmosphere. The anomalous structure of the troposphere’s warm cover not only exists in heavy haze pollution on the daily scale, but also provides seasonal, interannual and interdecadal strong signals for frequently occurring regional haze pollution. It is revealed that a close relationship existed between interannual variations of the TP’s heat source and the warm cover strong signal in the middle troposphere over EC. The warming TP could lead to anomalous warm cover in the middle troposphere from the plateau to the downstream EC region and even the entire East Asian region, thus causing frequent winter haze pollution in EC region. (Xu Xiangde, Cai Wenyue, Zhao Tianliang)

2.8 南海台风生成前48 h环流特征及热力与动力条件

利用1979—2019年4—11月中国气象局上海台风研究所热带气旋最佳路径资料和静止卫星红外云图资料，筛选出189例南海台风，结合欧洲中期天气预报中心1°×1°再分析资料，分析南海台风生成前48 h至生成时刻的天气环流和动力、热力条件。结果表明：南海台风生成于热带洋面大范围的高海表温度、高水汽含量和高不稳定层结区，其生成前的主要环境背景环流是赤道辐合带、西南季风或东风波等；台风生成前扰动中心常常处于其北侧风切变小而南侧风切变大的过渡带中，少数扰动中心倾向于风切变小值中心附近，风切变与扰动的发展之间无显著相关；扰动中心一般与垂直涡度中心重合，垂直涡度中心是表征扰动自身强弱的物理量，但垂直涡度自身的大小与未来扰动发展趋势关系不明显，而Okubo-Weiss（OW）指数则对于扰动的发展以及扰动位置确定有较好的指示意义；在扰动发展过程中，扰动中心附近存在一个贯穿整个对流层的位涡柱，低层扰动部分与位涡柱中的中低层位涡相互作用，有利于扰动发展。（高拴柱，张胜军，吕心艳）

3 暴雨和强对流研究

3 Research on heavy rainfall and strong convection

3.1 A double-moment SBU-YLIN cloud microphysics scheme and its impact on a squall line simulation

A double-moment version of the SBU-YLIN cloud microphysical scheme in WRF is introduced. It predicts the mass and number mixing ratios of cloud droplets, rains, cloud ice, and precipitating ice. In addition, a number of physical processes, like rain evaporation, collection between rain and snow are also optimized in the new scheme. The scheme is evaluated and compared with the original one-moment scheme for a squall line case. We found that the double-moment approach gives a better representation of rain evaporation,which is critical for the development, morphology, and evolution of the simulated squall line, especially for the enhanced trailing stratiform cloud and leading convective line. The relationship between key microphysical processes and squall line dynamics is investigated to identify the driving mechanisms of the descending rear inflow, cold pool, and slantwise updraft. Furthermore, formation of the transition zone in the simulated squall line strongly depends on the flexible description of ice particle properties, such as size, degree of riming and fall speed. (Zhao Xi, Lin Yanluan, Luo Yali)

3.2 A study of the fraction of warm rainfall in a pre-summer rain event over South China

The precipitation is divided into the warm-rain and cold-rain processes according to the microphysical mechanism, but which processes are more important in a heavy precipitation even? An approach that quantitatively separates the warm-rain from total-rain processes is proposed by adding a set of new variables in microphysics schemes of the Weather Research and Forecasting model. The fraction of rainfall that formed by warm-rain processes and the related microphysical characteristic are investigated by a heavy precipitation event in South China on 9 May 2016. The simulation using the Thompson microphysics reasonably reproduces the spatial distribution of precipitation and temporal evolution of rain bands. It is found that when the composite reflectivity is between 25 and 35 dBz, the lowest percentage of warm-rain fraction occurs (median of 20%−30%). While in the strong precipitation area, the contribution of warm-rain steadily increases with the median of −50% due to the continuous moisture supply. The similar characteristics by two other microphysics schemes (Morrison and CAMS) further verify the results. In addition, abundant supercooled water exists above the 0 level due to the high condensation rate. The strong updrafts in lower-middle layers are closely associated with the areas of water vapor condensation, implying that the phase-change processes should be responsible for the small-scale buoyancy production. The budget of rain water shows that the warmrain processes play a leading role in the initiating stage of convection, and the weak advection of rain water indicates that the transport of cloud body from surroundings to the precipitation area is quite limited. (Gao Wenhua, Xue Lulin, Liu Liping)

3.3 Analysis of a record-breaking rainfall event associated with a monsoon coastal megacity of South China using multisource data

Monsoon coastal cities often suffer from extreme rain-induced flooding and severe hazard. However, the associated physical mechanisms and detailed storm structures are poorly understood due to the lack of highresolution data. This study presents an analysis of a thunderstorm that produces extreme hourly rainfall (EXHR)of 219 mm over the Guangzhou megacity on the southern coast of China using integrated multiplatform observations and a four-dimensional variational Doppler radar analysis system. Results indicate that weak environmental flows and convectively generated weak cold pool facilitate the formation of a quasi-stationary storm, while onshore warm and moist flows in the boundary layer (BL) provide the needed moisture supply.The 219-mm EXHR is attendant by a shallow meso-gamma-scale vortex due to stretching of intense latent heating-induced convergence, which, in turn, helps organize convective updrafts into its core region. Lightning and dual-polarization radar observations reveal active warm-rain (but weak mixed-phase) microphysical processes, with raindrop size distribution (RSD) closer to marine convection. In contrast, another storm develops about 4 h earlier and only 35 km to the northwest, but with more lightning, higher cloud tops, more graupels, higher supercooled liquid water content and continental RSD, little evidence of rotation, and much less rainfall. They are attributable to the presence of larger convective available potential energy resulting from the urban heat island effects and less moisture supply in the BL. These results highlight the importance of using multisource remote sensing data sets in understanding the microphysical and kinematic structures of EXHRproducing storms. (Li Mingxin, Luo Yali, Zhang Da-Lin)

3.4 Atmospheric-boundary-layer-height variation over mountainous and urban sites in Beijing as derived from radar wind-profiler measurements

The evolution of the atmospheric boundary layer (ABL) varies greatly with terrain, so that the spatial and temporal variabilities of the ABL height remain poorly understood over complex terrain. Using radar wind-profiler measurements obtained from rural mountainous (Yanqing) and adjoining urban-plain (Haidian)landscapes of Beijing, China in 2019, ABL heights are calculated based on a normalized signal-to-noise-ratio threshold. The seasonally contrasting features of ABL height variation and growth rate over the two sites are revealed for clear-sky conditions. Interestingly, the ABL in spring remains suppressed during the morning and evolves rapidly in the afternoon over Haidian; however, a usual diurnal ABL evolution is observed over Yanqing. During the winter, more rapid evolution of the ABL is observed over Haidian, although on average the daytime ABL height remains less than 800 m above ground level. The growth rate of ABL height is found to undergo a more pronounced seasonal variation over Haidian while being relatively less variable over Yanqing. As expected, the lowest (highest) growth rate of 90 m h−1(188 m h−1) occurs in winter (summer) over Haidian. The analysis of the seasonal variations in wind profiles reveals deeper insights into the development of the local plain-to-mountain flow circulation over the region and possible implications on the contrasting seasonal ABL variations, particularly during the spring and summer. Additionally, the slower ABL evolution over Haidian in autumn and winter could be associated with an aerosol-induced stable ABL as well as stronger urban heat accumulation. The findings have implications for the better understanding of air pollution meteorology in regions with mountainous terrain. (Solanki Raman, Guo Jianping, Li Jian)

3.5 On the anomalous development of a series of heavy rainfall events from central to northern China during 19−21 July 2016

This study examines the development of a series of heavy rainfall events over four different geographical regions from the central to northern China on 19−21 July 2016, with the maximum 48-h-accumulated and hourly rainfall amounts of 875 and 139 mm, respectively. Results show that the heavy rainfall events occurred in an environment with an anomalous deep trough associated with unusual extratropical cyclogenesis during this warm month and a pronounced moisture anomaly compared to a 30-year climatology. The cyclogenesis coincided with dry-air intrusion from the lower stratosphere, and its merging with a northwest and a southwest vortex in the lower troposphere. After its formation, the cyclone moved northward on the east side of Mt.Taihang and then northeastward across Mt. Yanshan. The presence of the southwest vortex and an abnormally strong western Pacific subtropical high contributed to the maintenance and intensification of a southwesterly low-level jet (LLJ) carrying ample moisture, leading to the heavy rainfall in central China. The further enhanced southwesterly LLJ through the cyclogenesis and its accompanying southeasterly LLJ provided much needed precipitable water for the heavy rainfall events over North China. It was the quasi-geostrophic forcing of the extratropical cyclone, together with its cold/warm frontal systems, and especially its northward movement with the approaching airflows near-perpendicular to the general mountain orientations that provided optimized settings for the generation of a series of heavy rainfall events along the windward foothills of the major mountains in the northern China. (Xia Rudi, Zhang Da-Lin, Fu Shenming)

3.6 On the diurnal cycle of heavy rainfall over the Sichuan Basin during 10−18 August 2020

A sustained heavy rainfall event occurred over the Sichuan Basin in Southwest China during 10−18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air,often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall. (Xia Rudi, Luo Yali, Zhang Da-Lin)

3.7 On the localized extreme rainfall over the Great Bay Area in South China with complex topography and strong UHI effects

In this study, high-resolution surface and radar observations are used to analyze 24 localized extreme hourly rainfall (EXHR; > 60 mm h−1) events with strong urban heat island (UHI) effects over the Great Bay Area (GBA) in South China during the 2011−2016 warm seasons. Quasi-idealized, convection-permitting ensemble simulations driven by diurnally varying lateral boundary conditions, which are extracted from the composite global analysis of 3−5 June 2013, are then conducted with a multilayer urban canopy model to unravel the influences of the UHI and various surface properties nearby on the EXHR generation in a complex geographical environment with sea-land contrast, topography, and vegetation variation. Results show that EXHR is mostly distributed over the urban agglomeration and within about 40 km on its downwind side, and produced during the afternoon-to-evening hours by short-lived meso-γ- to meso-β-scale storms. On the EXHR days, the GBA is featured by a weak gradient environment with abundant moisture, and a weak southwesterly flow prevailing in the boundary layer (BL). The UHI effects lead to the development of a deep mixed layer with warm bubbles over the urban agglomeration, in which the lower-BL convergence and BL-top divergence is developed, assisting in convective initiation. Such urban BL processes and associated convective development with moisture supply by the synoptic low-level southwesterly flow are enhanced by orographically increased horizontal winds and sea breezes under the influence of the herringbone coastline, thereby increasing the inhomogeneity and intensity of rainfall production over the pi-shaped urban clusters. Vegetation variations are not found to be an important factor in determining the EXHR production over the region. (Sun Xiaoyan, Luo Yali, Gao Xiaoyu)

3.8 Spatiotemporal characteristics of atmospheric turbulence over China estimated using operational high-resolution soundings

Large-scale in situ observations are sorely lacking, leading to poor understanding of nationwide atmospheric turbulence over China. Nevertheless, high-resolution soundings have become available starting in 2011, providing a unique opportunity to investigate turbulence across China. Here, we calculated the mean turbulence dissipation rate from radiosonde measurements across China for the period 2011−2018 using the Thorpe analysis. The atmospheric layers that had stronger turbulence indicated by larger epsilon generally came with larger Thorpe length but with smaller Brunt-Vaisala frequency. Overall, the clear-air epsilon in the free atmosphere exhibited large spatial variability with a “south-high north-low” pattern. Large clear-air epsilon values were observed in both the lower stratosphere (LS) and upper troposphere (UT), especially over the Tibetan Plateau (TP) and its neighboring regions with complex terrain likely due to large-amplitude mountain waves. Particularly, less frequent but more intense clear-air turbulence was observed in both lower troposphere(LT) and UT over the TP, while more frequent, less intense clear-air turbulence was found in the northern China. The all-sky turbulence considering the moist-saturation effects was much stronger in the troposphere,notably in the southern China where convective clouds and precipitation of tentimes dominated. In the vertical direction, the altitude of peak clear-air epsilon in the troposphere was found to decrease poleward, broadly consistent with the meridional gradient of tropopause height in the Northern Hemisphere. A double-peak mode stood out for the profiles of clear-air epsilon at midlatitudes to the north of 30°N in winter: one peak was at altitudes of 15−18 km, and another at altitudes of 5−8 km. The strong shear instabilities around the westerly jet stream could account for the vertical bimodal structures. The seasonality of epsilon was also pronounced,reaching maxima in summer and minima in winter. Our results may help understand and avoid clear-air turbulence, as related to aviation safety among other issues. (Lyu Yanmin, Guo Jianping, Li Jian)

3.9 Technical note: First comparison of wind observations from ESA’s satellite mission Aeolus and ground-based radar wind profiler network of China

Aeolus is the first satellite mission to directly observe wind profile information on a global scale. After implementing a set of bias corrections, the Aeolus data products went public on 12 May 2020. However,Aeolus wind products over China have thus far not been evaluated extensively by ground-based remote sensing measurements. In this study, the Mie-cloudy and Rayleigh-clear wind products from Aeolus measurements are validated against wind observations from the radar wind profiler (RWP) network in China. Based on the position of each RWP site relative to the closest Aeolus ground tracks, three matchup categories are proposed,and comparisons between Aeolus wind products and RWP wind observations are performed for each category separately. The performance of Mie-cloudy wind products does not change much between the three matchup categories. On the other hand, for Rayleigh-clear and RWP wind products, categories 1 and 2 are found to have much smaller differences compared with category 3. This could be due to the RWP site being sufficiently approximate to the Aeolus ground track for categories 1 and 2. In the vertical, the Aeolus wind products are similar to the RWP wind observations, except for the Rayleigh-clear winds in the height range of 0−1 km.The mean absolute normalized differences between the Mie-cloudy (Rayleigh-clear) and the RWP wind components are 3.06 (5.45), 2.79 (4.81), and 3.32 (5.72) m/s at all orbit times and ascending and descending Aeolus orbit times, respectively. This indicates that the wind products for ascending orbits are slightly superior to those for descending orbits, and the observation time has a minor effect on the comparison. From the perspective of spatial differences, the Aeolus Mie-cloudy winds are consistent with RWP winds in most of East China, except in coastal areas where the Aeolus Rayleigh-clear winds are more reliable. Overall, the correlation coefficient R between the Mie-cloudy (Rayleigh-clear) wind and RWP wind component observation is 0.94(0.81), suggesting that Aeolus wind products are in good agreement with wind observations from the RWP network in China. The findings give us sufficient confidence in assimilating the newly released Aeolus wind products in operational weather forecasting in China. (Guo Jianping, Liu Boming, Gong Wei)

3.10 The impact of the observation data assimilation on atmospheric reanalyses over Tibetan Plateau and western Yunnan-Guizhou Plateau

Three modern atmospheric reanalyses with different input observation data (NOAA-CIRES 20th century reanalysis (20CR), Japanese 55-year reanalysis (JRA-55) and (JRA-55C) were compared against the independent radiosonde observations over the Tibetan Plateau (TP) and the western Yunnan-Guizhou Plateau(YGP) from the China-Japan Meteorological Disaster Reduction Cooperation (JICA/Tibet) Center Project in the summer of 2018 to investigate the effects of the assimilation of the observation data on the quality and accuracy of the reanalyses in the troposphere. The results indicate that the mean biases and mean root-meansquare errors of horizontal wind, temperature, and specific humidity significantly decreased when comparing the 20CR reanalysis (assimilating only surface pressure) to the JRA-55C (assimilating conventional surface and upper-air observations) and the JRA-55 (assimilating conventional and satellite observations), and the bias spreads of these aboveground variables in JRA-55C and JRA-55 were cut to almost half of those observed in 20CR. However, the mean biases and uncertainties varied little from JRA-55C to JRA-55. This means that the assimilation of conventional observation data plays a vital role in the quality of reanalyses for the troposphere over these data-sparse plateaus. It was also found that the temperature and specific humidity near the ground over TP showed larger mean biases and bias spans than those over YGP, likely due to the sparser surface observation over TP. (Bao Xinghua, Zhang Fuqing, Zhao Yang)

3.11 The significant role of radiosonde-measured cloud-base height in the estimation of cloud radiative forcing

The satellite-based quantification of cloud radiative forcing remains poorly understood, due largely to the limitation or uncertainties in characterizing cloud-base height (CBH). Here, we use the CBH data from radiosonde measurements over China in combination with the collocated cloud-top height (CTH) and cloud properties from MODIS/Aqua to quantify the impact of CBH on shortwave cloud radiative forcing (SWCRF).The climatological mean SWCRF at the surface (SWCRFSUR), at the top of the atmosphere (SWCRFTOA),and in the atmosphere (SWCRFATM) are estimated to be −97.14, −84.35, and 12.79 W m−2, respectively for the summers spanning 2010 to 2018 over China. To illustrate the role of the cloud base, we assume four scenarios according to vertical profile patterns of cloud optical depth (COD). Using the CTH and cloud properties from MODIS alone results in large uncertainties for the estimation of SWCRFATM, compared with those under scenarios that consider the CBH. Furthermore, the biases of the CERES estimation of SWCRFATM tend to increase in the presence of thick clouds with low CBH. Additionally, the discrepancy of SWCRFATM relative to that calculated without consideration of CBH varies according to the vertical profile of COD. When a uniform COD vertical profile is assumed, the largest SWCRF discrepancies occur during the early morning or late afternoon. By comparison, the two-point COD vertical distribution assumption has the largest uncertainties occurring at noon when the solar irradiation peaks. These findings justify the urgent need to consider the cloud vertical structures when calculating the SWCRF which is otherwise neglected. (Xu Hui, Guo Jianping, Li Jian)

3.12 The synoptic impacts on the convection initiation of a warm-sector heavy rainfall event over coastal South China prior to the monsoon onset: A numerical modeling study

A series of convection-permitting numerical experiments using the weather research and forecasting model with different model configurations are performed to investigate physical mechanisms governing convection initiation (CI) at the coast of South China (SC) during a warm-sector heavy rainfall event occurred before the South China Sea (SCS) monsoon onset. The simulation results are more sensitive to initial state from the different reanalysis products than the microphysics schemes, horizontal resolutions, initialization time, and lateral boundary conditions with the variation examined. It is found that the speeds of easterly to southeasterly winds in the boundary layer (BL) over the northern SCS determine the strength of horizontal convergence of the warm and moist airflows arriving at the coast. The midtropospheric divergence over the west coastal areas of SC and its southwest upstream impact vertical motion of the mid-to-lower tropospheric southwesterly airflows when approaching the coast. Differences in such features of upstream airflows in the experiments lead to distinct temporal evolutions of BL dynamic lifting and temperature stratification in the mid-to-lower levels at the CI location, and thus the level of free convection and convective available potential energy. The modeling evidence presented shows that the synoptic dynamic and thermodynamic characteristics of the upstream airflows in BL and mid-to-lower troposphere, with local modulation by the coastal mountains,play important roles in determining the time and location of CI, and the distribution and amount of rainfall.Studies on more cases are needed to gain more general and robust conclusions about physical mechanism governing the warm-sector coastal CI. (Bao Xinghua, Luo Yali, Gao Xiaoyu)

3.13 Typhoon disaster risk zoning for China’s coastal area

Previous studies on typhoon disaster risk zoning in China have focused on individual provinces or smallscale areas and lack county-level results. In this study, typhoon disaster risk zoning is conducted for China’s coastal area, based on data at the county level. Using precipitation and wind data for China and typhoon disaster and social data at the county level for China’s coastal area from 2004 to 2013, first we analyze the characteristics of typhoon disasters in China’s coastal area and then develop an intensity index of factors causing typhoon disasters and a comprehensive social vulnerability index. Finally, by combining the two indices, we obtain a comprehensive risk index for typhoon disasters and conduct risk zoning. The results show that the maximum intensity areas are mainly the most coastal areas of both Zhejiang and Guangdong,and parts of Hainan Island, which are similar to the distribution of typhoon disasters. The maximum values of vulnerability are in the northwest of Guangxi, parts of Fujian coastal areas and parts of the Shandong Peninsula. The comprehensive risk index generally decreases from coastal areas to inland areas. The high-risk areas are mainly distributed over Hainan Island, south-western Guangdong, most coastal Zhejiang, the coastal areas between Zhejiang and Fujian and parts of the Shandong Peninsula. (Zhu Jing, Lu Yi, Ren Fumin)

3.14 Vertical structures of temperature inversions and clouds derived from high-resolution radiosonde measurements at Ny-Ålesund, Svalbard

The knowledge of the vertical atmospheric structures in the Arctic region remains elusive, due largely to sparse long-term continuous profiling observations. Based on the temporally high-resolution (1 s) radiosonde measurements from April 2017 to September 2019 collected at the Ny-Ålesund (11.92°E, 78.92°N) station in the Arctic, we analyzed the characteristics of temperature inversion (TI) and clouds, including the diurnal and seasonal variabilities under different atmospheric circulations. Clouds mainly appear in the lower troposphere,with the largest contribution by double-layer clouds. The seasonal variation of vertical cloud distribution above 7 km seems closely linked to the seasonal variability of tropopause height. Besides, the diurnal variation of TI frequency exhibits significant seasonality, with a bimodal distribution in the vertical, with stronger TI intensity in summer. The lowest temperatures at the top of bottom of the elevated inversion are observed in winter,whereas the lowest temperature of the surface-based inversion top is observed in spring, which may be related to the seasonal variation of sea surface temperature. The characteristics of cloud and TI are further analyzed under the five typical circulation patterns. It is found that the low-pressure system and southerly wind in front of the trough are favorable for cloud formation in the lower troposphere, while the impact of synoptic pattern on clouds in the upper troposphere seems negligible, likely due to the cold environment. The TI associated with cyclone systems tends to be much thinner and weaker, owning to the conditionally unstable conditions.These findings provide key reference for the vertical structure of the inversion and cloud in the Arctic, which is expected to help improve cloud parameterization in numeric models. (Wang Ding, Guo Jianping, Xu Hui)

3.15 Vorticity and moisture budget analyses on a plateau vortex that cause an intense rainfall event within the Qaidam Basin

As one of the most seriously arid areas in China, the Qaidam Basin (QB) features a notable growing probability of intense rainfall under global warming. Compared to a normal/humid region, intense rainfall usually results in more severe disasters in an arid area. Considering that few studies focused on intense rainfall within the QB, there is an urgent need to understand the mechanisms governing intense rainfall in this region. A type of Tibetan Plateau vortex (TPV) associated intense rainfall within the QB was investigated in this study, which partly fills the existing research gaps in the field. The main findings are as follows: (a) The intense-rainfall-producing TPV formed and maintained in a favorable background environment which was characterized by a notable upper-level divergence north of a strong upper-level jet and a strong middle-level warm advection ahead of a shortwave trough over the Tibetan Plateau. (b) Vorticity budget indicates that two factors affected the vortex’s formation notably, one was the convergence-related vertical stretching, which dominated the vortex’s formation and the other was the import of the horizontal transport of anticyclonic vorticity which was the most detrimental factor for the formation of the TPV. Tilting and vertical transport only exerted weak effects on the TPV’s formation, since convective activities were relatively weak in this event.(c) Moisture budget shows that the southwestern and southern moisture transport channels, which were mainly driven by the wind field associated with the shortwave trough over the Tibetan Plateau, contributed similar to 70% to the total moisture income of the intense rainfall within the QB. The transport was accomplished primarily through the southern boundary of the QB, with the moisture mainly coming from the Indian Peninsula and Indochina Peninsula. (Li Wanli, Xia Rudi, Zhong Qi)

3.16 Warm-season mesoscale convective systems over eastern China: Convection-permitting climate model simulation and observation

Mesoscale convective systems (MCSs) are important warm-season precipitation systems in the eastern China. However, our knowledge of their climatology and capability in their simulation is still insufficient. This paper examines their characteristics over the 2008−2017 warm seasons using convection-permitting climate simulations (CPCSs) with a 3-km grid spacing that explicitly resolves MCSs, as well as a high-resolution gauge-satellite merged precipitation product. An object-based tracking algorithm is applied to identify MCSs.Results indicate that the MCS genesis and occurrence are closely related to the progression of the East Asian monsoon and are modulated by the underlying topography. On average, about 243 MCSs are observed each season and contribute 19% and 47% to total and extreme warm-season precipitation. The climatological attributes and variabilities are reasonably reproduced in the CPCS. The major model deficiencies are excessive small MCS occurrence and overmuch MCS rainfall, consequently overestimating the precipitation contributions, whereas observational uncertainties may play a role too. Both the observed and simulated MCS precipitation feature a nocturnal or morning maximum and an eastward delayed diurnal peak east of the Tibetan Plateau, in contrast to the dominant afternoon peak of non-MCS precipitation. The favorable comparison with observations demonstrates the capability of CPCSs in simulating MCSs in the Asian monsoon climate, and its usefulness in projecting the future changes of MCSs under global warming. The finding that non-MCS precipitation is responsible for the high biased afternoon precipitation provides helpful guidance for further model improvement. (Yun Yuxing, Liu Changhai, Luo Yali)

3.17 Contrasting effect of soil moisture on the daytime boundary layer under different thermodynamic conditions in summer over China

The investigation is still lacking concerning the effect of soil moisture (SM) on the evolution of planetary boundary layer (PBL) under different land conditions in a huge domain as large as China. We perform an explicit correlation analysis between daytime PBL height (PBLH) and SM for convective, neutral and stable boundary layer regimes (i.e., CBL, NBL, and SBL), respectively. A negative correlation exists between SM and daytime PBLH for CBL and NBL, exhibiting a spatial pattern of east strong west weak, albeit a positive correlation for SBL. The standard deviation of PBLH for CBL and NBL exhibits a spatial pattern of northwest high southeast low. Cloudy, humid, and stable atmosphere result in CBL shoaling. PBLH more correlates with the sensible heat flux (CBL: r=0.25; NBL: r=0.33) over dry areas, but over Northwest China the PBL depends more on meteorology likely owing to the extremely dry soil. The effect of SM on the evolution of PBL throughout China remains elusive. This study explores the correlation between PBLH and SM under different PBL regimes, using the 5-year record of high-resolution summertime soundings across China. The CBL and NBL heights during daytime are found to negatively correlate with SM. In contrast, the SBL height anticorrelates with SM, likely due to more fluid and moisture intermittent turbulence caused by enhanced SM in SBL. The spatial pattern of the standard deviation of CBL and NBL height exhibits a pronounced northwest high southeast low pattern. Our study also reveals more significant impact of sensible heat flux on CBL and NBL regimes over dry areas, compared to over wet areas, indicating that land surface processes are more coupled to the PBL evolution over dry areas. Besides, less cloud, dry and unstable atmosphere favor the development of CBL in wet areas, and PBLH is more dependent on meteorological quantities, but less on SM in dry areas, likely owing to extremely dry soil and bare land over Northwest China. (Xu Zhiqi, Chen Haishan,Guo Jianping)

3.18 Increased turbulence in the Eurasian upper-level jet stream in winter: Past and future

Anthropogenic activities have been increasingly strengthening the meridional temperature gradient in the upper atmosphere, which has a profound impact on the wind shear and turbulence in mid-latitudes. Here we examined the trends of the wintertime jet stream and clear-air turbulence at 250 hPa over the Eurasian.The zonal wind speed showed a significantly increasing trend in the region between 40°N and 60°N where the meridional temperature gradient between the equator and the pole was increasing sharply in the past 4 decades (1980−2019). In addition, there existed a clear trend toward stronger vertical shear at 250 hPa in the latitudinal zone between 30°N and 40°N where the zonal wind increased, while a declining trend was shown to the south of 30°N, according to three different reanalysis datasets, including ERA5, JRA55, and NCAR/NCEP,which may be related to the poleward shift of the subtropical jet stream. The turbulence, closely related to the occurrence of vertical wind shear, was found to become stronger in the latitudinal zone between 30°N and 40°N consequently. The projections based on phase 6 of Coupled Model Intercomparison Project models show that the zonal wind speed in the mid-latitudes will increase significantly for the period 2015−2100, especially between the latitudinal zone between 40°N and 60°N, and the corresponding vertical wind shear in this area tends to dramatically increase. The findings not only help us better understanding of turbulence occurrence in the past 40 years, but also have profound implications for the projection of turbulence occurrence under global warming over mid-latitude regions of the Northern Hemisphere. (Lyu Yanmin, Guo Jianping, Li Jian)

3.19 Investigation of near-global daytime boundary layer height using high-resolution radiosondes:First results and comparison with ERA5, MERRA-2, JRA-55, and NCEP-2 reanalyses

The planetary boundary layer (PBL) governs the vertical transport of mass, momentum, and moisture between the surface and the free atmosphere, and thus the determination of PBL height (BLH) is recognized as crucial for air quality, weather, and climate analysis. Although reanalysis products can provide important insight into the global view of BLH in a seamless way, the BLH observed in situ on a global scale remains poorly understood due to the lack of high-resolution (1 or 2 s) radiosonde measurements. The present study attempts to establish a near-global BLH climatology at synoptic times (00:00 and 12:00 UTC) and in the daytime using high-resolution radiosonde measurements over 300 radiosonde sites worldwide for the period from 2012 to 2019, which is then compared against the BLHs obtained from four reanalysis datasets, including ERA5, MERRA-2, JRA-55, and NCEP-2. The variations in daytime BLH exhibit large spatial and temporal dependence, and as a result the BLH maxima are generally discerned over the regions such as the western United States and western China, in which the balloon launch times mostly correspond to the afternoon.The diurnal variations in BLH are revealed with a peak at 17:00 local solar time (LST). The most promising reanalysis product is ERA5, which underestimates BLH by around 130 m as compared to radiosondes released during daytime. In addition, MERRA-2 is a well-established product and has an underestimation of around 160 m. JRA-55 and NCEP-2 might produce considerable additional uncertainties, with a much larger underestimation of up to 400 m. The largest bias in the reanalysis data appears over the western United States and western China, and it might be attributed to the maximal BLH in the afternoon when the PBL has risen. Statistical analyses further indicate that the biases of reanalysis BLH products are positively associated with orographic complexity, as well as the occurrence of static instability. To our best knowledge, this study presents the first near-global view of high-resolution radiosonde-derived boundary layer height and provides a quantitative assessment of the four frequently used reanalysis products. (Guo Jianping, Zhang Jian, Yang Kun)

3.20 Large-scale pattern of the wintertime diurnal temperature range variations over North America during 1979−2018

The atmospheric circulation and remote forcing of SST anomalies play an imporant role in the formation of largescale cloud cover and precipitation anomalies as well as the patterns of diurnal temperature range (DTR)variability. In this study, the dominant patterns of the wintertime DTR variations over North America (NA) and the underlying mechnisms are investigated based on observational and reanalysis datasets during 1979−2018.The first leading mode is characterized by a widespread same-sign loading in the majority of NA. The corresponding principal component (PC) shows an obvious interannual time-scale variations. The second mode displays a north-south dipole-like pattern in the NA and the corresponding PC exhibits a decline trend during 1979−2018. Both patterns of DTR variations are closely associated with the changes in local cloud cover and precipitation. Furthermore, clear effects are identified of El Niño-southern oscillation (ENSO) and Tropical/Northern Hemisphere (TNH) teleconnection on the formation of the two dominant patterns of wintertime DTR variabilty over NA through changing the atmospheric circulation and associated transportation of temperature and moisture. Our findings provide a new insight into understanding the DTR changes during recent decades.(Liu Lin, Chen Wen, Guo Jianping)

3.21 Regional variability of summertime raindrop size distribution from a network of disdrometers in Beijing

Regional raindrop size distribution (DSD) features are poorly understood due to the lack of observations.Here we investigate the regional variability of summertime DSD in Beijing, using the DSD observations from ten disdrometer sites from April to September 2017. The characteristics of DSD are analyzed for both convective and stratiform precipitation, mainly classified by rain rate (R). The shape (µ) and slope (λ)parameters follow a second-degree polynomial regression relationship for both stratiform and convective precipitation. On average, stratiform precipitation is found to have larger values of µ and λ than convective precipitation, whereas convective precipitation has a larger mass-weighted mean diameter (Dm) and a generalized intercept parameter (Nw). Interestingly, the north of Beijing has larger values of µ and λ, as opposed to Dmand Nwthat exhibit greater values in the south, which could probably be attributed to terrain differences.The rain rate dependence on DSD is analyzed as well. In general, the Dm-R and Nw-R relationships follow a power-law distribution, and both Dmand Nwincrease significantly with R. As the rain rate increases, Dmkeeps increasing to around 1.5 mm until an equilibrium state is reached. The findings obtained here could provide useful reference for better estimations of rainfall using the remote sensing techniques. (Han Yi, Guo Jianping,Yun Yuxing)

3.22 The impact of reforestation induced land cover change (1990−2017) on flood peak discharge using HEC-HMS hydrological model and satellite observations: A study in two mountain basins,China

Understanding the effect of land use and land cover (LULC) type change on watershed hydrological response is essential for adopting applicable measures to control floods. In China, the Grain to Green Program(GTGP) and the Natural Forest Conservation Program (NFCP) have had a substantial impact on LULC. We investigate the effect of these conservation efforts on flood peak discharge in two mountainous catchments.We used a series of Landsat images ranging from 1990 to 2016/2017 to evaluate the LULC changes. Further to this, the hydrological responses at the basin and sub-basin scale were generated by the Hydrologic Modeling System (HEC-HMS) under four LULC scenarios. Between 1990 and 2016/2017, both catchments experienced an increase in forest and urban land by 18% and 2% in Yanhe and by 16% and 8% in Guangyuan, respectively.In contrast, the agricultural land decreased by approximately 30% in Yanhe and 24% in Guangyuan,respectively. The changes in land cover resulted in decrease in flood peak discharge ranging from 14% in Yanhe to 6% in Guangyuan. These findings provide a better understanding on the impact of reforestation induced LULC change on spatial patterns of typical hydrological responses of mountainous catchment and could help to mitigate flash flood hazards in other mountainous regions. (Kabeja Crispin, Li Rui, Guo Jianping)

3.23 Assessing the surface-layer stability over China using long-term wind-tower network observations

Surface-layer stability is important in many processes, such as in the surface energy budget, atmospheric pollution, and boundary-layer parametrizations. Most previous studies on stability, however, conducted either theoretical or observational investigations at specific sites, thus leaving a gap with regard to the large-scale pattern. Here, wind-speed and temperature observations at multiple heights from the wind-tower network of China are used to estimate low-level stability during the 2009−2016 period. A series of data-quality-control procedures are conducted and data from 170 wind towers with more than 2 years of valid observations are selected. The degree of stability is classified by the Obukhov length, which is derived from the wind speed and temperature at 10 m and 70 m above ground level, combined with information regarding the roughness length. Overall, the occurrence frequency of surface-layer instability exhibits significant temporal and spatial variability, being particularly larger in spring and summer than in autumn and winter. The maximum frequency of summertime instability occurs in the time period 1000−1200 local solar time, approximately 2 h earlier than in autumn. Geographically, the peak instability frequency occurs much earlier in the day in Northwest China than in other regions, likely owing to the arid and semi-arid land cover. Also noteworthy is the steady increase in instability frequency observed during the period analyzed here, likely resulting from the reduction in the vertical gradient of wind speed. Our findings call for explicit consideration of stability variability in the windenergy industry and in fundamental boundary-layer investigations in China. (Li Jian, Guo Jianping, Xu Hui)

3.24 Study of the formation of the Arctic cell associated with the two-wave middle-high latitude circulation

The formation of the Arctic cell associated with the two-wave middle-high latitude circulation, which is a major atmospheric circulation common to the three-cell and four-cell mean meridional circulations in the Northern Hemisphere, is analyzed using a long period of reanalysis data. In the context of the two-wave middle-high latitude circulation, when the high near the Arctic region from 120°E to 80°W (AH120E80W)weakens and withdraws eastward and the low near the Arctic region from 80°W to 120°E (AL80W120E)strengthens and expands northeastward, the Arctic tends to be controlled by obvious low pressure and associated upward motion, leading to the formation of the Arctic cell. The eastward withdrawal of the AH120E80W is attributed to an eastward retreat of the North Pacific Low, because it promotes the strong anticyclonic wind shear associated with the maintenance of the AH120E80W to migrate eastward. The eastward retreat of the North Pacific Low is induced by the decrease in the width of the East Asian Trough,which results from the response of the high terrain in Central Asia to the weakening of middle-latitude westerly winds caused by a northward shift of the Azores high. On the other hand, the eastward withdrawal of the AH120E80W results in the decay of the Arctic high, causing the winds near the Arctic to change from easterly to westerly. At the same time, the northward shift of the Azores high promotes the strong Icelandic Low to expand poleward. The combination of the Arctic westerly winds and the poleward expansion of the strong Icelandic Low leads to the northeastward expansion of the AL80W120E. (Liang Zhaoming, Gao Shouting)

3.25 A dynamical and numerical study of the effects of the topography of the Tibetan Plateau and westerly wind speed on the East Asian Trough

The effects of the topography of the Tibetan Plateau (TP) and westerly wind speed on the East Asian Trough (EAT) are discussed with analysis using a dynamical model, and verified with numerical experiments based on real cases. The dynamical model utilizes a complex terrain function to approximate the TP topography, and derives a stream function generated by the TP terrain. The numerical experiments used for the verification consider the influence of different TP terrain heights and westerly wind speeds. The results show that the topography of the TP has a prominent contribution to the intensity of the EAT, but little effect on its position. The topography of the TP in the westerly winds causes a low-pressure trough to its east, which promotes a deepening of the EAT to the east side of the TP. In contrast, in the context of the TP, westerly wind speed has an important influence on the position of the EAT, but a minor impact on the deepening of the EAT. A larger westerly wind speed causes a more eastern position of the EAT. These findings indicate that the combination of large-scale topography and westerly wind speed plays a key role in the evolution of the EAT, in the terms of intensity and position. (Liang Zhaoming, Xu Pengcheng, Gao Shouting, Zuo Qunjie)

3.26 Organized warm-sector rainfall in the coastal region of South China in an anticyclone synoptic situation: Observational analysis

Organized warm-sector rainfall (OWSR) near the coast of South China tends to occur in certain synoptic situations characterized with either a low-level jet or an anticyclone, with the latter being less investigated.This paper fills the gap by analyzing 15 OWSR events that occurred in an anticyclone synoptic situation during the pre-summer rainy season of 2011–2016, based on high-resolution observational and reanalysis data. The results show that the anticyclone synoptic situation produces marked northerly boundary-layer winds inland and obvious northeasterly, easterly/southwesterly, and southeasterly boundary-layer winds near the coasts of the eastern Guangdong, western Guangdong, and Guangxi, respectively. The coastal boundary-layer winds promote favorable environmental conditions and strong convergence for convection initiation; consequently,OWSR is prone to occur near the coasts of the western Guangdong and Guangxi, but exhibits different formation and propagation features in the following two subareas. (1) The southeasterly boundary-layer winds tend to converge near the border area between Guangxi and Guangdong (BGG), promoting the formation of a stable convective line along the mountains. The convective line persists with support of upper-level southwesterly winds that facilitate convective cells to propagate along the convective line, producing heavy OWSR along the mountains near BGG. (2) In contrast, a west-east convective line tends to form and maintain near the coast of Yangjiang (YJ) area, about 200 km east of BGG, owing to stable convergence between the easterly (or southwesterly) and the northerly boundary-layer winds reinforced by the mountains near YJ.Moreover, the coupling of upper-level westerly winds with the easterly (southwesterly) boundary-layer winds facilitates expansion (eastward propagation) of the convective line, causing west–east-oriented heavy OWSR near the coast of YJ. In a word, this study reveals refined properties of OWSR initiation and development in the anticyclone synoptic situation, which may help improve the forecast skill of OWSR during the pre-summer rainy season in South China. (Liang Zhaoming, Gao Shouting)

3.27 1980—2017年南海季风爆发前后华南前汛期降水统计特征对比分析

利用1980—2017年华南地区303个国家级地面气象站逐小时降水数据、ERA-Interim再分析资料，分析华南前汛期（4—6月）降水统计特征，定义站点上短时（1～6 h）、中等时长（7～12 h）和长时（＞12 h）降水事件，对比降水量、频次和强度在南海季风爆发前后的变化，以及所定义的西部内陆、东部内陆、沿海地区的异同。结果表明：（1）南海季风爆发后，研究区域平均而言，3类降水事件的降水量增多、小时降水强度增强，短时、长时降水事件发生频次增多，而中等时长降水事件发生频次有所减少。（2）从空间分布来看，南海季风爆发后，小时降水强度在整个华南地区均增强，西部内陆时长大于6 h的降水事件尤为明显；降水事件的发生频次在西部内陆和沿海地区升高，而东部内陆时长大于6 h的降水事件发生频次降低，因此，季风爆发后西部内陆和沿海地区的总降水量均显著增大，而东部内陆的总降水量变化不大。（3）西部内陆降水事件主要在夜间开始发生，持续时间越长的事件越早开始，且由西向东逐渐推迟；东部内陆短时降水事件主要在14:00（北京时，下同）左右开始，季风爆发后更为明显，而时长大于6 h的降水事件的开始时间和峰值时间无明显的分布规律；沿海地区短时降水事件在季风爆发前主要于05:00—08:00时开始，季风爆发后，在海岸线约50 km以内仍然如此，而较远离海岸线的短时降水事件主要于14:00开始，沿海地区长时降水事件在季风爆发前、后都倾向于在夜间开始，并在日间出现峰值。（李争辉，罗亚丽）

3.28 北京地区一次飑线的组织化过程及热动力结构特征

2015年8月7日华北西北部的一次断线状对流系统向东南方向移动，并与平原地区多单体雷暴合并、组织，最终形成强飑线，造成北京地区出现较大范围的风雹和局地短时强降水天气。基于多源资料的研究结果表明：（1）飑线形成有上游线状对流发展移动、平原多个单体雷暴的新生和合并、线状对流并入本地多单体后组织成飑线3个阶段。第2阶段中，城区北部边缘地面热力分布不均，配合局地风场辐合，触发了雷暴。雷暴冷池范围不断扩大，温度梯度区向南扩展，造成新生对流向南传播。（2）飑线的组织化过程，呈现出两支强入流为典型特征的动力结构：一支位于雷暴冷池后侧中层（4500～5000 m），另一支位于低层飑线前侧，由强辐合区垂直于飑线指向云内。这两支强入流分别构成飑线前侧和后侧两个独立的顺时针垂直环流圈。后侧入流和前侧入流在同时加强，造成飑线前侧垂直环流不断加强，与之对应的环境垂直风切变也同步增强。这一动力过程形成了有利于飑线组织化的中尺度垂直切变环境，垂直风切变增大的本质实际上是飑线发展反馈的结果，同时也是驱动飑线快速向前移动和发展的重要因素。当后侧中层入流消失，前侧垂直环流也随之逐渐减弱，预示着飑线从成熟开始减弱消亡。（3）从热力结构看，下山的线状对流冷池与平原地区多单体雷暴的冷池合并，形成了扰动温度低于-8 、厚度加深到1.5 km的强冷池，其前沿的β中尺度锋面附近的辐合上升运动加强，进一步促进了飑线在平原地区发展加强，并出现阵风锋。（雷蕾，孙继松，陈明轩）

3.29 强台风“菲特”（1323）极端降水研究进展

台风暴雨灾害是台风三类灾害（暴雨、大风、风暴潮）之首，而台风极端降水是暴雨灾害的直接原因，对台风极端降水的研究有利于增强对台风极端降水机理的认识和提高极端降水的预报水平。强台风“菲特”（1323）具有登陆强度历史罕见、降雨强度大、影响范围广、引发灾害重等特点，本文对“菲特”极端降水特征及其形成机理研究进行了回顾和总结。“菲特”的强降水过程主要分为两个阶段，造成了杭州湾一带和浙闽交界处两个强降水中心。“菲特”极端降水的产生主要源于环境因子、地形和内部条件多尺度相互作用：环境因子涉及双台风作用、弱冷空气侵入、台风倒槽、垂直风切变和高空急流等，其中“丹娜丝”台风外围偏东气流源源不断的水汽输送是“菲特”极端降水形成的关键物理因子；山脉等地形增幅作用是浙江余姚等地出现历史性强降水的重要原因；水汽辐合和凝结与霰的融化和对流区雨滴的迁移是暴雨增幅重要的内部因素。（秦思，姚美，任福民）

3.30 山东半岛海风锋在一次飑线系统演变过程中的作用

2016年6月30日生成于华北南部的一次长生命期的强飑线过程，造成了山东地区大范围风雹天气。文中利用常规观测资料、区域自动气象站观测数据及雷达监测产品，分析了山东半岛复杂的海风锋特征在这次飑线系统的断裂、再组织化以及极端大风、冰雹灾害形成过程中的重要作用。结果表明：（1）初始对流是在地面冷锋辐合线上触发的弱对流，在对流系统向更不稳定区域移动时与水平对流卷相交，对流迅速发展，并组织成东西走向的直线型飑线。（2）飑线系统在平原地区继续向前移动的过程中发生断裂，这一过程与渤海湾在黄河三角洲形成的两条移动方向不同的海风锋以及飑线系统的阵风锋有关：向内陆推进的两条海风锋与阵风锋在飑线系统中段的前部相交，诱发新生单体，造成该处对流系统更快地向前传播，最终导致飑线系统断裂;与此同时，断裂后的西段风暴因低层暖湿入流被切断而逐渐减弱。（3）断裂后西段残留风暴系统出流阵风产生的新生风暴向东北方向发展，与断裂后的东段风暴的后向传播（向西南方向发展）机制相互作用，完成了飑线的再次组织化，形成了具有典型弓状特征、水平尺度更大、近似于东北—西南走向的飑线系统。（4）长生命期飑线系统造成的极端雷暴大风和最大冰雹出现在飑线再组织化初期，位于飑线系统“弓部”位置，地面极端雷暴大风是冷池密度流、后侧入流急流和水成物对应的前侧下沉气流共同作用的结果，其中与后侧入流急流几乎完全分离的、与水成物对应的前侧下沉气流在这次极端地面大风发生时可能起到了重要作用。（5）山东半岛东侧的黄海海风锋向内陆推进（东南向西北）过程中与自西北向东南移动的飑线相遇，加强了风暴前侧的抬升、水汽供给和组织化程度，为飑线的长时间维持提供了有利条件。（万夫敬，孙继松，孙敏）

3.31 中小尺度对流系统的高分辨率数值模拟近况和未来挑战

随着高速、大容量、并行计算能力的迅速增长和中尺度数值模式的不断完善，近年来不断涌现使用1 km乃至次千米网格距开展中、小尺度对流系统的数值模拟研究。这些数值模拟工作展现出目前观测手段还无法得到的动力学一致的高分辨率气象信息，大大提高了对一些中、小尺度对流系统内部结构和演变的理解。但高分辨率数值模拟的未来发展也面临着不少问题和挑战。本研究回顾了提高模式分辨率至1 km乃至次千米模拟中、小尺度对流系统的进展，并综述目前在高分辨率数值模拟中的资料同化和物理过程处理方法，同时指出中尺度模式中处理积云对流、大气边界层和辐射过程时值得注意的分辨率“灰色区”，介绍使用1 km和次千米分辨率模拟中、小尺度对流系统的几篇有代表性的工作，最后讨论了高分辨率数值模拟的未来发展和挑战。（黎慧琦，张大林）

4 台风研究

4 Typhoon research

4.1 台风快速更新短临预报系统TRANS v1.0业务运行

聚焦影响我国台风的风雨短临预报，集成 “随流型而变”的集合卡尔曼滤波同化方案、空间均匀雷达资料稀疏化算法（ESTM）等台风“973”项目关键技术，气科院牵头发展的台风快速更新短临预报系统（TRANS）于2021年1月实现业务化运行，实现沿海雷达资料同化，逐小时更新强风雨短临预报。TRANS成为台风逐小时定位、定强的重要依据，强风雨临近预报的重要参考。2020年TRANS系统相比欧洲中心ECMWF模式暴雨以上量级降水和10级大风预报准确率提高16%。该系统的持续升级优化将服务于我国台风业务预报和防灾减灾。（端义宏，冯佳宁，赵大军）

4.2 台风登陆前后强度结构变化机理研究

基于非定常的卡诺热机原理，建立了基于非定常卡诺热机和边界层动力过程的热带气旋（TC）增强的动力系统模型，能定量估计理想数值模拟和实际TC的增强率（即强度变化）。同时，为了弥补TC增强的动力系统模型动力过程缺失的不足，基于TC边界层内切向风的收支方程和热力学熵的准平衡假定，通过引入一个表征等熵面与等角动量面汇合的程度的因子，推导出另外一个时间依赖的TC增强理论模型。虽然基于不同假设，两个模型描述台风加强率的公式形式极为相似，这说明从能量学和动力学考虑，TC增强的基本动力过程具有一致性。这两个协调一致的TC增强率方程可以再现模拟TC生命史中的强度时间演变。将在后续的工作中考虑环境场的影响并试验用于实际TC的强度变率预报。为建立TC登陆后的强度衰减预报模型，系统评估了三个经典的登陆TC强度衰减模型，为后续建立更优化的适用于我国的登陆TC强度衰减模型奠定基础。（王玉清，徐晶，刘璐）

4.3 台风眼墙小尺度精细结构对模式水平分辨率的敏感性研究

利用超高分辨率大涡模拟结果分析，发现次公里级分辨对热带气旋（TC）风圈、垂直上升运动，强度的描述优于中尺度数值模拟。166 m能够开始部分分辨边界层滚涡，55 m分辨率可分辨不同尺度滚涡以及龙卷尺度的相互作用。边界层方案比如YSU方案能够在500 m分辨率上部分分辨滚涡，但是其结构以及波长尺度与大涡模拟结果相差较远。具有尺度依赖的边界层算法能够提高边界层方案对滚涡等小尺度系统的分辨能力，但是对其尺度依赖函数具有较强的敏感性。（王玉清，徐洪雄，赵大军）

4.4 基于台风事件随机模拟新技术的风险评估模型

面向台风防灾减灾需求，建立了基于统计学习方法的热带气旋（TC）风险评估模型，研究提出了一种多变量函数型数据分析方法，用于TC随机事件的全路径模拟，将贯穿TC生命史的各种属性，如中心坐标、最大风速等联合起来视作多元函数型随机变量，构建协同统一的一体化模型，据此进行TC全路径多要素的同步模拟。该方法在TC年发生概率模型、随机模拟模型优化等方面均有创新性的应用。综合评价表明，该方法可与风场模型和工程风险性模型结合使用，为政府和保险/再保险行业估计经济和保险损失提供科学依据。（徐晶）

4.5 吕宋岛地形影响下的台风结构变化及新眼墙形成机制

利用高分辨率模式研究典型台风穿过吕宋岛过程中的眼墙破坏和重组，发现台风登陆吕宋岛后由于摩擦和海面熵通量减小，原始眼墙收缩并且减弱消失，与此同时，在岛外海洋上的外雨带持续增强并且轴对称化，引起岛屿外围大眼墙生成。当台风中心过岛重新进入中国南海后，原始眼墙又重新产生。这种重新产生的眼墙结构是由涡旋内部残余对流和海洋熵通量增加所导致。（王玉清，王慧）

4.6 台风极端降水（TCER）的气候特征及季风涌对登陆台风极端降水的影响

研究发现影响我国的台风极端降水（TCER）频次（强度）有波动增多趋势。造成TCER的台风有两类路径，一是以西北行路径影响（登陆）台湾之后进而登陆我国东南沿海，二是以西北行或西行路径影响（登陆）海南岛及两广地区。研究季风涌对登陆台风极端降水影响发现整层水汽通量积分在极端暴雨发生前1～2天达到峰值；极端暴雨热带气旋（TC）整层水汽通量明显高于非极端暴雨TC，极端暴雨类TC比非极端暴雨类TC水汽通量辐合更强。极端暴雨类TC风场辐合主要是由西南低空急流叠加TC本体环流引起。动态季风涌指数对ERLTC有较好指示意义，有潜力成为一个业务预报新指标。（李英，赵大军）

4.7 A numerical study of typhoon Megi (2010). Part II: Eyewall evolution crossing the Luzon Island

Typhoon Megi (2010) experienced drastic eyewall structure changes when it crossed the Luzon Island and entered the South China Sea (SCS), including the contraction and breakdown of the eyewall after landfall over the Luzon Island, the formation of a new large outer eyewall accompanied by reintensification of the storm after it entered the SCS, and the appearance of a short-lived small inner eyewall. These features were reproduced reasonably well in a control simulation using the advanced weather research and forecasting(ARW-WRF) model. In this study, the eyewall processes of the simulated Megi during and after landfall have been analyzed. Results show that the presence of the landmass of the Luzon Island increased surface friction and reduced surface enthalpy flux, causing the original eyewall to contract and break down and the storm to weaken. The formation of the new large eyewall results mainly from the axisymmetrization of outer spiral rainbands after the storm core moved across the Luzon Island and entered the SCS. The appearance of the small inner eyewall over the SCS was due to the increased surface enthalpy flux and the revival of convection in the central region of the storm core. In a sensitivity experiment with the mesoscale mountain replaced by flat surface over the Luzon Island, a new large outer eyewall formed over the western Luzon Island with its size about one-third smaller after the storm entered the SCS than that in the control experiment with the terrain over the Luzon Island unchanged. (Wang Hui, Wang Yuqing)

4.8 An investigation of the impact of different turbulence schemes on the tropical cyclone boundary layer at turbulent gray-zone resolution

Accurately resolving turbulence in the tropical-cyclone boundary layer (TCBL) is crucial for realistic simulations of tropical cyclones (TCs), but how well the fine-scale structure of the simulated TCBL can be reproduced at gray-zone resolutions by state-of-the-art planetary boundary layer (PBL) parameterization schemes, and why the simulated fine-scale structure is so sensitive to the PBL scheme, are yet to be evaluated and understood. To address these issues, a series of numerical experiments under idealized conditions were conducted using the advanced research weather research and forecasting (WRF-ARW) model at 500 and 166 m grid spacing. An 18 h WRF-ARW large-eddy simulation (LES) with a 55 m grid spacing based on the nonlinear backscatter and anisotropy subfilter-scale (NBA-SFS) stress scheme was used as reference (RLES).Results from experiments using five different PBL schemes, that is, two conventional schemes-the Yonsei University (YSU) and the original MYNN (termed MYNN1)-two scale-aware schemes-the Shin-Hong (SH) and the revised MYNN (termed MYNN2)-and the coarse-resolution LES (CLES), were subsequently compared with the RLES. The YSU and S-H not only produced better simulations of TC intensity and structure than the other three PBL schemes, but also produced a fine-scale turbulent structure reasonably well compared with the RLES. Discrepancies in the simulated fine-scale structure among the PBL schemes result primarily from the different strength of vertical mixing, which could be reduced by the scale-aware option. Results from a series of sensitivity experiments with a 166-m grid spacing demonstrate that the scale-aware S-H can significantly improve the finer resolution simulations than the conventional YSU at relatively finer grid spacing at gray zone. (Xu Hongxiong, Wang Hui, Duan Yihong)

4.9 Associations between strong earthquakes and local rainfall in China

Strong earthquakes are a major cause of natural disasters and may also be related to heavy rainfall events. Both phenomena have received considerable attention in seismology and meteorology, two relatively independent disciplines, but we do not yet know whether there is a connection between them. We investigated the characteristics of daily rainfall over seismic areas in China. Our statistical analyses showed that there was a strong correlation between strong earthquakes (Ms≥6.0) and rainfall over the seismic area, with 74.9%of earthquakes in China accompanied by seismic epicenter rainfall and 86.6% by seismic area rainfall. The statistics also showed that the daily precipitation over the seismic area, including the epicenter, was mainly light rain, with only a few instances of torrential or storm rain, with 80% of the rainfall events lasting two or more days. The maximum cumulative precipitation corresponded well with the strong earthquakes occurring over steep terrain, such as the Taiwan central mountains and the eastern Tibetan Plateau. The earthquake area rainfall had a higher frequency than the 30-year climatological average and was dominated by earthquake events in the wet season. The WRF-ARW numerical simulation of seismic local rainfall during the devastating Ms 8.0 Wenchuan earthquake in May 2008 showed that the geothermal heat from the earthquake strengthened the local convergence of moisture and vertical motion near the epicenter and the upward transport of the sensible heat flux, which favored seismic rainfall. The results of this study show that rainfall in the seismic area is closely related to strong earthquakes and can be triggered and enhanced by geothermal heat. (Zhao Dajun,Chen Lianshou, Yu Yubin)

4.10 Contribution of the intensity of intraseasonal oscillation to the interannual variation of tropical cyclogenesis over the western North Pacific

The present study compares the contributions of seasonal, intraseasonal, and synoptic variations of environmental factors, i.e., mid-level moisture, vertical instability, and lower-level relative vorticity, to the interannual variation of tropical cyclone (TC) genesis over the western North Pacific (WNP) during July-August-September-October (JASO) of 1979−2015. It is found that the TC genesis frequency has a significant positive correlation with the intensity of intraseasonal oscillation, but a weak correlation with the intensity of synoptic variation of environmental factors during JASO in the WNP. In addition, the intraseasonal components of convection and lower-level vorticity at the location of TC genesis have a larger contribution than the interannual and synoptic components to the differences between more and less TC years over the WNP. The present results indicate that the intensity of intraseasonal oscillation of environmental factors has an important effect on the interannual variation of the TC genesis frequency over the WNP. This study enhances our understanding of the impacts of intraseasonal oscillation on the interannual variation of the WNP TC activity and indicates that the intensity of intraseasonal oscillation has the potential to be used in the seasonal prediction of the WNP TC genesis. (Cao Xi, Wu Renguang, Xu Jing)

4.11 Contribution of vertical advection to supergradient wind in tropical cyclone boundary layer: A numerical study

The existence of supergradient wind in the interior of the boundary layer is a distinct feature of a tropical cyclone (TC). Although the vertical advection is shown to enhance supergradient wind in the TC boundary layer (TCBL), how and to what extent the strength and structure of supergradient wind are modulated by vertical advection are not well understood. In this study, both a TCBL model and an axisymmetric full-physics model are used to quantify the contribution of the vertical advection process to the strength and vertical structure of supergradient wind in TCBL. Results from the TCBL model show that the removal of vertical advection of radial wind reduces both the strength and height of supergradient wind by slightly more than 50%. The removal of vertical advection of agradient wind reduces the height of the supergradient wind core by similar to 30% but increases the strength of supergradient wind by similar to 10%. Results from the fullphysics model show that the removal of vertical advection of radial wind or agradient wind reduces both the strength and height of supergradient wind but the removal of that of radial wind produces a more substantial reduction (52%) than the removal of that of agradient wind (35%). However, both the intensification rate and final intensity of the simulated TCs in terms of maximum 10-m wind speed show little differences in experiments with and without the vertical advection of radial or agradient wind, suggesting that supergradient wind contributes little to either the intensification rate or the steady-state intensity of the simulated TC. (Fei Rong, Wang Yuqing, Li Yuanlong)

4.12 Effect of the vertical diffusion of moisture in the planetary boundary layer on an idealized tropical cyclone

Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones. The separate impact of eddy diffusivity estimated by planetary boundary layer (PBL) parameterization on the tropical cyclones has not yet been systematically examined. We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme. Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones.Increasing the eddy diffusion of moisture increases the moisture content of the PBL, with three different effects on tropical cyclones: (1) a decrease in the depth of the PBL; (2) an increase in convection in the inner rain band and eyewall; and (3) drying of the lowest region of the PBL and then increasing the surface latent heat flux. These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms. The increased surface latent heat flux is mainly responsible for the decrease in pressure. Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones,but contributes little to the intensity of wind. This largely influences the wind-pressure relationship, which is crucial in tropical cyclones simulation. These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones, and provide guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations. (Xu Hongxiong, Zhao Dajun)

4.13 Impact of the monsoonal surge on extreme rainfall of landfalling tropical cyclones

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport (Qvt)during the ERLTC is significantly higher than that during the NERLTC. The Qvtreaches a peak 1−2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Qvtand the horizontal wind field during the ERLTC. The Qvtconvergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC,whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC. (Zhao Dajun, Yu Yubin, Chen Lianshou)

4.14 Increasing lifetime maximum intensity of rapidly intensifying tropical cyclones over the western North Pacific

The long-term trend in the annual mean lifetime maximum intensity (LMI) of rapidly intensifying tropical cyclones (RI-TCs) over the western North Pacific (WNP) is investigated in this study. During 1970−2019, a notable upward trend is observed in the average RI-TC LMI, which is primarily linked to a significant increase in the mean intensification rate prior to LMI. This intensification rate increase is caused by an increase in the mean magnitude of RI cases. By contrast there is no significant change in the RI ratio, which is calculated as the proportion of 24 h RI records to all 24 h records before a RI-TC reaches its LMI. Furthermore, there is a significantly greater RI magnitude at the west of 155°E, where the vast majority of RI cases occur on average.Over this region, there are significant increases in sea surface temperatures, TC heat potential, 700−500 hPa relative humidity and 200 hPa divergence during 1970−2019. Only a small region of significantly reduced 850−200 hPa vertical wind shear is observed to the northeast of the Philippines during 1970−2019. These results imply that both thermodynamic and dynamic variables play an important role in modulating RI magnitude over the WNP. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.15 Modulation of tropical cyclone formation over the western North Pacific by the ENSO combination mode

Previous studies have shown that a combination mode (C-mode) is formed by nonlinear interactions between the western Pacific warm pool annual cycle and El Niño-southern oscillation variability. We find a significant inverse relationship between the western North Pacific (WNP) tropical cyclone (TC) frequency and the C-mode index on monthly timescales from 1970 to 2019. During positive C-mode phases, TC formation is significantly suppressed over a region spanning 5°−20°N and 140°−175°E, primarily due to reduced 850 hPa relative vorticity and increased 850-200-hPa vertical wind shear. By contrast, there is significantly enhanced TC genesis over the region spanning 25°−30°N and 125°−135°E, mainly due to increased 850 hPa relative vorticity and 600 hPa relative humidity. All of these changes in atmospheric conditions are further linked to an anomalous large-scale anticyclone over the WNP generated by the C-mode. Our results suggest that C-mode variability can serve as a potential predictor for forecasting monthly WNP TC activity. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.16 Monsoon surges enhance extreme rainfall by maintaining the circulation of landfalling tropical cyclones and slowing down their movement

Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) in China can cause flash floods and other disastrous impacts, so investigating their genesis and mechanism of enhancement has been attracting considerable attention. This study demonstrates that the extreme rainfall of landfalling tropical cyclones(LTCs) possesses two key properties, namely, maintenance of the LTC circulation and a lagging (slowing down or looping) of its movement, and the monsoon surge can provide a positive contribution to these properties. Specifically, diagnostics show that the low-level cyclonic vorticity and upper-level divergence of ERLTCs are significantly stronger than those of NERLTCs (non-extreme-rainfall-producing LTCs). The continuous intensification of the cyclonic rotation in the lower troposphere before the occurrence of extreme rainfall is a significant feature that distinguishes ERLTCs from NERLTCs. Vorticity budget analysis further shows that the relative vorticity advection term contributes the most to the local increase and maintenance of vorticity in the middle and lower troposphere of ERLTCs under the influence of the southwest monsoonal surge, thus demonstrating that the monsoonal surge favors the maintenance of LTC circulation. On the other hand, the activity of the southwest monsoonal surge is mainly manifested in the zonal wind anomaly, and the corresponding strong westerly transport can significantly reduce the zonal component of the steering flow.As a result, the total steering flow can be weakened, which decreases the northwestward translation speed of ERLTCs, and thus the monsoonal surge favors a lagging (slowing down or looping) of LTC movement.These results reveal the mechanism of influence through which the monsoonal surge affects ERLTCs via its direct impacts on the maintenance of their circulation and lagging of their movement-two distinct evolutionary characteristics. (Zhao Dajun, Chen Lianshou, Yu Yubin)

4.17 Recent weakening of the interannual relationship between ENSO Modoki and boreal summer tropical cyclone frequency over the western North Pacific

This study shows that the impact of El Niño-southern oscillation (ENSO) Modoki on boreal summer tropical cyclone (TC) formation over the western North Pacific (WNP) has experienced decadal changes during the past few decades. The correlation between the ENSO Modoki index and TC frequency over the WNP was weak during 1975−1989, becomed strong and significant during 1990−2004, and becomed weak again during 2005−2019. Over the eastern part of the WNP, ENSO Modoki enhanced TC formation during 1990−2004 but did not significantly impact on the TC formation during 1975−1989 and 2005−2019. The difference in correlation strength primarily results from changes in large-scale features related to ENSO Modoki among the three subperiods (1975−1989, 1990−2004, and 2005−2019). El Niño Modoki during 1990−2004 was characterized by a tripole sea surface temperature (SST) pattern with maximum SST anomalies in the equatorial central Pacific, while during 1975−1989 and 2005−2019, the maximum SST anomalies were located over the subtropical northeastern Pacific. The two primary environmental variables likely leading to these observed relationships between ENSO Modoki and TCs were mid-level moisture (RH600) and low-level vorticity (VOR850). During 1990−2004, TC formation was enhanced both at the south of 20°N and north of 20°N. The increase in TC activity during El Niño Modoki at the south of 20°N was likely tied to greater RH600 and north of 20°N to larger cyclonic VOR850. In contrast, ENSO Modoki’s impacts on both VOR850 and RH600 were weak during 1975−1989 and 2005−2019. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.18 Sensitivity of fine-scale structure in tropical cyclone boundary layer to model horizontal resolution at sub-kilometer grid spacing

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone(TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7−10 h simulation of an idealized TC are examined using the weather research and forecasting (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex,the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future. (Xu Hongxiong, Wang Yuqing)

4.19 Statistical linkage between coastal El Niño-southern oscillation and tropical cyclone formation over the western North Pacific

This study investigates the modulation of tropical cyclone (TC) formation over the western North Pacific (WNP) by coastal El Niño-southern oscillation (ENSO). There is a significant inverse relationship between WNP TC frequency during July−October from 1961 to 2019 and simultaneous Nino 1+2 sea surface temperature anomalies. TC formation is significantly suppressed and enhanced over the subtropical and equatorial WNP during coastal El Niño, respectively, while TC formation exhibits opposite-signed anomalies during La Niña. This north-south dipolar pattern during coastal ENSO is distinct from the pattern observed in basin-wide ENSO events. Additional analyses show that coastal ENSO influences WNP TC formation through modulation of the large-scale environment. Changes in mid-level moisture and low-level vorticity appear to be the primary large-scale influences on TC formation during both coastal ENSO phases. These changes can be further linked to the anomalous large-scale circulation over the WNP during coastal ENSO. (Song Jinjie,Klotzbach Philip J., Duan Yihong)

4.20 Stochastic simulation of tropical cyclones for risk assessment at one go: A multivariate functional PCA approach

A multivariate functional principal component analysis approach to the full-track simulation of tropical cyclones (TCs) is developed for risk assessment. Elemental variables of TC along the track necessary for risk assessment, such as center coordinates, maximum wind speed, minimum central pressure and ordinal dates, can be simulated simultaneously at one go, using solely the best-track data with no data supplemented from any other sources. The simulation model is optimally determined by means of the ladle estimator. A TC occurrence model using the Conway-Maxwell-Poisson distribution is proposed as well, by which different dispersion features of annual occurrence can be represented in a unified manner. With the occurrence model,TCs can be simulated on an annual basis. The modeling and simulation process is programmed and fully automated such that little manual intervention is required, which greatly improves the modeling efficiency and reduces the turnaround time, especially when newly available TC data are incorporated periodically into the model. Comprehensive evaluation shows that this approach is capable of generating high-performance synthetic TCs in terms of distributional and extreme value features, which can be used in conjunction with wind field and engineering vulnerability models to estimate economic and insurance losses for governments and insurance/reinsurance industry. Tropical cyclones (TCs) are one of the biggest threats to life and property around the world. However, the infrequent nature of catastrophic TCs invalidates the standard actuarial loss estimation approaches. TC risk assessment requires estimation of catastrophic TCs having a very low occurrence probability, or equivalently a very long return period spanning up to thousands of years. Since reliable TC data are available only for recently decades, stochastic modeling and simulation turned out to be an effective approach to achieve more stable TC risk estimates for regions where little or no historical TC records exist. Here we present a novel model for the full-track simulation of TCs for risk assessment, via a machine learning approach called multivariate functional principal component analysis. Using this model, highperformance synthetic TCs can be generated in a fully automated manner such that little manual intervention is required, which greatly improves the modeling efficiency and reduces the turnaround time, especially when newly available TC data are incorporated periodically into the model. These synthetic TCs can be used in conjunction with wind field and engineering vulnerability models to estimate economic and insurance losses for governments and insurance/reinsurance industry. (Yang Chi, Xu Jing, Yin Jianming)

4.21 The cold avoidance of typhoons in their north turning over the South China Sea

Based on the typhoon best tracks of the China Meteorological Administration (CMA), ERA5 reanalysis data of ECMWF at 0.25° horizontal resolution, and NOAA optimal interpolated sea surface temperature (OISST V2) data, the dynamical compositing analysis is used to study the north turning at nearly 90° of 4 westward typhoons over the South China Sea (SCS). The composite analysis results show that: (1) As the typhoon goes westward into the SCS, the upper-level westerly trough moves eastward to the vicinity of 110°E in the mainland of China, and the western North Pacific subtropical high (SH) retreats eastward at the same time,which weakens the steering flow of typhoon and slowes down its movement. (2) The cold air guided by the westerly trough invades southwardly into the western part of SCS from the mainland leading to a descending and divergent airflow in the lower-to-middle atmospheric layers and enhancing the eastward pressure gradient force (PGF) in the west quadrant of the typhoon, which blocks and repesls the typhoon from moving any further westward. (3) Due to the cold air intrusion, the vertical atmospheric stratification in the west quadrant of the typhoon becomes static and stable, which may suppress the convection, impeding a typhoon’s westward motion. (4) With the cold air involving to the south of the typhoon, the direction of the PGF on the typhoon switches from eastward to northward, and the SH falling southward enhances the southwesterly airflow on the south of the typhoon at the same time. The remarkable increase of the northward steering airflows of the typhoon results in an abrupt northward turn. (5) In addition, the sea surface temperature (SST) and the ocean heat content (OHC) on the western part of the SCS are also reduced, attributed to the cold air cooling, and the typhoon is likely to avoid the cold ocean and approachs a relatively warmer region. This study suggests that cold avoidance during the westward movement of typhoons is worthy of consideration in the operational forecast of typhoon tracks. (Lai Shaojun, Li Ying, He Fen, Wang Yufei)

4.22 The intensity dependence of tropical cyclone intensification rate in a simplified energetically based dynamical system model

In this study, a simple energetically based dynamical system model of tropical cyclone (TC) intensification is modified to account for the observed dependence of the intensification rate (IR) on the storm intensity.According to the modified dynamical system model, the TC IR is controlled by the intensification potential(IP) and the weakening rate due to surface friction beneath the eyewall. The IP is determined primarily by the rate of change in the potential energy available for a TC to develop, which is a function of the thermodynamic conditions of the atmosphere and the underlying ocean, and the dynamical efficiency of the TC system.The latter depends strongly on the degree of convective organization within the eyewall and the inner-core inertial stability of the storm. At a relatively low TC intensity, the IP of the intensifying storm is larger than the frictional weakening rate, leading to an increase in the TC IR with TC intensity in this stage. As the storm reaches an intermediate intensity of 30−40 m s−1, the difference between IP and frictional weakening rate reaches its maximum, concurrent with the maximum IR. Later on, the IR decreases as the TC intensifies further because the frictional dissipation increases with TC intensity at a faster rate than the IP. Finally, the storm approaches its maximum potential intensity (MPI) and the IR becomes zero. The modified dynamical system model is validated with results from idealized simulations with an axisymmetric nonhydrostatic, cloudresolving model. (Wang Yuqing, Li Yuanlong, Xu Jing)

4.23 The performance of three exponential decay models in estimating tropical cyclone intensity change after landfall over China

In this study, the performance of three exponential decay models in estimating intensity change of tropical cyclones (TCs) after landfall over China is evaluated based on the besttrack TC data during 1980–2018.Results indicate that the three models evaluated can reproduce the weakening trend of TCs after landfall, but two of them (M1 and M2) tend to overestimate TC intensity and one (M3) tends to overestimate TC intensity in the first 12 h and underestimate TC intensity afterwards. M2 has the best performance with the smallest errors among the three models within 24 h after landfall. M3 has better performance than M1 in the first 20 h after landfall, but its errors increase largely afterwards. M1 and M2 show systematic positive biases in the southeastern China likely due to the fact that they have not explicitly included any topographic effect. M3 has better performance in the southeastern China, where it was originally attempted, but shows negative biases in the eastern China. The relative contributions of different factors, including landfall intensity, translational speed, 850-hPa moist static energy, and topography, to model errors are examined based on classification analyses. Results indicate that the landfall intensity contributes about 18%, translational speed, moist static energy and topography contribute equally about 15% to the model errors. It is strongly suggested that the TC characteristics and the time-dependent decay constant determined by environmental conditions, topography and land cover properties, should be considered in a good exponential decay model of TC weakening after landfall. (Liu Lu,Wang Yuqing,Wang Hui)

4.24 The role of boundary layer dynamics in tropical cyclone intensification. Part I: Sensitivity to surface drag coefficient

This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification using numerical simulations. The hypothesis is that although surface friction has a negative effect on TC intensification due to frictional dissipation (direct effect), it contributes positively to TC intensification by determining the amplitude and radial location of eyewall updrafts/convection (indirect effect). Results from a boundary layer model indicate that TCs with a larger surface drag coefficient (CD) can induce stronger and more inwardly penetrated boundary layer inflow and upward motion at the top of the boundary layer. This can lead to stronger and more inwardly located condensational heating inside the radius of maximum wind with higher inertial stability and is favorable for more rapid intensification. Results from full-physics model simulations using TC Model version 4 (TCM4) demonstrate that the intensification rate of a TC during the primary intensification stage is insensitive to CD if CD is changed over a reasonable range. This is because the increased/reduced positive contribution by the indirect effect of surface friction to TC intensification due to increased/reduced CD is roughly offset by the increased/reduced negative (direct) dissipation effect due to surface friction. However, greater surface friction can significantly shorten the initial spinup period through stronger frictional moisture convergence and Ekman pumping and thus expedite moistening of the inner-core column of the TC vortex but is likely to lead to a weaker storm in the mature stage. (Li Tsung-Han, Wang Yuqing)

4.25 The role of boundary layer dynamics in tropical cyclone intensification. Part II: Sensitivity to initial vortex structure

In Part I of this series of studies, we demonstrated that the intensification rate of a numerically simulated tropical cyclone (TC) during the primary intensification stage is insensitive to the surface drag coefficient. This leads to the question of what is the role of the boundary layer in determining the TC intensification rate given sea surface temperature and favorable environmental conditions. This part attempts to answer this question based on a boundary layer model and a full-physics model as used in Part I. Results from a boundary layer model suggest that TCs with a smaller radius of maximum wind (RMW) or of lower strength (i.e., more rapid radial decay of tangential wind outside the RMW) can induce stronger boundary layer inflow and stronger upward motion at the top of the boundary layer. This leads to stronger condensational heating inside the RMW with higher inertial stability and is thus favorable for a higher intensification rate. Results from full-physics model simulations indicate that the TC vortex initially with a smaller RMW or of lower strength has a shorter initial spinup stage due to faster moistening of the inner core and intensifies more rapidly during the primary intensification stage. This is because the positive indirect effect of boundary layer dynamics depends strongly on vortex structure, but the dissipation effect of surface friction depends little on the vortex structure. As a result, the intensification rate of the simulated TC is very sensitive to the initial TC structure. (Li Tsung-Han,Wang Yuqing)

4.26 The simulation of five tropical cyclones by sample optimization of ensemble forecasting based on the observed track and intensity

The quality of ensemble forecasting is seriously affected by sample quality. In this study, the distributions of ensemble members based on the observed track and intensity of tropical cyclones (TCs) were optimized and their influence on the simulation results was analyzed. Simulated and observed tracks and intensities of TCs were compared and these two indicators were combined and weighted to score the sample. Samples with higher scores were retained and samples with lower scores were eliminated to improve the overall quality of the ensemble forecast. For each sample, the track score and intensity score were added as the final score of the sample with weight proportions of 10 to 0, 9 to 1, 8 to 2, 7 to 3, 6 to 4, 5 to 5. These were named as tr, 91, 82,73, 64, and 55, respectively. The WRF model was used to simulate five tropical cyclones in the northwestern Pacific to test the ability of this scheme to improve the forecast track and intensity of these cyclones. The results show that the sample optimization effectively reduced the track and intensity error, 55 usually had better performance on the short-term intensity prediction, and tr had better performance in short-term track prediction. From the overall performance of the track and intensity simulation, 91 was the best and most stable among all sample optimization schemes. These results may provide some guidance for optimizing operational ensemble forecasting of TCs. (Li Jihang, Zhang Zhiyan, Liu Lu)

4.27 川藏高原一次混合型强对流天气的观测特征

利用中国气象局地面自动气象站、探空、天气雷达等观测资料和ERA-Interim再分析资料，分析2016年9月8日川藏高原一次强对流天气过程。结果表明:该过程多站出现8级雷暴大风、10 mm以上小时强降水且伴有最大直径为18 mm的冰雹，是川藏高原一次混合型强对流过程。对流系统发生在500 hPa弱冷平流和低层切变线影响下，中低层深厚湿层、环境中等强度对流有效位能和垂直风切变为超级单体的形成和维持提供有利条件。初始北侧多单体和南侧弱对流在地面辐合线上生成，向东南移入适宜环境后，北侧多单体发展成线状对流系统，与南侧单体合并且促使其迅速发展成超级单体。成熟超级单体低层具有清晰的前侧入流缺口、钩状回波和中气旋特征。强回波区随高度前倾，呈显著的上冲云顶突起、回波悬垂和有界弱回波区。风暴内中层径向辐合、上升气流减弱和反射率因子核心快速下降预示下击暴流的产生。中层干空气的夹卷和水凝物快速下落的拖曳作用加强下沉气流，结合峡谷地形的狭管效应，引起地面大风。（王黉，李英，文永仁）

4.28 台风“山竹”（2018）远距离暴雨的成因分析

热带气旋远距离暴雨（TRP）往往成为高影响天气，是业务预报难点。本文用地面、探空观测资料、雷达遥感资料以及NCEP一日四次0.5°×0.5°再分析资料，对2018年第22号台风“山竹”登陆广东期间在长江三角洲（简称长三角）地区引起的远距离暴雨过程进行分析。结果表明：（1）这是一次发生在副热带高压（简称副高）控制范围内的热带气旋远距离暴雨，低层受台风倒槽影响。（2）这次过程第一阶段暴雨主要是在强的对流不稳定条件下，由对流层低层“山竹”倒槽中的辐合线触发对流产生，同时对流层高层“山竹”的极向流出汇入加大了中纬度西风风速，在长三角地区上空产生辐散，有利于上升运动的维持。第二阶段，对流不稳定条件有所减弱，但前一阶段强回波产生的低层偏北外出气流与东南风形成辐合线，辐合线上还有中γ尺度的涡旋产生，又促进了对流发展。850 hPa台风倒槽北端形成一个低涡，500 hPa副高边缘发展出一个短波槽，暴雨的动力条件更为有利。（3）长三角的3个强降水中心分别在长江口、杭州湾北岸的嘉兴沿海及宁波沿海，都是在水陆边界附近。（4）远距离暴雨区的涡度收支诊断发现：暴雨的初始扰动主要由近地层水平辐合辐散项提供，850 hPa的水平辐合辐散项和扭曲项共同作用形成和加强低涡，并通过垂直运动上传使中层700～500 hPa附近涡度增长，进而发展出500 hPa短波槽。850 hPa涡度来自于台风倒槽和副高边缘的偏南急流。（5）在这次远距离暴雨过程中，台风“山竹”与海上西太平洋副高之间形成偏南低空急流，向长三角输送水汽，这与典型TRP事件相似。不同之处在于：典型TRP中暴雨的初始扰动一般由西风槽提供，而这次过程主要由低空台风倒槽和偏南急流提供，涡度上传形成高空短波槽，是不同于典型TRP事件的一个物理过程。（陈淑琴，李英，范悦敏）

4.29 台风“利奇马”（1909）双眼墙特征及长时间维持机制

利用CIMSS微波卫星产品和多普勒天气雷达资料，分析超强台风“利奇马”（1909）的长时间双眼墙特征，并采用集合卡尔曼滤波方法同化雷达径向风资料，诊断台风“利奇马”双眼墙的三维结构演变特征。结果表明：在双眼墙演变过程初期，受强垂直风切变和中高层干空气入侵的影响，外眼墙对流减弱，呈非对称特征。Sawyer-Eliassen方程诊断结果显示：台风“利奇马”（1909）内、外眼墙次级环流之间的相互作用不明显，不同于发生眼墙替换过程的台风，其外眼墙处非绝热加热引起的下沉运动发生在内眼的眼心，内眼墙的上升运动并未受到外眼墙次级环流抑制。另外，在强垂直风切变条件下，非对称的外眼墙不能持续增强收缩并取代内眼墙，因此双眼墙结构得以长时间维持。可见，台风“利奇马”（1909）外眼墙的非对称结构和特殊的次级环流分布是其双眼墙能够长期维持的重要原因。（刘涛，端义宏，冯佳宁）

5 雷电研究

5 Lighting research

5.1 雷电野外科学试验

雷电团队克服新冠疫情影响，于5—8月在广州从化开展了第16年度的人工触发闪电试验。期间建设了新型人工引雷平台，成功触发闪电8次，改进了弱电流测试方案；围绕引雷试验场建设了低功耗太阳能供电的闪电中低频电场探测阵列；完成了针对广州塔雷电流直接测量的方案设计和工程建设；升级实时低频电场探测阵列，引入混合基线GPU三维定位算法实现快速全闪定位；在海南和西藏那曲新建了新一代三维全闪探测网络，关注海陆雷暴和高原雷暴的闪电活动探测。通过上述技术发展和试验开展，显著提升了雷电团队对闪电过程的综合数据获取能力和数据质量。（吕伟涛，郑栋，张阳，樊艳峰，马颖）

5.2 雷电探测技术研发

在VHF闪电探测和定位方面，提出了利用DBM_EEMD方法对宽带干涉仪VHF信号进行带通截断等质量控制，通过广义互相关、信号阈值和相似度约束等方法实现不同天线信号的精准匹配的方法，对闪电通道解析能力获得显著提升。在低频闪电探测和定位方面，引入深度学习技术，提出了基于编码特征的脉冲匹配新方法，大大提高了匹配效率和定位速度。评估了粤港澳闪电定位系统和广东电网地闪定位系统在粤港澳大湾区范围内的地闪探测性能。提出了一种考虑辐射源密度和通道重复放电的辐射源连接方法，能够获得体现时间和空间信息的闪电放电尺度参量，更好地描述闪电放电过程和受影响区域。（张阳，范祥鹏，刘恒毅）

5.3 雷电物理研究

发现广州塔触发式上行闪电的正极性连续先导产生之前，普遍存在先导企图发展过程，并指出触发式上行正先导在初始速度上比人工触发闪电高一个量级；发现有14%的高建筑物负地闪回击事件其继后回击光脉冲峰值大于首次回击。提出了一种闪击距离估算方法，发现雷击建筑物的高度和顶部几何形状是影响闪击距离的关键因素；在正地闪回击后的水平通道上发现了类“针”状结构，指出其是正地闪回击后期和连续电流期间大量负电荷被输送至水平通道上的结果。提出了以集合经验模态分解方法为基础的高能云内脉冲（EIP）放电事件电场波形分解方法，指出EIP的发生机制是逃逸电子雪崩产生的大电流过程；发现EIP与地源伽马射线闪（TGF）之间的密切关系，将任一EIP亦是TGF的可能性由原来的超过37%提到了74%～100%。（樊艳峰，武斌，齐奇，吕凡超）

5.4 雷暴电学研究

给出了高原、中国中东部和喜马拉雅山南麓雷暴结构、电荷区大小以及起电效率概念图，提出闪电频次随着雷暴对流增强持续增大而闪电尺度则先增大再减小的观点，指出闪电频次和尺度的反向关系出现在当对流强度超过一定阈值时，发现冬季雷暴正地闪主要发生在正常极性和反极性电荷结构中，不支持先前的倾斜偶极子假说和雷暴消亡阶段的单层正电荷区假说。利用闪电数据以及卫星云顶亮温和云分类数据建立了亚太区域雷暴特征数据集，揭示我国陆地和毗邻海域以及西太平洋地区的雷暴活动和雷暴云结构特征。发现全球闪电探测网（WWLLN）和星基光学成像仪（LIS）探测的高原闪电活动时空分布存在差异，指出高原雷暴云电属性存在区域差异和季节性变化。发现一类只发生在高原山地雷暴环境中的山顶与其上部雷暴主负电荷区之间的一种特殊放电过程，它可以产生上百kA的瞬态电流；揭示中尺度对流系统层云区闪电分布位置、尺度以及相互之间的关系。（郑栋，张文娟，王飞）

5.5 雷电预警预报技术研发与应用

发现湍流耗散率是指示闪电初生以及区分雷暴和非雷暴的有效参量；针对闪电活动短时预报，发展了基于自动气象站、历史闪电观测和WRF模式模拟的多源时空数据深度神经网络闪电预报框架（LightNet+）。针对闪电活动临近预警，利用闪电监测数据以及雷达数据，构建了一个基于主从时空预测网络模型（MSTNet）的雷电临近预警方法；对比Gatlin算法和σ算法，指出2σ闪电跃增算法更适于基于闪电资料预警北京冰雹天气；实现基于CMA-meso的中国南海区域闪电活动预报，30 km邻域半径下，6～24 h预报时效下的CSI评分超过0.2。初步完成基于人工智能的雷电短时预报系统的研制，找到适合不同评估标准的预报概率阈值，并于2021年5月起在广东省气象安全技术支持中心开展运行试验，取得较好效果。（姚雯，孟青，徐良韬）

5.6 A comprehensive study on the improved radio-frequency magnetic field measurement for the initial upward leader of a negative rocket-triggered lightning flash

The spectrum analysis of the lightning current in the experiment campaign of 2019 reveals that the lightning current waveform contains rich medium-frequency (MF) radiation signals in the initial stage.However, there is a lack of resolution for MF signals by using conventional magnetic sensors. The bandwidth of radio-frequency magnetic field measurement is improved by extending to 20 kHz to 1.2 MHz in the Guangdong Comprehensive Observation Experiment on Lightning Discharge (GCOELD). During the previously noticed quiet period that can only maintain the upward propagation with relatively small-scale breakdown, magnetic pulses of quiet period (MPQPs) are discerned more clearly than the previous experiment in GCOELD. Aided by the improvement of a magnetic sensor, this paper captures richer magnetic field signals radiated from the weak discharge of the precursory phase than previous experiments in GCOELD. The analysis shows that both aborted UPLs and UPLs are caused by weak discharge pulses called initial precursor pulses (IPPs), which are very similar to the amplitude of the streamer discharge obtained in the laboratory.In summary, the signals detected by an improved magnetic sensor will provide an important reference for exploring the pulse characteristics of the whole discharge process and formation mechanism of the UPL in the initial stage of triggered lightning. (Shi Tao, Lu Gaopeng, Fan Yanfeng)

5.7 A new method for connecting the radiation sources of lightning discharge extension channels

The connections of lightning radiation sources along channels are greatly affected by the radiation source density, and the channel length is geometrically scaled when neglecting repeated discharges in the same channel. Based on lightning mapping array (LMA) three-dimensional radiation source location data of two lightning flashes, this study presents a radiation source connection method considering the source density and repeated discharges in channels that includes two steps: the connection of radiation sources and the connection of segments. After increasing the spatial connection threshold determined by the source density, the stability of the channel scale under different detection capabilities (source densities) is improved compared with that of the traditional fixed spatial threshold method. The channel growth rate of the low-density case reaches 120.99%, which is close to the real situation, and the connection shape is consistent with the real situation.For repeated discharge paths, by limiting the time interval of the radiation source connection, the optimized threshold obtained in this paper can distinguish between discharges occurring in the same channel at different times. Compared with the geometric scale, the discharge scale is significantly larger (similar to 2.8 times the geometric scale in one case) and can better characterize both the lightning discharge process and the affected area. These comprehensive results show that the proposed method can reduce the number of incorrect connections, increase the channel length, and obtain a more realistic discharge scale. (Li Yurui, Zhang Yang,Zhang Yijun)

5.8 A positive cloud-to-ground flash caused by a sequence of bidirectional leaders that served to form a ground-reaching branch of a pre-existing horizontal channel

High-speed video and electric field change data were used to analyze the initiation and propagation of four predominantly vertical bidirectional leaders making connection to a predominantly horizontal channel previously formed aloft. The four bidirectional leaders sequentially developed along the same path and served to form a positive branch of the horizontal in-cloud channel, which became a downward positive leader producing a 135-kA positive cloud-to-ground (+CG) return stroke. The positive (lower) end of each bidirectional leader elongated abruptly at the time of connection of the negative (upper) end to the pre-existing channel aloft. Thirty-six negative streamer-like filaments (resembling recently reported needles) extended sideways over similar to 110 to 740 m from the pre-existing horizontal channel at speeds of similar to(0.5−1.9)×107m s−1, in response to the injection of negative charge associated with the +CG. (Wu Bin,Lyu Weitao, Qi Qi)

5.9 Application of ensemble empirical mode decomposition in low-frequency lightning electric field signal analysis and lightning location

The application of empirical mode decomposition (EMD) in the analysis and processing of lightning electric field waveforms acquired by the low-frequency e-field detection array (LFEDA) in China has significantly improved the capabilities of the low-frequency/very-low-frequency (LF/VLF) time-of-arrival technique for studying the lightning discharge processes. However, the inherent mode mixing and the endpoint effect of EMD lead to certain problems, such as an inadequate noise reduction capability, the incorrect matching of multistation waveforms, and the inaccurate extraction of pulse information, which limit the further development of the LFEDAs positioning ability. To solve these problems, the advanced ensemble EMD (EEMD)technique is introduced into the analysis of LF/VLF lightning measurements, and a double-sided bidirectional mirror (DBM) extension method is proposed to overcome the endpoint effect of EMD. EEMD can effectively suppress mode mixing, and the DBM extension method proposed in this article can effectively suppress the endpoint effect, thus greatly improving the accuracy of a simulated signal after a 25500-kHz bandpass filter.The resulting DBMEEMD algorithm can be used in the LFEDA system to process and analyze the detected electric field signals to improve the systems lightning location capabilities, especially in terms of accurate extraction and location of weak signals from lightning discharges. In this article, a 3-D image of artificially triggered lightning obtained from an LF/VLF location system is reported for the first time, and methods for further improving the location capabilities of the LF/VLF lightning detection systems are discussed. (Fan Xiangpeng, Zhang Yijun, Krehbiel Paul R.)

5.10 Characteristics of negative leader propagation area of lightning flashes initiated in the stratiform regions of mesoscale convective systems

To investigate the characteristics of extension areas (mainly the propagation areas of negative leaders in this study), the lightning location data of 254 lightning flashes initiated in the stratiform regions (stratiform lightning flashes) of 14 mesoscale convective systems (MCSs) are analyzed. The results show that most of the flashes have a relatively small lightning area (LA) (≤100 km2), although they are initiated in the stratiform regions. In small or developing MCSs, most negative leaders of stratiform lightning flashes concentrate within the 9−12 km altitude range. In other MCSs with a large-sized and developed stratiform region, besides being in this high-altitude range, the negative leaders are also found to propagate more frequently in a low-altitude range of 5−7 km. Further analysis indicates that most of the stratiform lightning flashes with a large LA (＞100 km2)propagate their negative leaders within the high-altitude range, no matter where they are initiated. Moreover,the stratiform lightning flashes with or near the largest LA tend to be initiated 4−6 km below their negative leaders, while most of the stratiform lightning flashes usually propagate their negative leaders horizontally within ± 1 km of the first detected very-high-frequency (VHF) radiation source. It is inferred that some insitu electrifications occurring before and during the formation of the high reflectivity layers in the low-altitude range contribute to these flashes, although the influence of the advection charges from the convective regions still cannot be totally ruled out. (Wang Fei, Zhang Yijun, Dong Wansheng)

5.11 Electromagnetic characteristics of upward leader initiated from the Canton Tower: A comparison with rocket-triggered lightning

By using the synchronous observation data obtained at the Tall-Object Lightning Observatory in Guangzhou (TOLOG) of 2019, the electromagnetic characteristics of upward positive leader (UPL) ascending from the 600 m high Canton Tower are examined, and are compared with the magnetic field (B-field) radiation of UPL in rocket-triggered lightning. Before the inception of sustained UPL, small electric field (E-field) pulses are superposed on the fast E-field changes. The timescale of B-field pulses corresponding to the E-field pulses ranges from 5 to 9 µs, and the inter-pulse interval is about 30 µs, which are both similar to the B-field pulses associated with the precursors of rocket-triggered lightning. Measurements show that the precursor-like stage is likely common for upward lightning initiated from the Canton Tower. Moreover, the UPL channel of towerinitiated upward lightning extends significantly in the first several ms with the initial average two-dimensional(2-D) velocity of 8.77×105m s−1(3-D velocity of 11.0×105m s−1), which is one order of magnitude faster than the UPL initiated from the wire tip of rocket-triggered lightning, indicating that initiation of UPL from Canton Tower benefits from a substantial E-field enhancement of nearby lightning discharges. However, this favorable condition is rapidly consumed during the UPL development, causing the average 2-D velocity of UPL to decrease rapidly and maintain at about 0.5×105m s−1. It is noted that the variation in the 2-D speed of sustained UPL differs from tower-initiated upward lightning reported in the literature, which is possibly associated with the physical and geometric properties of the tower. (Fan Yanfeng, Lyu Weitao, Lu Gaopeng)

5.12 Fast and fine location of total lightning from low frequency signals based on deep-learning encoding features

Lightning location provides an important means for the study of lightning discharge process and thunderstorms activity. The fine positioning capability of total lightning based on low-frequency signals has been improved in many aspects, but most of them are based on post waveform processing, and the positioning speed is slow. In this study, artificial intelligence technology is introduced for the first time to lightning positioning, based on the low-frequency electric-field detection array (LFEDA). A new method based on deep-learning encoding features matching is also proposed, which provides a means for fast and fine location of total lightning. Compared to other LFEDA positioning methods, the new method greatly improves the matching efficiency, up to more than 50%, thereby considerably improving the positioning speed. Moreover,the new algorithm has greater fine-positioning and anti-interference abilities, and maintains high-quality positioning under low signal-to-noise ratio conditions. The positioning efficiency for return strokes of triggered lightning was 99.17%, and the standard deviation of the positioning accuracy in the X and Y directions was approximately 70 m. (Wang Jingxuan, Zhang Yang, Tan Yadan)

5.13 First documented downward positive cloud-to-ground lightning initiated by an upward negative lightning

An interesting downward positive cloud-to-ground lightning (DPCG) initiated by an upward negative cloud-to-ground lightning (UNCG) was recorded by the instruments installed at the Tall-Object Lightning Observatory in Guangzhou. The characteristics of the discharges before and after the DPCG return stroke were investigated in detail using both optical and electric field change data. The UNCG was triggered from the tip of the Canton Tower (600 m high) by a distant +CG lightning. A sequence of intracloud (IC) discharges developed toward and connected to several pre-existing channels of the UNCG. As a result, some positive charge was pumped from remote thundercloud regions to the region around the intersection of the pre-existing and new channels. A branch of one IC channel developed from the intersection point toward the ground, transformed into a downward positive leader, terminated on the ground, and resulted in a positive return stroke. After that,the discharge processes in the cloud continued to neutralize the remaining positive charge. The UNCG served to facilitate the IC discharges that supplied positive charges for the DPCG. Most of the charge involved in the UNCG-initiated DPCG originated from remote positively charged regions in the cloud. These observations constitute evidence for a new scenario of the initiation of +CG lightning. Key Points A downward +CG initiated by an upward tall-object -CG was recorded for the first time. The characteristics of the discharges before and after the +CG were investigated using optical and electric field change data. The observed sequence of events constitutes evidence for a new scenario of the initiation of downward +CG (Jiang Ruijiao, Lyu Weitao, Wu Bin)

5.14 Lightning fatalities in China, 2009−2018

The statistical characteristics of 1789 deaths, 1552 injuries and 1904 disasters caused by lightning based on the 2009 to 2018 National Lightning Disaster Compilation of Mainland China were analyzed. The results showed that males accounted for 53% of casualties. Lightning disasters were more common in the east and south than in the west and north. The number of lightning disasters and casualties in the south accounted for 82.98% and 82.94% of the totals, respectively. May to August of each year is the intensive period of lightning disasters in China, and the number of deaths, injuries and disasters caused by lightning during these four months accounted for 84.80%, 79.45% and 82.77% of the total numbers per year, respectively. From 2009 to 2018, the annual injury and death rates per million people in China were 0.13 and 0.12, respectively. After population weighting, the high death rate of lightning disaster shows a certain trend of transferring from the concentrated area to the sparsely populated area, and after area weighting, the casualty density was higher in small provinces and lower in large provinces. The analysis of the environments in which lightning disaster casualties occurred found that environments closely related to agriculture, such as farmland (35%) and paddy fields or ponds (4%), accounted for nearly 40% of the casualties; accordingly, rural farmers were the main victims, accounting for 80.96% of the total casualties. Lightning protection and disaster reduction measures in rural agricultural areas should be the focus of future work. (Yin Qiyuan, Liu Hengyi, Fan Xiangpeng)

5.15 New insights into the correlation between lightning flash rate and size in thunderstorms

Thunderstorms over the Tibetan Plateau (TP), Central and Eastern China (CEC), and southern foothills of the Himalayas (SHF), have the correlations between lightning flash rate and size disagreeing with the previously published negative correlation in thunderstorms with different dynamics. There is a positive correlation when comparing TP and CEC thunderstorms. Compared with SHF thunderstorms, TP thunderstorms have much lower flash rate and similar flash size. It is explored that the TP and SHF thunderstorms have the weakest and strongest convection, respectively, and both have small effective charge regions (ECRs). The CEC and SHF thunderstorms contain more ECRs than TP thunderstorms. The weak convection in TP thunderstorm should be responsible for the common occurrence of small flash rate and size. The results suggest that the negative correlation between flash rate and size may require the dynamic intensity of compared thunderstorms to exceed a certain threshold. (Zheng Dong, Zhang Yijun)

5.16 Spatiotemporal lightning activity detected by WWLLN over the Tibetan Plateau and its comparison with LIS lightning

Herein, we compared data on the spatiotemporal distribution of lightning activity obtained from the World Wide Lightning Location Network (WWLLN) with that from the lightning imaging sensor (LIS). The WWLLN and LIS both suggest intense lightning activity over the central and southeastern Tibetan Plateau (TP) during May—September. Meanwhile, the WWLLN indicates relatively weak lightning activity over the northeastern TP, where the LIS suggests very intense lightning activity, and it also indicates a high-density lightning center over the southwestern TP that is not suggested by the LIS. Furthermore, the WWLLN lightning peaks in August in terms of monthly variation and in late August in terms of 10-day variation, unlike the corresponding LIS lightning peaks of July and late June, respectively. Other observation data were also introduced into the comparison. The blackbody temperature (TBB) data from the Fengyun-2E geostationary satellite (as a proxy of deep convection) and thunderstorm-day data support the spatial distribution of the WWLLN lightning more.Meanwhile, for seasonal variation, the TBB data are more analogous to the LIS data, whereas the cloud-toground (CG) lightning data from a local CG lightning location system are closer to the WWLLN data. It is speculated that the different WWLLN and LIS observation modes may cause their data to represent different dominant types of lightning, thereby leading to differences in the spatiotemporal distributions of their data. The results may further imply that there exist regional differences and seasonal variations in the electrical properties of thunderstorms over the TP. (Ma Ruiyang, Zheng Dong, Zhang Yijun)

5.17 Turbulence characteristics of thunderstorms before the first flash in comparison to nonthunderstorms

This study evaluates how clouds evolve into thunderstorms in terms of the turbulence characteristics producing the first flash. Observations of 57 (39) isolated thunderstorm (non-thunderstorm) cells during 2016−2017 in South China are provided by an S-band polarimetric radar and three independent lightning location systems. The vertical turbulence characteristics of clouds associated with thunderstorms are obviously different from non-thunderstorms. For thunderstorms, the maximum of the eddy dissipation rate (mean value in each height layer) in the entire height is 0.19 m2s−3, which occurs at the first flash stage, and the achievable height of turbulence exceeds the −30 layer. For non-thunderstorms, however, the maximum is 0.12 m2s−3,and the achievable turbulence height hardly exceeds the −10 layer. Additionally, the turbulence intensities of the locations where the initial discharge pulse events of the first flashes occur are weak. These turbulence characteristics are useful for lightning nowcasting. (Zhao Chuanhong, Zheng Dong, Zhang Yijun)

5.18 Two-dimensional striking distance of lightning flashes to a cluster of tall buildings in Guangzhou

Based on the high-speed video records of 54 lightning flashes striking on a cluster of tall buildings in Guangzhou and the return stroke peak currents provided by lightning location systems, this paper analyzed the two-dimensional (2D) first-return-stroke striking distances (SD) of lightning flashes to buildings with a height ranging from 100 to 600 m. Three methods, including one proposed in this study, have been used to estimate the 2D SD. The results with different methods basically agree with each other and show that the height and the top geometry of the structure on which lightning terminates are key factors that affect the SD. The correlation between the SD and the peak return stroke current appeared very scattered, particularly in the cases when each building was considered separately. Besides, we found that: (a) the estimated 2D average initiation speed of upward connecting leader (UCL) on different buildings are similar, ranging from 4.9 to 23×104m s−1, with an average of 13.4×104m s−1; (b) about 87% (27/31) flashes that struck on buildings with simple top shape only have one upward leader observed, while for buildings with complicated top shape, only 26% (6/23) flash cases have one upward leader observed; (c) for the 36 flashes to four tall buildings with heights higher than 300 m,the 2D average speed ratio of the downward leader and the UCL is 0.74 during the last 0.1 ms before the first return stroke. (Qi Qi, Lyu Weitao, Wang Daohong.)

5.19 Winter positive cloud-to-ground lightning flashes observed by LMA in Japan

Using a lightning mapping array (LMA), we have observed 24 positive cloud-to-ground (CG) flashes occurred in three thunderstorm days. These flashes can be apparently grouped into 5 clusters according to their occurrence times. We have obtained the charge structures for both the individual flashes and clusters.It was found that 4 out of 5 clusters of positive CG flashes exhibited either inverter dipolar or tri-polar charge structures. This result indicates that the high percentage of positive CG flashes in Hokuriku winter thunderstorms originated from the inverted charge structure rather than the various deformations of normal charge structures widely accepted in literatures. The flash positive charge appeared to distribute usually in a layer with its thickness of around 1 km, its horizontal area of more than 100 km2and its bottom altitude of around 1−2 km above the ground. For each of the flash, we have also obtained its duration, length, duration before the first stroke and convex area. It was found that all flashes with a large peak return stroke current had short durations before their first return strokes. (Wang Daohong, Zheng Dong, Wu Ting)

5.20 CMA＿FEBLS低频三维全闪探测技术研究及观测10年进展

三维全闪探测已经成为了深入认识闪电物理机制和雷暴电活动规律的重要手段。中国气象局雷电野外科学试验基地（CMA_FEBLS）自主研发了低频电场探测阵列（LFEDA），并持续开展了针对广东地区雷暴全闪电活动的综合观测试验，在精细化三维定位算法和基于三维全闪数据的闪电放电过程研究方面取得了若干研究结果。（1） LFEDA具备雷暴电活动的无死时间捕获能力以及优于百米的定位精度和一定的通道定位能力。（2）发展了基于简单脉冲特征的三维全闪定位算法，并进一步将经验模态分解及衍生方法引入到闪电信号处理中，提高了定位的精细化水平；发展了融合到达时差和时间反转技术的定位方法，提升了抗干扰能力、降低了对站网的要求。（3）实现了典型放电事件的电流波形反演，获得了窄偶极性放电事件（NBE）、初始击穿脉冲（IBP）和爆发式脉冲簇（RBPs）的放电特征；研究发现绝大多数闪电始发于IBP，而作为始发的NBE则具有更大的孤立性、更大的相对幅度和更快的发展速度。（4）发现随着起始放电高度增加，初始阶段持续时间和步长增加，发展速度和脉冲丰度下降；始发于强对流区时始发阶段通常有更大的速度和脉冲丰度。（张阳，王敬轩，郑栋）

5.21 FY-4A LMI观测的“利奇马”（2019）台前飑线闪电活动及其与对流演变的关系

为研究风云四号A星闪电成像仪（FY-4A LMI）闪电资料在强对流天气的监测预警能力，以2019年台风“利奇马”台前飑线为例，利用FY-4A LMI闪电资料、FY-4A云顶亮温资料（TBB）、地基闪电定位资料（ADTD）、组网雷达组合反射率因子资料和东南沿海自动站风雨资料，研究“利奇马”台前飑线全闪电活动的时空分布特征及其与飑线内对流演变的关系。结果表明：FY-4A LMI闪电频次的时空变化与台前飑线的演变过程相一致，LMI闪电爆发对台前飑线强度增强具有提早约1 h的指示作用。在闪电活动与台前飑线对流的演变关系上，LMI闪电与卫星TBB深对流及雷达强回波的时空演变存在较好的相关性。LMI观测的闪电频数与强回波（35～55 dBz）顶高具有对应关系，与-72 ℃冷云区面积及35 dBz以上雷达组合反射率因子面积的变化特征相同。闪电活动集中位于TBB低值区的左侧和前部的亮温梯度大值区，对地面雷暴大风和强降水的可能发生位置具有判识作用。LMI与ADTD的比较发现二者所揭示的“利奇马”台前飑线闪电活动特征基本一致。（林小红，张文娟，范能柱）

5.22 不同闪电跃增算法在北京地区应用效果对比

基于S波段多普勒天气雷达基数据、北京闪电定位网全闪定位数据和北京地区降雹的人工观测结果，对比分析Gatlin和σ两种闪电跃增算法在不同配置下对北京地区2015—2018年共177次冰雹天气过程的预警效果。结果表明：不同倍数的σ算法预警结果差别很大，2σ（要求当前闪电频数变化率超过之前平均闪电频数变化率两倍标准差）在σ算法中的预警效果最佳;不同N（总闪频数变化率的数量）配置下的Gatlin算法的预警结果差别不大，其中当N = 6时的预警效果最佳。2σ算法的命中率、虚警率和临界成功指数分别为80.2%，41.6%和51.1%，N = 6的Gatlin算法的相应结果分别为82.5%，62.0%和35.2%。另外，详细分析了一次多单体雷暴过程和一次飑线过程中两种算法的应用情况，结果也表明Gatlin算法比2σ算法的命中率略高，但虚警率偏高很多，临界成功指数偏低。综合Gatlin算法和σ算法对冰雹预报结果评估情况，发现2σ闪电跃增算法更适于对北京冰雹天气的预警，对提升闪电数据在北京地区冰雹预报业务的可用度有一定参考价值。（田野，姚雯，尹佳莉）

5.23 广州高建筑物雷电回击光脉冲特征分析

为了深入认识负地闪放电过程中光辐射信号的特性，对广州高建筑物雷电观测站所获得的回击光脉冲波形进行了分析。对观测到的88例负地闪事件中的184次回击（包括60次下行闪电首次回击、58次下行闪电继后回击、66次上行闪电继后回击）的光脉冲特征进行了统计分析。结果表明：下行闪电首次回击光脉冲10%～90%上升时间T1的算术平均值/中值为32.5/31.4 μs，20%～80%上升时间T2的算术平均值/中值为22.6/22.4 μs，半峰宽度T3的算术平均值/中值为131.1/117.0 μs。下行闪电继后回击光脉冲T1的算术平均值/中值为30.4/27.7 μs，T2的算术平均值/中值为19.5/17.6 μs，T3的算术平均值/中值为153.6/142.6 μs。在21例下行多回击负地闪事件中，光脉冲回击间隔时间在12.6～368.6 ms范围之间，算术平均值为78.7 ms，有14%闪电事件存在继后回击光脉冲峰值大于首次回击的情况。上行闪电继后回击光脉冲T1的算术平均值/中值为27.5/24.3 μs，T2的算术平均值/中值为17.0/15.7 μs，T3的算术平均值/中值为132.2/124.5 μs。总体上，下行闪电首次回击的光脉冲上升时间最长、下行闪电继后回击次之、上行闪电继后回击最小;下行闪电继后回击脉冲半峰宽度比下行闪电首次回击及上行闪电继后回击的更大。（黄晓磊，吕伟涛，武斌）

5.24 华南飑线系统对流与层云区闪电起始和通道位置处的云微物理特征

利用广州S波段双偏振雷达观测数据和低频电场探测阵列三维闪电定位数据，分析了2017年5月4日和5月8日华南地区两次飑线过程中闪电起始和通道位置处的雷达偏振参量和降水粒子特征。两次飑线中约80%的闪电起始和通道（统称闪电放电）定位于对流区。对流区闪电放电位置处的雷达反射率（ZH）要比层云区平均大4～5 dBz，其它偏振参量的平均值较为接近。闪电放电位置处的ZH中值随高度增加而减小，但差分反射率（ZDR）、差分传播相移率（KDP）和共极化相关系数（CC）在-10 ℃层以上随高度变化不大；-10 ℃层以下，对流区闪电放电位置对应ZDR和KDP随高度下降明显增大。闪电起始位置的平均ZH比闪电通道位置处的平均ZH大1～2 dBz，但前者在对流区内对应ZH分布峰值区间为25～30 dBz，弱于后者的30～35 dBz；同时，它们的对比关系在-20 ℃层上下不同。对流区内闪电放电位置处的主导性粒子是霰和冰晶，它们的区域占比接近。在层云区内，闪电放电位置主要是干雪和冰晶，干雪区域的占比显著大于冰晶。（赵川鸿，郑栋，张义军）

5.25 基于闪电聚类方法的西北太平洋区域雷暴活动特征

利用2010—2018年全球闪电探测网（WWLLN）观测资料，采用基于闪电密度的空间聚类算法（DBSCAN）建立了西北太平洋地区雷暴数据集，研究了该区域雷暴的时空分布特征，并进行海陆差异对比。研究结果表明，在合理设定DBSCAN参数阈值的条件下，基于WWLLN闪电聚类的雷暴与天气雷达观测在时空分布和过程演变上具有一致性。西北太平洋区域的日均雷暴数为3869，雷暴的闪电密集区平均面积为557.91 km2，平均延展尺度为31.99 km，平均每小时每个雷暴闪电频次为33次。在空间分布上，东南亚沿海地区与热带岛屿的雷暴活动最强，南海的雷暴活动强于深海。距离海岸线越近的海域其雷暴面积越大。在季节分布上，整个区域雷暴活动在夏季（6—8月）达到全年最强，南海雷暴活动6月达到峰值，而日本东部近海海域的雷暴活动则在冬季达到最强。我国内陆南方地区雷暴3月开始显著增多，雷暴平均面积达到最大，但雷暴平均闪电频次5月才达到峰值。在日变化方面，陆地雷暴活动呈现典型的单峰型特征，大部分雷暴发生在午后及傍晚。海洋雷暴日变化则较为平缓，南海具有其独特的雷暴日变化特征。（周鑫，张文娟，张义军）

5.26 雷暴闪电活动特征研究进展

从一般雷暴、灾害性雷暴和台风的闪电活动特征以及雷暴闪电尺度特征四个方面对相关研究进行梳理。一般雷暴通常具有正常极性电荷结构，云/地闪比例在3左右（中纬度地区），地闪中正地闪占比为10%左右，负地闪位置往往更集中于对流区。灾害性雷暴倾向具有活跃的云闪，低比例的地闪，易出现反极性电荷结构，正地闪比例偏高。闪电活动与灾害性天气现象之间存在关联性，部分雹暴过程具有两次闪电活跃阶段。台风中大部分闪电发生在外雨带，眼壁/外雨带闪电爆发很可能预示气旋强度的增强以及路径的改变。由闪电持续时间、通道空间扩展所表征的闪电尺度与雷暴对流强度相关。弱对流雷暴或雷暴的弱对流区域可能由水平扩展、垂直分层的电荷分布形态主导，闪电频次低，闪电空间尺度大；强对流雷暴或雷暴的强对流区域可能由交错分布的小电荷区主导，闪电频次高，闪电尺度小。（郑栋，张文娟，姚雯）

5.27 雷暴云特征数据集及我国雷暴活动特征

基于FY-2E气象卫星相当黑体亮度温度（TBB）和云分类数据（CLC）及全球闪电探测网（WWLLN）闪电数据，通过对TBB不超过-32 ℃的云区进行椭圆拟合，定义1 h内上述云区或椭圆区域有WWLLN闪电发生的个例为雷暴云，获得雷暴云时间、位置、形态、结构、闪电活动等特征参量，构建雷暴云特征数据集，并基于该数据集初步分析了我国陆地和毗邻海域的雷暴活动特征。研究表明：我国华南、西南、青藏高原东、中部和南海雷暴最为活跃，华北和东北地区是北方雷暴活动较强的区域。雷暴活动时间变化海陆差异明显，陆地雷暴活动峰值出现在6—8月，南海雷暴活动一个峰值出现在5月左右，另一峰值出现在8月后，且纬度越低出现越晚。陆地大部分地区雷暴活动在14:00—20:00（北京时）达到峰值，毗邻海域雷暴活动峰值主要出现在早上。雷暴云TBB不超过-32 ℃面积符合对数正态分布，峰值区间位于1×103～1×104km2，平均值为3.0×104km2。南海雷暴云面积最大，陆地上大于雷暴云面积平均值1.2×105km2的区域主要分布于我国地形的第一阶梯和柴达木盆地。（马瑞阳，郑栋，姚雯）

5.28 热带气旋闪电活动特征研究综述

针对热带气旋（TC）闪电已有研究，首先从闪电活动分布特征、眼壁闪电爆发对TC强度和路径的指示、外雨带闪电活动与雨带对流结构的关系3个方面进行了总结；其次从动力—微物理方面对TC闪电的形成原因和特征机理进行了梳理；最后提出当前研究中存在的两个关键问题，并对后续研究内容进行了展望。基于地基和空基相结合的综合闪电探测得到的闪电属性特征参量，有望建立一个明确的、具有代表性的闪电活动-TC强度变化关系。利用沿海地区架设的三维闪电定位系统结合地基双偏振雷达，针对登陆台风强对流过程开展的综合观测研究，将有助于推进闪电观测资料在台风中小尺度强对流监测、预警和资料同化中的应用。（张文娟，张义军，郑栋）

5.29 铷原子簇自发磁矩的试验观测及理论分析

在碱金属原子簇磁性的研究中，存在自由原子簇含有的原子个数及其磁矩难以准确确定的问题，本文采用光磁共振光谱检测手段，对工作温度约为328 K的饱和铷蒸汽样品中单原子分子87Rb1和14种簇粒子（87Rb）n'（n' = 2，3，…，15）的磁矩进行了深入研究。试验结果表明：在同一外磁场下，14种簇粒子（87Rb）n'的共振频率fn'与87Rb1的共振频率f*之间存在fn'= f*/n'的数值关系，并且各簇粒子的磁矩值与振幅值均随n'的大小和奇、偶性呈现不同性质的变化规律。运用分子轨态理论通过87Rbn= 87Rbn-1+87Rb联合原子簇构造模式，给出14种簇粒子87Rbn（n = 2，3，…，15）的基态和最低激发态的电子组态和分子态项型，分析了各分子态的稳定性和发生可见塞曼效应的可能性。进一步基于双原子分子磁矩公式计算，发现当n = n'时87Rbn的磁矩值与（87Rb）n'的磁矩值严格吻合（平均相对误差仅为0.6765%），证实了（87Rb）n'和87Rbn的对应关系。（邸淑红，张阳，杨会静）

5.30 粤港澳大湾区两套闪电定位系统地闪探测性能的对比分析

根据2014—2018年粤港澳闪电定位系统（GHMLLS）和广东电网地闪定位系统（GDLLS）两套闪电定位系统的观测资料，对粤港澳大湾区范围内二者的地闪探测性能进行评估和对比。结果表明：在粤港澳大湾区，对于地闪回击记录，从各月份记录和空间密度的分布上来看，GDLLS的探测数都多于GHMLLS，二者的比值为1.24；两套系统总回击、负回击、正回击的月分布和日变化特征基本一致；两系统地闪回击的匹配记录在大湾区大部分区域内的偏差量都在1.5 km内；对于匹配后的负、正回击电流峰值，二者有较强的相关性，相关系数分别为0.99和0.98，GDLLS的负、正回击电流峰值分别为GHMLLS的1.47倍和1.45倍。（张悦，吕伟涛，陈绿文）

6 模式和再分析资料

6 Model and reanalysis data

6.1 A large-eddy simulation study on the diurnally evolving nonlinear trapped lee waves over a twodimensional steep mountain

The diurnally evolving trapped lee wave over a small-scale two-dimensional steep mountain is investigated in large-eddy simulations based on a fully compressible and nonhydrostatic model (Icosahedral Nonhydrostatic, ICON) with triangular grids of 50-m-edge length. An idealized atmospheric profile derived from a realistic case is designed to account for influences from the stagnant layer near the surface, the stability of the atmospheric boundary layer (ABL) and the upper-level jet. First, simulations were done to bridge from the linear regime to the nonlinear regime by increasing the mountain height, which showed that largeramplitude lee waves with longer wavelength can be produced in the nonlinear regime than in the linear regime.Second, the effects of the stagnant layer near the surface and the ABL stability were explored, which showed that the stagnant layer or the stable ABL can play a similar wave-absorbing role in the nonlinear regime as in linear theories or simulations. Third, the role of the upper-level jet was explored, indicating that a stronger(weaker) upper-level jet can help to produce longer (shorter) lee waves. The stable ABL with a stagnant layer can more (less) efficiently absorb the longer (shorter) lee waves due to the stronger (weaker) jet, so that the wave response is more sensitive to the wave-absorption layer when an upper-level jet is present. Finally,the momentum budget was analyzed to explore the interaction between the upper and lower levels of the troposphere, which showed that the momentum flux due to the upward-propagating waves and trapped waves varies with the upper-level jet strength and low-level stagnancy and ABL stability. (Xue Haile, Giorgetta Marco A.)

6.2 A new perspective on evaluating high-resolution urban climate simulation with urban canopy parameters

The “state-of-the-art” urban climate models have not been evaluated against dense meteorological networks under various weather conditions. In this study, we conducted high-resolution urban climate simulations in Beijing and investigated the relationship between their performances and urban canopy parameters (UCPs). The latest version of single-layer (UCM) and multi-layer (BEP) urban canopy models were tested separately. Results show that model performances are insensitive to rainfall events in summer, but change significantly with wind conditions in winter. Inclusion of UCPs has a limited impact on air temperature simulations. In terms of wind speed simulations, consistent overestimations by UCM and BEP are found in both summer and winter. The overestimation by BEP reduces significantly when UCPs are available, especially on windy days in winter. On the other hand, performance of UCM can degrade over urban areas with canopy parameters. Wind speed biases are found to correlate significantly with UCPs. The accuracy of wind speed simulations by UCM and BEP increases with urban fraction and building surface ratio. This indicates poor model performances over low-density urban areas, which requires enhanced aerodynamic parameterizations to better account for UCPs. This study reveals the limitation of current urban climate simulations and provides guidance for future model developments. (Yu Miao, Chen Xuan, Yang Jiachuan)

6.3 A study of extrapolation nowcasting based on IVAP-retrieved wind

In this study, we propose a new way to obtain motion vectors using the integrating velocity-azimuth process (IVAP) method for extrapolation nowcasting. Traditional tracking methods rely on tracking radar echoes of a few time slices. In contrast, the IVAP method does not depend on the past variation of radar echoes; it only needs the radar echo and radial velocity observations at the latest time. To demonstrate it is practical to use IVAP-retrieved winds to extrapolate radar echoes, we carried out nowcasting experiments using the IVAP method, and compared these results with the results using a traditional method, namely, the tracking radar echoes by correlation (TREC) method. Comparison based on a series of large-scale mature rainfall cases showed that the IVAP method has similar accuracy to that of the TREC method. In addition, the IVAP method provides the vertical wind profile that can be used to anticipate storm type and motion deviations. (Luo Yi,Liang Xudong, Wang Gang)

6.4 Advances in research on the ITCZ: Mean position, model bias, and anthropogenic aerosol influences

The zonal-mean position of the intertropical convergence zone (ITCZ) and its shift in the meridional direction significantly influence both the tropical and even global climate. This work reviews three aspects of the progress in ITCZ-relevant research: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean positions relative to the equator; 2) causes of the double-ITCZ problem (pervasive in climate models) and the efforts to solve it; and 3) the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ. According to recent studies, the north-of-the-equator location of the annualand zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean, induced by the Atlantic overturning circulation. A quantitative relationship between the ITCZ shift and the anomalous crossequatorial energy transport in the atmosphere has been found. Presently, the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models. Recently,some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models; whereas others have found that improving model resolutions and convective parameterizations in climate models, such as entrainment rate, rain-droplet re-evaporation, and convection triggering function, can alleviate the double-ITCZ bias. Therefore, it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models, which is not yet well-understood. In addition, anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ, but through various physical mechanisms. Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere, whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation. (Zhang Hua, Ma Xinyu, Zhao Shuyun)

6.5 An investigation into the vertical structures of low-altitude atmosphere over the Central Taklimakan Desert in summer

In this study, 1-month continuous radiosonde observational data were applied to present the low-altitude vertical structures and their evolutions over the Central Taklimakan Desert (CTD). The primary focus is to highlight the vertical structures near the ground with the high-resolution (10 m in height and 6 h in time)radiosonde data. One of the unique features evident in our results is an obvious diurnal transition in lower layers near the ground due to strong surface heating or cooling. Unlike a traditional vertical structure in the boundary layer observed over a nondesert surface, both superadiabatic and inversion layers are distinct during the day. More specifically, the superadiabatic layer is obvious in the daytime because of strong solar radiation over the desert, and the superadiabatic can reach up to 0.2 km. In contrast, an apparent inversion layer forms in the nighttime due to the surface cooling. It is found that the surface forcing mainly dominates the structures in the boundary layer. At last, the vertical structures from the observations are compared with those from the ERA-Interim and MERRA2 reanalysis data sets. The results indicate that both reanalysis products can provide similar vertical profile patterns and diurnal variations. However, the diurnal transitions of temperature and wind profiles over the CTD are underestimated severely by both reanalysis data. Besides, the reanalysis data sets completely miss the superadiabatic near the ground in the daytime. (Yin Jinfang, Gu Haodong, Huang Jie)

6.6 Challenges in developing finite-volume global weather and climate models with focus on numerical accuracy

High-resolution global non-hydrostatic gridded dynamic models have drawn significant attention in recent years in conjunction with the rising demand for improving weather forecasting and climate predictions. By far it is still challenging to build a high-resolution gridded global model, which is required to meet numerical accuracy, dispersion relation, conservation, and computation requirements. Among these requirements, this review focuses on one significant topic—the numerical accuracy over the entire non-uniform spherical grids.The paper discusses all the topic-related challenges by comparing the schemes adopted in well-known finitevolume-based operational or research dynamical cores. It provides an overview of how these challenges are met in a summary table. The analysis and validation in this review are based on the shallow-water equation system. The conclusions can be applied to more complicated models. These challenges should be critical research topics in the future development of finite-volume global models. (Xie Yuanfu, Qin Zilong)

6.7 Changes in anthropogenic particulate matters and resulting global climate effects since the Industrial Revolution

In order to quantify air pollution effects on climate change, we investigated the climate response associated with anthropogenic particulate matters (PMs) by dividing fine PM (PM2.5, particle size ≤2.5 µm)and coarse particulate matter (CPM, particle size >2.5 µm) in great detail in this work, with an aerosol-climate coupled model. We find that the changes in PM2.5and CPM are very different and thus result in different, even opposite effects on climate, especially on a regional scale. The column burden of PM2.5increases globally from 1850 to the present, especially over Asia’s southern and eastern parts, whereas the column concentration of CPM increases over high-latitude regions and decreases over South Asia. The resulted global annual mean effective radiative forcing (ERF) values due to PM2.5and CPM changes are -1.21 W m−2and -0.24 W m−2,respectively. Increases in PM2.5result in significant cooling effects on the climate, whereas changes in CPM produce small and even opposite effects. The global annual mean surface air temperature (SAT) decreases by 0.94 K due to PM2.5increase. Coolings caused by increased PM2.5are more apparent over the Northern Hemisphere (NH) terrain and ocean at mid- and high- latitudes. Increases in SATs caused by increased CPM are identified over high latitudes in the NH, whereas decreases are identified over mid-latitude regions. Strong cooling due to increased PM2.5causes a southward shift of the Intertropical Convergence Zone (ITCZ),whereas the Hadley circulation associated with CPM is enhanced slightly over both hemispheres, along with the weak movement of corresponding ITCZ. The global annual mean precipitation decreases by approximately 0.11 mm day−1due to the increased PM2.5. Generally, PM2.5concentration changes contribute more than 80%of the variation caused by all anthropogenic aerosols in ERF, SAT, cloud fraction, and precipitation. (Yang Dongdong, Zhang Hua, Wang Zhili)

6.8 Discontinuous Galerkin isogeometric analysis of convection problem on surface

The objective of this work is to study finite element methods for approximating the solution of convection equations on surfaces embedded in R-3. We propose the discontinuous Galerkin (DG) isogeometric analysis(IgA) formulation to solve convection problems on implicitly defined surfaces. Three numerical experiments shows that the numerical scheme converges with the optimal convergence order. (Wang Liang, Xiong Chunguang, Yuan Xinpeng)

6.9 Effects of mosaic representation of land use/land cover on skin temperature and energy fluxes in Noah-MP land surface model over China

The representations of land use/land cover (LULC) play an important role in land surface models (LSMs)for the simulation of the energy flux partition, soil moisture redistribution, and runoff generation. This study was designed to investigate the regional effects of mosaic LULC representations on skin temperature (Ts) and energy fluxes over China at three horizontal resolutions and how these effects changed with climate regimes,using Noah with multiparameterization (Noah-MP) LSM. The current officially released Noah-MP only considered the most abundant LULC type within one model grid. In this study, the mosaic method considering all the LULC types existing in one model grid was implemented into Noah-MP. Against the reference data(including MODIS land surface temperature products, FLUXCOM energy flux data and Numerical Terra dynamic Simulation Group evapotranspiration data), the mosaic method generally performed better than the default method and reduced the root-mean-squared-error of Tsand energy fluxes significantly over urban region. The mosaic method affected the Tsand energy fluxes by changing leaf area index and soil moisture,mainly by the former. The warm (monthly mean air temperature larger than 10 ) and relatively humid climate(annual total precipitation larger than 200 mm) could enlarge the effect of mosaic method on Tsand energy fluxes. The mosaic method reduced discrepancies of Tsand energy fluxes among three horizontal resolutions(0.0625°, 0.25°, and 0.50°), especially over the heterogeneous vegetated and urban region. Key Points Mosaic representation of land use/land cover was implemented in Noah-MP to obtain more accurate land surface information Noah-MP incorporated with the mosaic method improved the skin temperature and energy fluxes.Warm climate regimes enlarged the effect of mosaic method on skin temperature and energy fluxes through LAI changes. (Zhang Guo, Li Jianduo, Zhou Guangsheng)

6.10 Effects of organic soil in the Noah-MP land-surface model on simulated skin and soil temperature profiles and surface energy exchanges for China

Inclusion of the thermal and hydraulic effect of soil organic matter plays an important role in land-surface models (LSMs) for simulating soil temperature and surface energy exchanges. The current officially released Noah with multiparameterization (Noah-MP) LSM, implemented in the community weather research and forecasting (WRF) model, does not include a parameterization for soil organic matter. In this study, the thermal and hydraulic effect of soil organic matter was parameterized into Noah-MP LSM. Based on the profiled soil organic matter data for China and the observations of skin temperature and soil temperature profiles from more than 2000 surface meteorological stations, we investigated the effects of organic soil using the Noah-MP LSM with and without the profiled soil organic matter at the regional scales. Compared with the simulation without the inclusion of organic soil parameterization, the Noah-MP LSM simulation with the profiled soil organic matter improved the skin temperature and soil temperature profiles, especially soil temperature in deep soils under cold and arid regions. The realistic representation of snow depth and the snow insulation dependency on snow depth were confirmed to be a pre-requisite in Noah-MP in the high latitudes. By incorporating the profiled soil organic matter, the Noah-MP LSM enlarged the regional mean sensible heat flux (SH) and lower the regional mean latent heat flux (LH). In the warm and humid regions, a relatively smaller effect of organic soil on soil temperature could lead to a larger effect on SH and LH, especially LH. (Zhang Guo, Chen Yueli, Li Jianduo)

6.11 Evaluating the impacts of cloud microphysical and overlap parameters on simulated clouds in global climate models

The improvement of the accuracy of simulated cloud-related variables, such as the cloud fraction, in global climate models (GCMs) is still a challenging problem in climate modeling. In this study, the influence of cloud microphysics schemes (one-moment versus two-moment schemes) and cloud overlap methods(observation-based versus a fixed vertical decorrelation length) on the simulated cloud fraction was assessed in the BCC_AGCM2.0_CUACE/Aero. Compared with the fixed decorrelation length method, the observationbased approach produced a significantly improved cloud fraction both globally and for four representative regions. The utilization of a two-moment cloud microphysics scheme, on the other hand, notably improved the simulated cloud fraction compared with the one-moment scheme; specifically, the relative bias in the global mean total cloud fraction decreased by 42.9%−84.8%. Furthermore, the total cloud fraction bias decreased by 6.6% in the boreal winter (DJF) and 1.64% in the boreal summer (JJA). Cloud radiative forcing globally and in the four regions was improved by 0.3%−1.2% and 0.2%−2.0%, respectively. Thus, our results showed that the interaction between clouds and climate through microphysical and radiation processes is a key contributor to simulation uncertainty. (Wang Haibo, Zhang Hua, Xie Bing)

6.12 Evaluation of a flexible single ice microphysics and a Gaussian probability-density-function macrophysics scheme in a single column model

Scale-aware parameterizations of subgrid scale physics are essentials for multiscale atmospheric modeling.A single-ice (SI) microphysics scheme and Gaussian probability-density-function (Gauss-PDF) macrophysics scheme were implemented in the single-column global-to-regional integrated forecast system model (SGRIST)and they were tested using the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) and the Atmospheric Radiation Measurement Southern Great Plains Experiment in 1997 (ARM97). Their performance was evaluated against observations and other reference schemes. The new schemes simulated reasonable precipitation with proper fluctuations and peaks, ice, and liquid water contents, especially in lower levels below 650 hPa during the wet period in the TWP-ICE. The root mean square error (RMSE) of the simulated cloud fraction below 200 hPa was 0.10/0.08 in the wet/dry period, which showed an obvious improvement when compared to that, i.e., 0.11/0.11 of the original scheme. Accumulated ice water content below the melting level decreased by 21.57% in the SI. The well-matched average liquid water content displayed between the new scheme and observations, which was two times larger than those with the referencing scheme. In the ARM97 simulations, the SI scheme produced considerable ice water content, especially when convection was active.Low-level cloud fraction and precipitation extremes were improved using the Gauss-PDF scheme, which displayed the RMSE of cloud fraction of 0.02, being only half of the original schemes. The study indicates that the SI and Gauss-PDF schemes are promising approaches to simplify the microphysics process and improve the low-level cloud modeling. (Li Jiabo, Peng Xindong, Li Xiaohan)

6.13 Evaluation of CMIP6 global climate models for simulating land surface energy and water fluxes during 1979−2014

This study examined the overall performance of the climate models in the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the key energy and water fluxes over land. For this purpose, this study selected multiple land flux products as reference data sets and assessed the global spatial means, patterns, trends, seasonal cycles, and regional mean estimates of the sensible heat (SH), latent heat (LH),net radiation (RN), runoff (RF), and precipitation (PR) simulated by 32 CMIP6 models in recent decades. The global (Antarctica, Greenland, and hot deserts are not included) mean SH, LH, RN, RF, and PR simulated by the CMIP6 models are 37.55 ± 4.81 W m−2, 49.88 ± 5.31 W m−2, 89.10 ± 4.45 W m−2, 351.31 ± 95.28 mm yr−1,and 948.35 ± 88.77 mm yr−1, respectively. The ensemble median of CMIP6 simulations (CMIP6-MED) can provide robust estimates of global and regional land fluxes, which are within the ranges given by the reference data sets, and highly consistent spatiotemporal patterns of these fluxes. The comparison of CMIP6-MED with the first preferred reference data sets shows that CMIP6-MED generally overestimates the water and energy fluxes over land, except for the simulated RF and PR in the Amazon region. The most disagreements between CMIP6-MED and the reference data sets occur in South America (particularly the Amazon region) and the Tibetan Plateau. Finally, the sources of model biases are discussed. It is suggested that current land flux products should be widely used to optimize the structures and parameters of climate models in future work. (Li Jianduo, Miao Chiyuan, Wei Wei)

6.14 Evaluation of five reanalysis products with radiosonde observations over the Central Taklimakan Desert during summer

To provide guidance for the use of reanalysis data in the Central Taklimakan Desert (CTD), five reanalysis products are evaluated based on the radiosonde data obtained from two field experiments during summer for the first time in the CTD, including the European Center for Medium-Range Weather Forecasts (ECMWF)reanalysis version 5 (ERA5), ECMWF reanalysis-interim (ERA-Interim), Japanese 55-years reanalysis(JRA55), modern-era retrospective analysis for research and applications version 2 (MERRA2), and the National Centers for Environmental Prediction-Department of Energy reanalysis version 2 (NCEP2). The results show that reanalysis temperature (T), specific humidity (Q), geopotential height (GPH), and wind field(U and V components) are consistent with the radiosonde observations in terms of the vertical distribution. In general, ERA5 has the best performance in the CTD during the study period, followed closely by ERA-Interim.However, NCEP2 produces the largest error. The errors of all the reanalysis data show significant diurnal variations, and the diurnal variations differ from each other. Moreover, the results indicate that the reanalysis datasets have the largest deviation at 850 hPa (near the ground), which means that in the desert region complex interactions may exist between the land surface and the atmosphere. Therefore, more attention should be paid to the description of complex interactions between land and atmosphere over the moving-sand desert region in the numerical models. (Huang Jie, Yin Jinfang, Wang Minzhong)

6.15 High-order conservative and oscillation-suppressing transport on irregular hexagonal grids

A third-order numerical scheme was developed for 2D irregular hexagonal meshes for the advection problems in this study. The scheme is based on a multi-moment constrained finite-volume method (MCV) in Cartesian coordinates and entails the introduction of a general integration method over a hexagonal cell. Unlike in the conventional finite-volume method, various discrete moments, that is, point value and volume-integrated average, are adopted as computational constraints to achieve high-order computation. The high-order spatial reconstruction can therefore be built in a local space, which considerably reduces the stencil length. The numerical scheme is tested using various idealized experiments. Compared with the existing schemes, this scheme is demonstrated to be flexible for application in irregular hexagonal meshes without increasing cost or compromising on accuracy. The general integration formulation based on a third-order polynomial helps to expand the application to arbitrary hexagons that does not require the use of centroids as computational points or Voronoi tessellation. It is also convenient to define the orthogonal wind components in the Cartesian system to directly drive the atmospheric transport. (Jiao Han, Peng Xindong, Che Yuzhang)

6.16 Impact of parameterizing the turbulent orographic form drag on convection-permitting simulations of winds and precipitation over South China during the 2019 pre-summer rainy season

A turbulent orographic form drag (TOFD) parameterization, which accounts for the unresolved drag induced by the subgrid orographic variance, is implemented in the Weather Research and Forecasting (WRF)model with a horizontal grid spacing of 3 km. Its impact on the surface wind and precipitation forecasts over South China during a pre-summer rainy season (April−June 2019) is evaluated based on 3-hour observations from more than 2500 stations, by comparing the results from two series of parallel simulation with the TOFD parameterization turned on or off, respectively. Results show that the seasonal mean root mean square error(RMSE) and bias of the surface wind have been significantly reduced by more than 7% and 5%, respectively.The low-level wind is also shown improved by comparing the two series of simulations with 12-hour observations from 26 sounding stations. Consequently, the equitable threat score (ETS) is increased by the TOFD parameterization for most rainfall-intensity categories, and the six-category mean ETS shows an improvement of more than 3%. The mechanism of the TOFD impact on low-level wind and precipitation is investigated as well. The low-level southwesterlies from the South China Sea flow over the rough land in South China are found to be weakened, which leads to the low-level wind convergence and more precipitation over those regions. Moreover, this effect is more apparent during the daytime. (Xue Haile, Zhou Xu, Luo Yali)

6.17 Modulation of snow on the daily evolution of surface heating over the Tibetan Plateau during winter: Observational analyses

Studying the daily evolution of turbulent fluxes modulated by snowfall over the Tibetan Plateau (TP) is of great importance to understand the features of the change in the TP heat source/sink and its contribution to Asian atmospheric circulation and weather processes. However, the lack of data over the TP restricts the detailed studies. Based on observations from four sites of the Third TP Atmospheric Scientific Experiment,the process of surface energy balance impacted by snow is investigated. The results show that the surface albedo largely increases on the first day of snow and then slowly decreases. Correspondingly, the sensible heat(H) flux sharply decreases after snow and then gradually recovers to the original level during the following approximately 10 days. The latent heat (LE) flux becomes more active and stronger after snowfall and persists for a longer period than H, since the soil moisture may still contribute to a high LE after snowmelt.As the synergistic result of H and LE modulated by snow, the surface turbulent heating (i.e., the sum H and LE) of the TP decreases at the early period of snow events and then even enhances to a higher level after the snowmelt than before snow. Comparison analyses reveal that the impact of snow on the H and LE over the TP is much stronger than over similar latitude low-altitude regions in North America and Europe, which may be partly attributed to the larger and more drastic change of the surface net solar radiation associated with snow processes in the TP. The ERA5 and CFS reanalysis data sets fail to reproduce the modulation of snow on the heat fluxes, which suggests that the physical schemes of the models should be further improved based on the observational analyses over TP. This study may help further understand the detailed physical processes of modulation of snow events on Asian weather processes during winter and is also conducive to the improvement of surface parameterization schemes of models. (Xin Yufei, Liu Ge, Chen Yueli)

6.18 Potential driving factors on surface solar radiation trends over China in recent years

The annual mean surface solar radiation (SSR) trends under all-sky, clear-sky, all-sky-no-aerosol, and clear-sky-no-aerosol conditions as well as their possible causes are analyzed during 2005−2018 across China based on different satellite-retrieved datasets to determine the major drivers of the trends. The results confirm clouds and aerosols as the major contributors to such all-sky SSR trends over China but play differing roles over sub-regions. Aerosol variations during this period result in a widespread brightening, while cloud effects show opposite trends from south to north. Moreover, aerosols contribute more to the increasing all-sky SSR trends over the northern China, while clouds dominate the SSR decline over the southern China. A radiative transfer model is used to explore the relative contributions of cloud cover from different cloud types to the alltypes-of-cloud-cover-induced (ACC-induced) SSR trends during this period in four typical sub-regions over China. The simulations point out that the decreases in low-cloud-cover (LCC) over the North China Plain are the largest positive contributor of all cloud types to the marked annual and seasonal ACC-induced SSR increases, and the positive contributions from both high-cloud-cover (HCC) and LCC declines in summer and winter greatly contribute to the ACC-induced SSR increases over East China. The contributions from mediumlow-cloud-cover (mid-LCC) and LCC variations dominate the ACC-caused SSR trends over the southwestern and South China all year round, except for the larger HCC contribution in summer. (Wang Qiuyan, Zhang Hua,Yang Su)

6.19 Reconstruction of missing data in weather radar image sequences using deep neuron networks

Missing data in weather radar image sequences may cause bias in quantitative precipitation estimation(QPE) and quantitative precipitation forecast (QPF) studies, and also the obtainment of corresponding highquality QPE and QPF products. The traditional approaches that are used to reconstruct missing weather radar images replace missing frames with the nearest image or with interpolated images. However, the performance of these approaches is defective, and their accuracy is quite limited due to neglecting the intensification and disappearance of radar echoes. In this study, we propose a deep neuron network (DNN), which combines convolutional neural networks (CNNs) and bi-directional convolutional long short-term memory networks(CNN-BiConvLSTMs), to address this problem and establish a deep-learning benchmark. The model is trained to be capable of dealing with arbitrary missing patterns by using the proposed training schedule. Then the performances of the model are evaluated and compared with baseline models for different missing patterns.These baseline models include the nearest neighbor approach, linear interpolation, optical flow methods,and two DNN models three-dimensional CNN (3DCNN) and CNN-ConvLSTM. Experimental results show that the CNN-BiConvLSTM model outperforms all other baseline models. The influence of data quality on interpolation methods is further investigated, and the CNN-BiConvLSTM model is found to be basically uninfluenced by less qualified input weather radar images, which reflects the robustness of the model. Our results suggest good prospects for applying the CNN-BiConvLSTM model to improve the quality of weather radar datasets. (Gao Lihao, Zheng Yu, Wang Yaqiang, Xia Jiangjiang, Chen Xunla, Li Bin, Luo Ming, Guo Yuchen)

6.20 Classification of the circulation patterns related to strong dust weather in China using a combination of the Lamb-Jenkinson and K-means clustering methods

Sand and dust storms (SDSs) cause major disasters in the northern China. They have serious impacts on human health, daily life, and industrial and agricultural production, in addition to threatening the regional ecological environment and social economy. Based on meteorological observational data and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 dataset for spring 2000–2021, we used the Lamb-Jenkinson circulation classification method to classify the three major areas influencing SDSs in the northern China. We also used the K-means clustering method to classify the overall circulation pattern in the northern China. Our results show that the circulation types favoring SDSs in the southern basin of Xinjiang are southwesterly winds (SW), cyclones (C), and anticyclones (A). The circulation types favoring SDSs in the western Inner Mongolia and southern Mongolia are northwesterly winds (NW), northerly winds (N), cyclones(C), and anticyclones (A). The circulation types favoring SDSs in the central Inner Mongolia are northwesterly winds (NW), northerly winds (N), southwesterly winds (SW), and anticyclones (A). The 500 hPa and surface circulation patterns in China can be divided into nine types. Among them, five dominant circulation patterns favor strong SDSs: a cold high-pressure region and cold front (T1), a Mongolian cyclone (T2), a mixed type of Mongolian cyclone and cold front (T3), a thermal depression and cold front (T5), and a cold front (T8). During 2000‒2004, the T8 circulation pattern occurred most frequently as the main influencing circulation. From 2005 to 2010, the T3 and T8 circulation patterns dominated. Circulation patterns T1 and T3 dominated during 2011–2015 and 2016–2020, respectively. We analyzed the main circulation patterns for four SDS events occurring in 2021 by combining the Lamb-Jenkinson and k-means methods. The SDS events in 2021 were closest to the T3 circulation pattern and were mainly influenced by Mongolian cyclones and surface cold fronts. The main propagation paths were westerly and northwesterly. (Yi Ziwei, Wang Yaqiang, Chen Wencong, Guo Bin, Zhang Bihui, Che Huizheng, Zhang Xiaoye)

6.21 IPCC AR6对地球气候系统中反馈机制的新认识

气候反馈反映了气候系统内部对外界干扰的适应过程，在很大程度上影响对未来气候变化的预估。本文对政府间气候变化专门委员会（IPCC）第六次评估报告（AR6）中有关气候反馈的内容进行了梳理。相比第五次评估报告（AR5），AR6对云反馈的认识有了较大的提高，尤其是副热带海洋上空低云的反馈。AR6认为在高信度上云反馈参数为正值，即对气候变化起到一种放大效应。不过，云反馈的不确定范围在所有反馈机制中依然是最大的。除了普朗克反馈外，其他反馈机制（包括水汽、温度直减率、地表反照率、云、生物地球物理和非CO2生物地球化学反馈）均在正值区间或零附近，总体上对气候变化起到放大效应。AR6对总的气候反馈的估计值为-1.16 W/（m2.℃），5%～95%的置信区间为（-1.81～-0.51） W/（m2.℃）。随着气候平均态的增暖，气候反馈参数很可能会更靠近正值。（赵树云，孔铃涵，张华）

6.22 IPCC AR6报告解读：地球能量收支、气候反馈和气候敏感度

文中对IPCC第六次评估报告（AR6）第一工作组（WGI）报告的第七章关于地球能量收支、气候反馈和气候敏感度中的重要内容进行了凝练，并简要总结该方面的最新研究成果和结论。评估显示，自工业革命以来，人类活动造成的有效辐射强迫（ERF）为2.72（1.96～3.48）W/m2，其中，均匀混合温室气体的贡献为3.32（3.03～3.61）W/m2，气溶胶的贡献为-1.1（-1.7～-0.4）W/m2。净的气候反馈参数为-1.16（-1.81～-0.51）W/（m2.℃），云仍然是气候反馈整体不确定性的最大来源。平衡态气候敏感度（ECS）和瞬态气候响应（TCR）可用于评估全球平均地表温度对强迫的响应，是衡量全球气候响应的有效手段。ECS和TCR的最佳估计分别为3.0（2.0～5.0）℃和1.8（1.2～2.4）℃。（张华，王菲，赵树云）

6.23 尺度自适应大气边界层参数化改进及其对一次海雾的数值模拟研究

大气边界层湍流运动是地球大气运动最重要的能量输送过程之一。当数值模式分辨率接近活跃含能湍涡长度尺度时，湍流运动被部分解析，被称为“灰色区域”，传统的边界层方案不适合此时模式湍流问题的描述。为了提高模式边界层方案在包括“灰色区域”的不同网格尺度上的描述能力，适应不同分辨率模式的需要，在雷诺平均湍流理论基础上，修正Mellor-Yamada-Nakanishi-Niino （MYNN）方案湍流长度尺度参数和非局地湍流的参数表达，改进湿度和温度在“灰色区域”的湍流输送参数化及对网格尺度的自适应能力。利用改进的MYNN尺度自适应方案，分别采用3 km和1 km、1.5 km和0.5 km分辨率单向嵌套网格WRF中尺度模式，对2014年2月26日的一次黄海海雾过程进行模拟试验，检验不同分辨率下改进后的MYNN大气边界层参数化方案的合理性和对海雾的模拟效果。尺度自适应MYNN大气边界层参数化方案在千米级网格尺度上获得稳定、合理的湍流垂直输送计算结果。参照雾区卫星云图，不同分辨率模式低层云水混合比模拟结果具有稳定表现，模拟的雾区分布和温度、湿度等物理量结构都较好地再现了再分析“观测事实”，初步表明该参数化方案有较高的网格尺度自适应能力。（卢绪兰，彭新东）

6.24 次季节波动对青藏高原及其下游东亚季风区降水的影响

影响青藏高原及其下游季风区的次季节波动对我国乃至亚洲地区的洪涝灾害起着不容忽视的作用。本文回顾了近40年来在青藏高原及下游东亚季风区的次季节降水方面取得的进展，主要从影响降水的次季节尺度波动的特征、来源和传播机制对研究成果进行了归纳。回顾表明，青藏高原及下游东亚季风区的降水主要受到来自欧亚大陆及孟加拉湾——南海地区30～60天及准双周次季节波动的强烈影响，同时高原次季节波动能够直接与间接地影响下游季风区降水。本文有助于系统理解高原及下游东亚季风区次季节大气振荡及降水，并基于现有研究提出了该领域值得进一步研究的重点和方向，具有一定的科学意义和参考价值。（杨琳韵，王淑瑜，符淙斌）

6.25 复杂地形对大理地区风场的影响研究——兼论观测、理论和数值模拟配合解答科学问题

引用了一次基于真实地形和大气背景的大涡模拟，阐述了大理大风成因并就如何配合使用观测资料、理论研究和模式模拟来研究小尺度高影响天气现象进行了讨论。通过大涡模拟的方式补充了观测的稀疏和理论的缺失，解答了大理三处地方大风形成的机制。同时又反过来分别对观测、理论和模式发展提出了新要求。（1）背风波是否真实存在，具有何种日变化和季节变化特征，如要提供更加坚实的证据，就必须要有长期的三维流场观测。（2） 背风波为什么会有周期性增强伸展和减弱缩短的现象，这要求对这种特殊的大振幅背风波的机理进行研究，而机理研究要求进行理论研究和理想模拟研究，因此本次大涡模拟对模式和理论也提出了新要求。（3）背风波的这种周期性变化会对动量在垂直和水平方向上的分布造成何种影响，其导致的背风波转子会对水汽和热量通量造成何种影响，这要求建立这种大振幅背风波的参数化方案或者使用超高分辨率大范围模拟，这对模式研发也提出了新要求。（薜海乐）

6.26 基于功率谱的风廓线雷达回波强度定标方法

风廓线雷达已在我国得到大范围的业务布网应用，现有业务产品主要为风场信息。为了充分发挥风廓线雷达的作用，获取更多的天气过程信息，该文提出仅使用风廓线雷达返回信号功率谱进行数据定标（DCNP）的方法。使用雷达系统噪声功率对返回信号功率谱单位幅度进行标校计算，基于标校后的雷达探测功率谱分布数据计算回波强度功率谱密度分布、回波强度、大气折射率结构常数。利用2017年北京风廓线雷达、2016年南京风廓线雷达和2018年梅州风廓线雷达观测数据，对我国业务运行的3种主要型号风廓线雷达进行算法评估试验。定标方法的计算结果稳定，风廓线雷达不同探测模式之间的一致性较好。使用每个测站定标结果与相邻天气雷达数据进行比较，风廓线雷达回波强度定标结果与天气雷达也有较好的一致性。DCNP方法与基于信噪比（SNR）的强度计算方法进行比较，与SNR方法相比，DCNP方法定标结果更加稳定可靠。（李丰，阮征，王红艳）

6.27 冬季稳定性降水相态预报研究进展

冬季降水无论对地面的人类生产生活还是对高空飞机航行都可能造成严重灾害，降水相态预报的准确性决定了冬季降水预报的成功。该文较系统地回顾了近几十年来降水相态的预报方法与技术的研究成果。降水相态预报的方法大致分为3类：第1类是基于观测或者数值预报建立的指标或回归方程，其中某些方法高度依赖于数值预报模式的准确率；第2类是数值模式预报微物理方案法和集合预报法；第3类是基于观测和数值预报资料的人工智能预报法。近年来降水相态模式预报产品的预报准确率不断提高，成为降水相态预报中一个重要的产品支撑。但如何将降水相态形成机制的微物理研究成果用于改善数值预报模式降水相态预报的技巧以及利用人工智能等技术提高降水相态预报的准确率等方面还需不断努力。（赵琳娜，慕秀香，马翠平，王秀娟，李缔华）

6.28 中国气象局野外科学试验基地管理服务平台

中国气象局野外科学试验基地管理服务平台是中国气象局气象科学决策支撑应用集约化建设项目的一个子系统，由中国气象科学研究院承担建设。平台开发采用前后端分离技术，通过nginx+tomcat方式有效解耦。前端开发采用vue+ElementUI框架，使用异步调用后端接口方式实现前后端数据交互和呈现。后端采用快速、敏捷的Springboot开发框架，应用tomcat服务实现接口发布，与前端实现数据交互。平台提供了两类管理服务：（1）面向基地工作人员和各级管理人员等内部注册用户，平台提供了基地管理信息、设备信息和数据信息的填报、审核与共享服务。通过预置的数据分析模型，生成分类或综合数据分析图、表，也可按需生成数据查询结果。（2）面向普通用户，平台网站提供了基地综合信息展示、基地短视频宣传和基地全景科普服务。通过浏览器呈现野外试验基地全景的视觉效果。用户使用鼠标控制环视方向，犹如置身现场环境中，在三维窗口中浏览外场试验基地的环境、场景、仪器设备、观测数据和集成的研究成果。2021年该平台完成了与气政通平台的集成，实现了31个基地在线信息填报，多个基地全景虚拟现实展示和短视频制作。（高梅）

7 卫星研究与应用

7 Satellite research and application

7.1 An observational study on the local climate effect of the Shangyi Wind Farm in Hebei Province

Zhangjiakou is an important wind power base in Hebei Province, China. The impact of its wind farms on the local climate is controversial. Based on long-term meteorological data from 1981 to 2018, we investigated the effects of the Shangyi Wind Farm (SWF) in Zhangjiakou on air temperature, wind speed, relative humidity,and precipitation using the anomaly or ratio method between the impacted weather station and the nonimpacted background weather station. The influence of the SWF on land surface temperature (LST) and evapotranspiration (ET) using MODIS satellite data from 2003 to 2018 was also explored. The results showed that the SWF had an atmospheric warming effect at night especially in summer and autumn (up to 0.95 ). The daytime air temperature changes were marginal, and their signs were varying depending on the season. The annual mean wind speed decreased by 6%, mainly noted in spring and winter (up to 14%). The precipitation and relative humidity were not affected by the SWF. There was no increase in LST in the SWF perhaps due to the increased vegetation coverage unrelated to the wind farms, which canceled out the wind farm-induced land surface warming and also resulted in an increase in ET. The results showed that the impact of wind farms on the local climate was significant, while their impact on the regional climate was slight. (Liu Yonghong, Dang Bing, Xu Yongming)

7.2 Comparing the thermal structures of tropical cyclones derived from Suomi NPP ATMS and FY-3D microwave sounders

Accurate information on the thermal structures of tropical cyclones (TCs) is essential for monitoring and forecasting their intensity and location. In this study, a scene-dependent 1-D variation (SD1DVAR)algorithm is developed to retrieve atmospheric temperature and moisture profiles under all-weather conditions.In SD1DVAR, the background and observation error matrix varies according to the scattering intensity.Especially, the observation error matrix increases in precipitating atmospheres due to a larger uncertainty in the forward operator. With the data from the advanced technology microwave sounder (ATMS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite, SD1DVAR can retrieve better thermal structures in the storm life cycle than the NOAA Microwave Integrated Retrieval System (MIRS). Comparing with the aircraft dropsonde observations, the temperature and humidity errors from SD1DVAR are about 3 K and 20%,respectively, whereas those from MIRS are around 4−5 K and 30%, respectively. SD1DVAR is also applied for microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) onboard the FengYun3D(FY-3D) satellite. The MWTS and MWHS data sets are first combined into a single comprehensive microwave suite (CMWS) data stream and then used to retrieve the hurricane thermal structures. It is shown that the hurricane structure from CMWS is very similar to that from ATMS. However, due to the availability of 118-GHz measurements from the CMWS, the hurricane temperature vertical structure is better resolved, and the humidity error is also reduced by about 5%. (Hu Hao, Han Yang)

7.3 Discrete ordinate adding method (DOAM), a new solver for advanced radiative transfer modeling system (ARMS)

Satellite data assimilation requires a computationally fast and accurate radiative transfer model. Currently,three fast models are commonly used in the numerical weather prediction (NWP) models for satellite data assimilation, including the Radiative Transfer for TIROS operational vertical sounder (RTTOV), community radiative transfer model (CRTM), and advanced radiative transfer modeling system (ARMS). ARMS was initiated in 2018 and is now becoming the third pillar supporting many users in NWP and remote sensing fields. Its radiative transfer solvers (e.g. doubling adding method) are inherited from CRTM. In this study,we propose a discrete ordinate adding method (DOAM) to solve the radiative transfer equation including both solar and thermal source terms. In order to accelerate the DOAM computation, the single scattering approximation is used in the layer with an optical depth less than 10−8or a single scattering albedo less than 10−10. From principles of invariance, the adding method is then applied to link the radiances between the layers. The accuracy of DOAM is evaluated through four benchmark cases. It is shown that the difference between DOAM and the DIScrete ordinate radiative transfer (DISORT) decreases with an increase of stream number. The relative bias of the 4-stream DOAM ranges from -5.03% to 5.92% in the triple layers of a visible wavelength case, while the maximum bias of the 8-stream DOAM is only about 1%. The biases can be significantly reduced by the single scattering correction. Comparing to the visible case, the accuracy of the 4-stream DOAM is much higher in the thermal case with a maximum bias -1.69%. Similar results are also shown in two multiple-layer cases. In the MacBook Pro (15-inch, 2018) laptop, the 2-stream DOAM only takes 1.68 seconds for calculating azimuthally independent radiance of 3000 profiles in the hyper-spectral oxygen A-band (wavelength ranges from 0.757 µm to 0.775 µm), while the 4-stream DOAM takes 4.06 seconds and the 16-stream DOAM takes 45.93 seconds. The time of the 2-, 4- and 16-stream DOAM are 0.86 seconds,1.09 seconds and 4.34 seconds for calculating azimuthally averaged radiance. DISORT with 16 streams takes 1521.56 seconds and 127.64 seconds under the same condition. As a new solver, DOAM has been integrated into ARMS and is used to simulate the brightness temperatures at the microwave humidity sounder (MWHS)as well as the microwave radiation imager (MWRI) frequencies. The simulations by the DOAM are compared to those by the Doubling Adding method and the accuracy of both solvers shows a general agreement. All the results show that the DOAM is accurate and computational efficient for applications in NWP data assimilation and satellite remote sensing. (Shi Yining, Yang Jun, Weng Fuzhong)

7.4 Impact of hematite on dust absorption at wavelengths ranging from 0.2 to 1.0 µm: An evaluation of literature data using the T-matrix method

Hematite is the absorbing mineral component of dust aerosols in the shortwave spectral region. However,dust shortwave absorption related to hematite suffers from significant uncertainties. In this study, we evaluated available hematite complex refractive index data in the literature on determining the dust effective refractive index at wavelengths ranging from 0.2 to 1.0 µm using rigorous T-matrix methods. Both spherical and superspheroidal dust with hematite inclusions were examined to compute the dust optical properties and associated effective refractive indices. We found that the imaginary part of the effective refractive index retrieved from all available hematite complex refractive index data is larger than the measured effective values from Di Biagio et al. The result obtained using the hematite refractive index from Hsu and Matijevic is closest to but approximately two times larger than Di Biagio et al. Our results emphasize the importance of accurate measurements of mineral refractive indices to clarify the dust absorption enigma. (Zong Ruirui, Weng Fuzhong, Bi Lei)

7.5 Impacts of urban spatial layout and scale on local climate: A case study in Beijing

Based on the meteorological data (2009−2018) acquired by high-density automatic meteorological stations (AMSs) in Beijing, the influences of urbanization on urban heat island (UHI), wind, and humidity were described by UHI, wind speed ratio (Wsr), and specific humidity ratio (qr), respectively. For the 37 AMSs in central Beijing, the relationships between UHI, Wsr, and qrand six spatial morphological parameters (building height (BH), building density (BD), floor area ratio, sky view factor, frontal area index, and roughness length(RL)) and two land surface parameters (vegetation coverage and impervious cover) in the range of 200−3000 m from the station were studied. The scale effect of each parameter on local climate was also studied. The results showed that individually, these parameters account for 46.8%−79.6% of the change in UHI, 25.6%−52.8% of the change in Wsr, and 25.7%−29.3% of the change in qr. The impact of spatial morphological parameters on local climate has surpassed that of land surface parameters. The parameters that make the largest contributions to the annual average UHI, Wsr, and qrare RL, BH, and BD, respectively. The optimal influencing ranges of the spatial morphological parameters on UHI, Wsr, and qrare 800−1000 m, 1600−2600 m, and 1200−1400 m,respectively. (Liu Yonghong, Xu Yongming, Weng Fuzhong)

7.6 Influence of the urban spatial layout of central Beijing on the atmospheric humidity field

Based on the meteorological data (2009−2018) acquired by high-density automatic meteorological stations in the central urban area (CUA) of Beijing, this study adopted an urban-rural ratio method to construct a qrto quantify the impact of urbanization on the spatial and temporal distribution of specific humidity (q). And the urban morphological parameters such as building height (BH), building density (BD), floor area ratio (FAR),sky view factor (SVF), and land surface parameters including vegetation coverage (VC) and impervious coverage (IC) with 500-m spatial resolution in CUA were calculated, and furthermore, the relationships between the six spatial layout parameters and another indicator, urban heat island (UHI) intensity, and qrwere studied. The results show that q of CUA is 81% to 114% of that of the suburbs, and the urban dry island(UDI) effect and urban wet island (UWI) effect coexist throughout the year. The maximum UDI occurs during autumn daytime and the maximum UWI appears at night in winter. The UDI effect is prone to occur in areas with VC < 11%, IC >85%, BH > 15 m, and BD > 24%, while the UWI effect is more likely to appear in areas with VC > 35% and FAR < 0.3. The contributions of the six urbanization parameters to the spatial change of qrin different time periods are 6.2% to 33.5%, and VC is the largest (33.5%), followed by BD (31.9%) and SVF(29.9%), and the main factor affecting the annual average qris BD with the contribution of 18.9%. Compared with the urbanization parameters, the UHI is more important and the contribution to the change of q can be up to 35.7%. The results show that these spatial layout parameters are not sufficient to explain the main change in q,and more other parameters need to be considered. (Liu Yonghong, Xu Yongming, Han Xiuzhen)

7.7 Retrieval of oceanic total precipitable water vapor and cloud liquid water from Fengyun-3D microwave sounding instruments

Fengyun-3D (FY-3D) satellite is the latest polar-orbiting meteorological satellite launched by China and carries 10 instruments onboard. Its microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) can acquire a total of 28 channels of brightness temperatures, providing rich information for profiling atmospheric temperature and moisture. However, due to a lack of two important frequencies at 23.8 and 31.4 GHz, it is difficult to retrieve the total precipitable water vapor (TPW) and cloud liquid water path(CLW) from FY-3D microwave sounder data as commonly done for other microwave sounding instruments.Using the channel similarity between Suomi National Polar-orbiting Partnership (NPP) advanced technology microwave sounder (ATMS) and FY-3D microwave sounding instruments, a machine learning (ML) technique is used to generate the two missing low-frequency channels of MWTS and MWHS. Then, a new dataset named as combined microwave sounder (CMWS) is obtained, which has the same channel setting as ATMS but the spatial resolution is consistent with MWTS. A statistical inversion method is adopted to retrieve TPW and CLW over oceans from the FY-3D CMWS. The intercomparison between different satellites shows that the inversion products of FY-3D CMWS and Suomi NPP ATMS have good consistency in magnitude and distribution. The correlation coefficients of retrieved TPW and CLW between CMWS and ATMS can reach 0.95 and 0.85, respectively. (Han Yang, Yang Jun, Hu Hao)

7.8 The potential of satellite sounding observations for deriving atmospheric wind in all-weather conditions

Atmospheric wind is an essential parameter in the global observing system. In this study, the water vapor field in Typhoon Lekima and its surrounding areas simulated by the weather research and forecasting (WRF)model is utilized to track the atmospheric motion wind through the Farneback Optical Flow (OF) algorithm.A series of experiments are conducted to investigate the influence of temporal and spatial resolutions on the errors of tracked winds. It is shown that the wind accuracy from tracking the specific humidity is higher than that from tracking the relative humidity. For fast-evolving weather systems such as typhoons, the shorter time step allows for more accurate wind retrievals, whereas for slow to moderate evolving weather conditions, the longer time step is needed for smaller retrieval errors. Compared to the traditional atmospheric motion vectors(AMVs) algorithm, the Farneback OF wind algorithm achieves a pixel-wise feature tracking and obtains a higher spatial resolution of wind field. It also works well under some special circumstances such as very low water vapor content or the region where the wind direction is parallel to the moisture gradient direction. This study has some significant implications for the configuration of satellite microwave sounding missions through their derived water vapor fields. The required temporal and spatial resolutions in the OF algorithm critically determine the satellite revisiting time and the field of view size. The brightness temperature (BT) simulated through community radiative transfer model (CRTM) is also used to track winds. It is shown that the error of tracking BT is generally larger than that of tracking water vapor. This increased error may result from the uncertainty in simulations of brightness temperatures at 183 GHz. (Zhang Yijia, Hu Hao, Weng Fuzhong)

7.9 北京城市空间形态对热岛分布影响研究

在城市尺度上探究城市空间形态布局对城市热岛（UHI）影响研究，对于城市规划中通风环境改善、生态宜居城市建设具有重要意义。以北京为例，利用2009—2018年高密度自动气象站逐小时气温资料和2018年NPP/VIIRS夜光卫星资料，分析了UHI时空分布特征；利用2017年1∶2000基础地理信息和Landsat8卫星资料，开展了北京主城区建筑高度（BH）、建筑密度（BD）、建筑高度标准差（BSD）、容积率（FAR）、迎风截面积指数（FAI）、粗糙度长度（RL）、天空开阔度（SVF）、城市分数维（FD）等8个空间形态参数和植被覆盖度（VC）、不透水盖度（IC）、反照率（AB）等3个陆表参数的提取，并在城市尺度上开展了这些参数与UHI之间空间相关性及对UHI变化影响研究。结果显示：2009—2018年北京主城区年均、四季以及夜晚02:00 UHI均存在一个较为固定的形态，年均、春、夏、秋、冬、白天14:00和夜晚02:00UHI分别为1.81 ℃、1.50 ℃、1.43 ℃、2.16 ℃、2.17 ℃、0.48 ℃和2.77 ℃；8个空间形态参数在一年中大部分时段与UHI存在明显空间相关性，这种相关性在冬季强于其他季节，在夜晚02:00强于白天14:00，排名前三的分别为SVF、FAR和BD。空间形态参数已超越陆表参数成为UHI变化的重要驱动因子，11种参数对UHI变化的单独贡献为13.7%～63.7%，其中夏季、冬季和全年时段贡献最大的空间形态参数分别是BD（43.7%）、SVF（63.7%）和SVF（45.4%），贡献最大的陆表参数分别是VC（42.6%）、AB（57.1%）和VC（45.3%）；夏季、冬季和全年时段多个参数对UHI变化的综合贡献分别为51.4%、69.1%和55.3%，主导要素分别为BD、SVF和BD。（刘勇洪，徐永明，张方敏）

7.10 卫星微波大气遥感温湿廓线及应用进展

简要回顾了星载微波探测仪对大气温湿廓线遥感应用技术进展，对统计回归反演方法、一维变分反演方法以及最新的场景自适应反演方法的发展进行了介绍。最后通过介绍新型微波探测仪搭载平台，对可能的新反演产品和技术进行展望。（胡皓，翁富忠）