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 Applications of QC and merged Doppler spectral density data from Ka-band cloud radar to microphysics retrieval and comparison with airplane in situ observation

The new Chinese Ka-band solid-state transmitter cloud radar (CR) uses four operational modes with different pulse widths and coherent integration and non-coherent integration numbers to meet long-term cloud measurement requirements.The CR and an instrument-equipped aircraft were used to observe clouds and precipitation on the east side of Taihang Mountain in Hebei Province in 2018.To resolve the data quality problems caused by attenuation in the precipitation area,we focused on developing an algorithm for attenuation correction based on rain drop size distribution (DSD) retrieved from the merged Doppler spectral density data of the four operational modes following data quality control (QC).After dealiasing Doppler velocity and removal of range sidelobe artifacts,we merged the four types of Doppler spectral density data.Vertical air speed and DSD are retrieved from the merged Doppler spectral density data.Finally,we conducted attenuation correction of Doppler spectral density data and recalculated Doppler moments such as reflectivity,radial velocity,and spectral width.We evaluated the consistencies of reflectivity spectra from the four operational modes and DSD retrieval performance using airborne in situ observation.We drew three conclusions:First,the four operational modes observed similar reflectivity and velocity for clouds and lowvelocity solid hydrometeors; however,three times of coherent integration underestimated Doppler reflectivity spectra for velocities greater than 2 m s–1.Reflectivity spectra were also underestimated for low signal-tonoise ratios in the low-sensitivity operational mode.Second,QC successfully dealiased Doppler velocity and removed range sidelobe artifacts,and merging of the reflectivity spectra mitigated the effects of coherent integration and pulse compression on radar data.Lastly,the CR observed similar DSD and liquid water content vertical profiles to airborne in situ observations.Comparing the CR and aircraft data yielded uncertainty due to differences in observation space and temporal and spatial resolutions of the data.(Liu Liping,Ding Han,Dong Xiaobo)

1.2 Algorithms for Doppler spectral density data quality control and merging for the Ka-band solidstate transmitter cloud radar

The Chinese Ka-band solid-state transmitter cloud radar (CR) can operate in three different work modes with different pulse widths and coherent integration and non-coherent integration numbers to meet the requirement for long-term cloud measurements.The CR was used to observe cloud and precipitation data in the southern China in 2016.In order to resolve the data quality problems caused by coherent integration and pulse compression,which are used to detect weak clouds in the cloud radar,this study focuses on analyzing the consistencies of reflectivity spectra using the three modes and the influence of coherent integration and pulse compression,developing an algorithm for Doppler spectral density data quality control (QC) and merging based on multiple-mode observation data.After dealiasing Doppler velocity and artefact removal,the three types of Doppler spectral density data were merged.Then,Doppler moments such as reflectivity,radial velocity,and spectral width were recalculated from the merged reflectivity spectra.Performance of the merging algorithm was evaluated.Three conclusions were drawn.Firstly,four rounds of coherent integration with a pulse repetition frequency (PRF) of 8333 Hz underestimated the reflectivity spectra for Doppler velocities exceeding 2 m s–1,causing a large negative bias in the reflectivity and radial velocity when large drops were present.In contrast,two rounds of coherent integration affected the reflectivity spectra to a lesser extent.The reflectivity spectra were underestimated for low signal-to-noise ratios in the low-sensitivity mode.Secondly,pulse compression improved the radar sensitivity and air vertical speed observation,whereas the precipitation mode and coherent integration led to an underestimation of the number concentration of big raindrops and an overestimation of the number concentration of small drops.Thirdly,a comparison of the individual spectra with the merged reflectivity spectra showed that the Doppler moments filled in the gaps in the individual spectra during weak cloud periods,reduced the effects of coherent integration and pulse compression in liquid precipitation,mitigated the aliasing of Doppler velocity,and removed the artefacts,yielding a comprehensive and accurate depiction of most of the clouds and precipitation in the vertical column above the radar.The recalculated moments of the Doppler spectra had better quality than those merged from raw data.(Liu Liping,Zheng Jiafeng)

1.3 Statistical characteristics of raindrop size distribution in South China summer based on the vertical structure derived from VPR-CFMCW

The raindrop size distribution (DSD) characteristics during the precipitation season are analyzed using data collected by an OTT Particle Size Velocity (Parsivel) disdrometer and a Vertical Pointing Radar with C-band Frequency Modulation Continuous Wave (VPR-CFMCW) technology.The two datasets were collected at the same site in Longmen,Guangdong Province,which is the precipitation center of South China,from June to July in 2016 and 2017.We evaluate different fitting methods for the gamma model function and choose a nonlinear least-squares method to fit DSD.Based on the radar reflectance obtained by VPR-CFMCW,the precipitating clouds that occur during the summer precipitation season in South China are classified into four types (i.e.,convective,stratiform,mixture,and shallow).The characteristic parameters and the gamma model parameters of different precipitation types are compared.Avoiding the limitations of rainfall classification at the surface,the new classification quantifies the characteristics of mixture and shallow precipitation.The results show that the stratiform precipitation makes up 43.1% of the summer precipitation processes in South China,and the contribution of convective precipitation to total rainfall is 62.7%.The precipitation parameters of the four types of precipitation,such as the rain rate (R),the mass-weighted mean diameter (D-m),the radar reflectance (Z),and the liquid water content (LWC),follow the pattern:convective＞mixture＞stratiform＞shallow.The DSD characteristics of the four precipitating cloud types are investigated.For the DSD of convective and mixture precipitation,the spectra width is similar but the rain drop concentration of the mixture is smaller.For the DSD of stratiform and shallow clouds,the rain drop concentrations are similar,but the spectra widths of the shallow clouds are smaller.In addition,the relationships between mu-Lambda,D-m-N-w,D-m-R,and Z-R are obtained.These new relationships will help improve the accuracy of precipitation estimation and deepen the understanding of the characteristics of surface precipitation microphysical parameters for different types of precipitating clouds in South China.(Huo Zhaoyang,Ruan Zheng,Wei Ming)

1.4 Dual CCD detection method to retrieve aerosol extinction coefficient profile

The profile of aerosol extinction coefficient can help understand the air pollution transportation and development of the atmospheric boundary layer.The charge-coupled device (CCD)-laser aerosol detection system (CLADS) was widely used to measure the profile of aerosol extinction coefficient,which has excellent resolution near the ground.Traditionally,a constant aerosol scattering phase function and single scattering albedo (SSA) is assumed when retrieving the profile of aerosol extinction coefficient using the measured signals from CLADS.Sensitivity studies in this research show that aerosol scattering phase function leads to an uncertainty up to 462% of the retrieved profile of aerosol extinction coefficient,while SSA leads to an uncertainty up to 25%.A new method is proposed to derive the profile of aerosol extinction coefficient by using two CCD cameras.The aerosol scattering phase function can be determined by minimizing the difference between profiles of aerosol extinction coefficient from the two CCD cameras without any assumption.The profile of aerosol extinction coefficient can be retrieved with high accuracy by using our optimized aerosol scattering phase function.This method is validated by simulation studies where the relative difference between the pre-parameterized aerosol extinction profile and retrieved aerosol extinction profile is below 6%.This dual CCD detection system is employed in a field measurement and proved to be reliable.Our proposed method can obtain more accurate profile of aerosol extinction coefficient for further works about air pollution and atmospheric boundary layer development.(Lian Shaopeng,Bian Yuxuan,Zhao Gang)

1.5 Analysis of dual-polarimetric radar variables and quantitative precipitation estimators for landfall typhoons and squall lines based on disdrometer data in the southern China

Typhoon rainstorms often cause disasters in the southern China.Quantitative precipitation estimation (QPE) with the use of polarimetric radar can improve the accuracy of precipitation estimation and enhance the typhoon defense ability.On the basis of the observed drop size distribution (DSD) of raindrops,a comparison is conducted among the DSD parameters and the polarimetric radar observation retrieved from DSD in five typhoon and three squall line events that occurred in the southern China from 2016 to 2017.A new piecewise fitting method (PFM) is used to develop the QPE estimators for landfall typhoons and squall lines.The performance of QPE is evaluated by two fitting methods for two precipitation types using DSD data collected.Findings indicate that the number concentration of raindrops in typhoon precipitation is large and the average diameter is small,while the raindrops in squall line rainfall have opposite characteristics.The differential reflectivity (Z(DR)) and specific differential phase (K-DP) in these two precipitation types increase slowly with the reflectivity factor (Z(H)),whereas the two precipitation types have different Z(DR) and K-DP in the same Z(H).Thus,it is critical to fit the rainfall estimator for different precipitation types.Enhanced estimation can be obtained using the estimators for specific precipitation types,whether the estimators are derived from the conventional fitting method (CFM) or the PFM,and the estimators fitted using the PFM can produce better results.The estimators for the developed polarimetric radar can be used in operational QPE and quantitative precipitation foresting,and they can improve disaster defense against typhoons and heavy rains.(Zhang Yonghua,Liu Liping,Bi Shuoben)

1.6 青藏高原雅鲁藏布大峡谷水汽通道入口处雨滴谱分布特征

利用墨脱2019年6—10月的雨滴谱仪数据，对雅鲁藏布大峡谷水汽通道入口处的降水及雨滴谱特征进行了统计分析。结果表明，雨强＜5 mm/h的降水占了降水频次的98%，且占总降水量的80%。雨滴谱的谱宽随着降水率的增加而增加，雨强大于1 mm/h的雨滴谱呈现双模态结构。从拟合的伽马分布散点图来看，墨脱地区的层状云降水表现出较宽的滴谱范围，log10Nw的范围为2.1～4.6，Dm为0.4～1.7 mm。平均值分别为3.59和0.77 mm，与那曲、华北地区的平均值比较接近。而对流云降水具有海洋性降水的特征，表现出数浓度较大、质量加权直径较小的特点，log10Nw的范围为3.1～4.7，Dm为0.9～2.4 mm。平均值分别为4.08和1.31 mm，接近于华东和华南的对流云伽马参数分布平均值。对于层状云来说，拟合的Z-R关系与那曲的接近，而对流云拟合的Z-R关系与墨脱的差异性较大，这与雨滴谱分布特征是一致的。（王改利，王志恩，刘黎平）。

1.7 基于灾情信息的1981—2017年北京地区降雹特征

对1981—2017年北京地区1010个高精度冰雹灾情信息进行了统计分析。从年代际变化看，1981—1990年平均降雹日数为10 d，1991—2000年和2001—2010年年平均降雹日数均有所减少（5.67 d和4.33 d），而从2011年起，年平均降雹日数急剧增加到21 d。1981—1995年年平均最大冰雹直径总体呈增加趋势，2002年开始总体较小。从年变化看，冰雹日数的年变化呈明显的单峰型即初夏峰型，4月起降雹日数逐渐增加并在6月达到峰值，其后缓慢下降。从空间分布看，北京地区的降雹分布十分广泛，但高频次降雹区域主要集中在北京西北部延庆区，平均每年至少发生两次降雹，此外降雹高值区还出现在城区的海淀区。2010年后，降雹范围明显增大，同时降雹分布也由相对集中变为相对均匀。（虎雅琼，边宇轩，黄梦宇）

1.8 利用Ka波段毫米波雷达功率谱反演云降水大气垂直速度和雨滴谱分布研究

利用中国气象科学研究院2016年华南云降水试验中Ka波段毫米波雷达探测一次层状云降水过程，开展了云内大气垂直速度和雨滴谱的反演研究，并与地面激光雨滴谱仪和微降水雷达的测量雨滴谱结果进行对比分析。首先，采用小粒子示踪法从功率谱密度中反演大气垂直速度以得到静止空气条件下的功率谱密度，进而利用粒子下落末速度-粒子直径关系反演出雨滴谱，最后进行标准化的Gamma分布拟合。研究表明：（1）云降水从零度层到地面1 km，主要由下沉气流主导，近地面大气浮游粒子和直流干扰造成的晴空杂波会影响雷达的功率谱分布；受动态范围限制，回波强度过饱和现象会影响近地面大气垂直速度的反演结果。（2）毫米波雷达CR、微雨雷达MRR和地面雨滴谱仪测量回波强度存在一定差异，MRR相较于CR与地面雨滴谱仪测量偏差较小；在稳定降水时CR和MRR功率谱密度对比较为一致。（3）CR和MRR反演雨滴谱对比试验中，雨滴谱反演对大气垂直速度十分敏感，大气垂直速度的变化会使CR反演雨滴谱随着高度增加数浓度量级变大、粒子平均半径变小。CR反演的雨滴谱与MRR反演结果基本一致，验证了CR功率谱反演雨滴谱方法的可靠性。（4）CR与地面雨滴谱仪雨滴谱拟合参数的对比表明，CR大气垂直反演的雨滴谱与地面雨滴谱相比粒子平均直径Dm较小，数浓度则较为一致。（马宁堃，刘黎平，郑佳锋）

1.9 杭州“12·05”降雪天气过程的偏振雷达观测分析

国内利用新一代多普勒天气雷达已经开展了较多冬季天气过程的分析研究，但随着国内雷达网逐渐升级到双线偏振雷达，如何将偏振参量应用于冬季业务预报成为了目前需要解决的问题。利用杭州临安C波段双线偏振雷达观测的2015年12月5日一次降雪过程资料及地面和探空资料，通过提出的基于零度层亮带识别的降雪相态识别方法和降雪累计时间统计方法，分析了雷达参量、零度层亮带的时空演变及统计的降雪累计时间分布特征，并与地面和探空资料对比，探索了双线偏振雷达在冬季降雪预报中的优势。结果表明：（1）冬季降水相比夏季连续性降水回波强度偏弱，雨、雪的差分反射率因子、相关系数差别不大。（2）零度层亮带在某些方位并不是水平的，大多数情况下为偏离雷达站的不规则环状或者线状，且某些时刻还有垂直零度层亮带存在。（3）降雪累计时间分布与实际降雪厚度分布基本一致，统计方法为地区降雪厚度的估测提供了可能。（4）零度层亮带的演变与地面和探空温度的空间、时间变化一致，相比单极化雷达使用双线偏振雷达观测零度层亮带更加可靠。（魏玮，刘黎平，吴翀）

1.10 利用相控阵及双偏振雷达对2016年6月3日华南一次强对流过程的分析

结合X波段相控阵雷达（XPAR）和S波段双偏振天气雷达（SPOL）及地面和探空观测资料，分析了2016年6月3日发生在华南的一次超级单体中小尺度特征。结果表明：（1）SPOL的多普勒参量和双偏振参量可以观测到超级单体的强度、速度和相态分布，以及超级单体的入流和出流影响钩状回波的形成。（2）XPAR可以获取时空分辨率远高于SPOL的观测资料，可以弥补SPOL仰角层严重不足的缺陷，观测到超级单体完整的垂直结构，可更精确地描述超级单体的短时演变。（3）超级单体钩状回波附近给地面带来了降温、大风和强降水，超级单体经过地区均有明显降温和风区。研究结果证明相控阵雷达和双偏振雷达对认识对流单体中小尺度系统的发展演变有较大帮助。（于明慧，刘黎平，吴翀）

1.11 风廓线雷达资料在GRAPES-Meso模式中的同化应用研究

以未来业务化应用为目标，进行了业务数值预报模式GRAPES-Meso（Global/Regional Assimilation and Prediction System）中的风廓线雷达资料同化应用研究。基于2015年7月的全国风廓线雷达观测数据，首先建立了面向同化应用的风廓线雷达资料两步质量控制方案。通过对比分析质量控制前后风廓线雷达观测资料集与欧洲中期天气预报中心再分析资料ERA-Interim的差值场特征，论证了质量控制方案的合理性，两步质控后风场误差显著减小，同时观测背景差更接近高斯分布，符合数值同化应用假设。将质量控制后的风廓线雷达资料应用于GRAPES-3DVar系统，开展有、无风廓线雷达资料同化的对比试验，通过批量试验和台风“莲花”个例分析来探讨风廓线雷达资料同化对数值预报的影响。研究表明，在循环同化过程中加入风廓线雷达资料对数值模式初始场有一定改善，风场、温度场、湿度场的分析误差均有减小，从而使短期降水（0～12 h）的预报技巧得以提高。针对台风暴雨个例分析结果表明，风廓线雷达资料同化能有效地调整台风降水区的动力结构和水汽分布，在模式中形成更有利于对流发展的环境条件，从而更好地预报降水的位置与强度。（王丹，阮征，王改利）

1.12 X波段双线偏振雷达数据质量分析及控制方法

利用双偏振参量在弱降水过程中性质均一、随时空变化缓慢的特征，选取北京、佛山地区弱降水过程的观测资料，通过将较长时间观测结果沿径向或方位累积的方法，分析双偏振参量测量精确度受地物、避雷针、旋转及俯仰关节的影响，并提出相应的质控方法。得出以下结论：（1）差分反射率（ZDR）、相关系数（ρhv）及差分传播相位（ΦDP）比水平反射率（Z）对地物更敏感，其中在地物处ρhv小于0.85，ZDR低于-1 dB。根据降雨与地物之间偏振参量特征的不同，将ρhv长时间累积能有效地识别地物回波。（2）每根避雷针对双偏振参量影响的方位和幅值是近似一致的。在以避雷针为中心的±15°的方位范围内，ZDR增大0.4～1.5 dB，ρhv降低0.01以下，Z降低1～2 dBz，且均在避雷针中心处影响达到极值。通过基于上述方位的统计订正可以较好地去除避雷针对双偏振参量的影响。（3）雷达旋转关节的异常会导致ZDR在水平方向上不平稳变化，而俯仰关节异常会使ZDR在高、低仰角差距较大，通过ZDR沿方位一段时间的累积得到各层仰角ZDR变化曲线，用此曲线来实现ZDR的误差标定。通过检验，本文提出的质量控制方法有效地提升了X波段双偏振雷达的数据质量，为其在业务中的进一步推广提供了支持。（王超，吴翀，刘黎平）

2 青藏高原天气研究

2 Research on weather over the Tibetan Plateau

2.1 热源驱动下青藏高原云降水上游-下游对流“组织化”模型

青藏高原为“亚洲水塔”的对流云敏感区。高原南侧来自低纬海洋“大三角扇形”水汽输送构成了高原云降水的水汽供应源，且此“亚洲水塔”水汽源可跨越赤道追溯到南半球。夏半年，原上空大气的物理属性与赤道低纬地区有许多相似之处。有关文献指出青藏高原东部夏季旺盛的中尺度对流活动和巨大的积雨云“烟囱效应”向上层大气持续地输送着热量和水汽。从动力学角度讲，在对流边界层中浮力是驱动湍流的主要机制。这种湍流不是完全无规则的，而往往是有组织成热泡和卷流之类可识别的结构。青藏高原中部和东部爆米花状组织化异常对流云系可揭示出与高原地形效应、空气密度小与湍流异常有关联的特殊湍流—对流运动物理机制。

青藏高原是全球唯一的对流层中部“中空热岛”，隆升的高原地形和强大的表面辐射加热形成了局地上升对流和高耸入对流层中部的热源柱。统计发现1948—2018年青藏高原“热岛”（高温“暖心”）气温纬向偏差值可达4.5 ℃以上。无疑，占中国约1/4面积持续存在如此强度的热岛已超越了世界上任何超级城市群落所产生的热力效应。通过流函数解析风场分析发现，与视热源相关的低层500 hPa涡度场高原区域呈逆时针旋转气旋环流，高层200 hPa则明显为顺时针旋转反气旋环流。从卫星遥感动态图像可发现上层对流云团往往围绕高原中心顺时针旋转，显然其与高原区域高层反气旋环流系统相关联。青藏高原“中空热岛”300～100 hPa呈高、低层互为反向环流结构，其高层潜力释放的类似台风“CISK”的自激反馈机制，导致高原高频组织化异常对流云降水系统，显然此类似台风结构，大尺度三维立体涡旋系统与夏季高原特殊“空中热岛”的驱动效应亦密切相关。另外，全球 500 hPa以上整层水汽含量场亦可描述出青藏高原亦为全球唯一的“湿岛”特征，这反映青藏高原亦是全球对流层云降水核心区。通过分析1961—2015年夏季中国区域地面观测站低云量资料可发现，雅鲁藏布江、三江源与高原东南缘区域是中国区域低云量的极值区。有关青藏高原陡峭南坡水汽爬升的“热驱动力”研究发现，夏季青藏高原整层视热源（热源柱）导致高层辐散驱动低层水汽流辐合，高原南坡—高原平台“两阶梯耦合爬升模型”可实现源自低纬海洋暖湿气流在陡峭南坡爬升的协同驱动，为青藏高原对流组织化异常云降水提供强水汽源。

夏季高原特殊“空中热岛”的驱动效应印证了青藏高原这一热驱动形成的三维特殊的涡旋结构对“亚洲水塔”大气水分循环起着核心作用。这不仅可揭示出此特殊的涡旋结构亦对“亚洲水塔”云降水活动起着关键影响效应，而且亦是青藏高原“世界屋脊”上游—下游云降水组织化对流触发的重要机制。计算发现，长江流域暴雨频数与全国低云量相关分布特征呈现从青藏高原延伸至长江下游地区带状高相关结构。这充分表明长江流域云降水组织化对流系统与上游高原“亚洲水塔”“热驱动”机制具有重要相关关系。

研究1979—2016年夏季青藏高原对流源东移轨迹发现，东移至下游长江流域的对流系统可能源于青藏高原，也与前人的研究吻合。青藏高原大气视热源对局地与下游区域云降水过程水汽输送流型等均呈显著影响；夏季在高原热力作用下对流层低层形成了辐合带，随后辐合带在青藏高原东部边缘激发出气旋性涡旋，伴随着充足的水汽输送，气旋性涡旋东移发展，在长江中下游上空演变成为中尺度强对流云系统。统计分析发现，上述高层反气旋相关环流系统向东延展，在长江流域上空高层呈东—西反气旋型辐散带；中低层则为东—西辐合带，此类三维环流相关结构有助于在长江流域产生组织化对流降水雨带。有关研究结果表明，长江流域暴雨与特大暴雨（23.4%）发生前期青藏高原上空水汽通量涡旋位移特征，统计发现存在明显的水汽通量涡旋结构东移影响到长江流域异常降水事件。在长江洪涝过程中，青藏高原地区中部和东部往往会出现爆米花状对流云高频突发现象，研究表明青藏高原水汽输送通道及其对流云团亦是影响中国区域旱涝形成的重要因素。上述研究可综合描述出高原热源驱动相关环流涡旋，尤其高层带状向下游延伸的反气旋型辐散结构亦是“激发”下游“组织化”对流及东亚区域天气灾害事件的关键动力机制之一。（徐祥德）

2.2 冬季区域大气重污染过程天气气候背景“大地形”“背风坡”大气动力、热力三维结构综合影响模型

京津冀及周边地区位于青藏高原“二阶梯”大地形的太行山东侧和燕山南侧的“半封闭”地形，是中国东部南—北向带状平原的最北端。深化京津冀秋冬季大气重污染气象成因认识，为大幅减少重污染事件提供天气、气候变化影响的科学认知，以回答京津冀地区大气污染长期变化趋势的主要气象驱动因素等关键科学问题，需从冬季京津冀特殊地理特征背景下大地形效应及其气候变化影响的视角，阐述区域大气动力、热力三维结构对大气重污染生消过程的影响。研究表明，高原背风坡垂直环流结构季节性特征与中国区域霾日空间分布季节差异相吻合，西风带背景下秋冬季高原大地形东侧存在类似的顺时针下卷环流圈，类似背风坡弱风区“避风港”效应，其恰对应霾日频发峰值区。春夏季背风坡则为逆时针垂直环流圈，其伴随上升气流特征；不同季节霾日频数峰值大小依次为冬、秋、春、夏季。秋冬季盛行的西北季风与大地形效应协同作用是造成京津冀冬季霾年际变化显著差异的重要因子。在冬季，值得注意的是高原大地形东侧不仅背风坡垂直环流出现“反转”而且下游区域大气垂直热力结构亦出现“逆转”，大地形东侧大气对流层中层呈现了“上暖下冷”的大尺度“暖盖”气温距平结构特征，“背风坡”地形效应与“暖盖”热力结构使中国东部上空大气层结变得更加稳定，由此有利于边界层下压、大气逆温频次上升，为该区域大气重污染过程形成提供了关键性气候背景。

历史数据分析表明，特别是从20世纪90年代中后期开始，到21世纪以来秋冬季盛行西北季风作用和大气扩散能力减弱，且近年来受气候变暖影响，对流层中层气温增幅，气温距平“暖盖”结构年代际特征更为显著。研究表明，冬季青藏高原对应冷源，其视热源柱与中国东部区域平均霾日频数存在显著的相关性，我们仍需思考青藏高原对气候变暖响应是否强化了上述“暖盖”气象条件，而与中国东部频繁发生的霾事件是否存在关联。敏感性模拟亦揭示出青藏高原变暖的热力异常因素可影响中国东部雾霾发生频率的机制，证实了气候变暖背景下该区域雾霾频发的年代际变化趋势与青藏高原的热源结构变化呈显著相关。通过分析研究亦进一步证实了青藏高原的热源结构与对流层中层大尺度“暖盖”特征存在十分明显的关联性。中国东部处于高原大地形东侧，呈“半封闭”地理特征，京津冀为南-北向带状平原的最北端点。区域性大气污染输送亦是不可忽视的因素。从大气环流形势场上看，中国东部，尤其北方不利气象条件往往污染区发生在低层，大多位于高压系统后部或低压系统主模态的控制下偏南气流的强辐合区内，且对流层中层气温距平“暖盖”结构和近地层高湿特征显著。另外，统计表明持续性大范围污染天气过程与位势高度距平场上中高层阻塞高压的特殊气象背景密切相关，这类特定阻塞高压环流形势的维持甚至可形成长达5天持续性区域重污染天气。由此，“大地形”“背风坡”影响下不同尺度大气环流模态与对流层“暖盖”热力结构可构成冬季区域大气重污染过程特有的动力、热力三维结构综合影响模型。（徐祥德）

2.3 Roles of oceanic moisture exports in modulating summer rainfall over the middle-lower Yangtze River Basin:Inter-annual variability and decadal transition

Precipitation over the mid-lower reaches of the Yangtze River Basin (YRB),which is a typical East Asian monsoon region in China,is strongly impacted by oceanic moisture exports (OMEs) and features variability at multiple spatial-temporal scales,resulting in frequent floods and droughts.We focus on two key issues of OME and its relevance to the summer rainfall variability over the YRB:determination of the individual contributions of OME from three specific oceanic sub-regions,i.e.,the Arabian Sea (ARS),Bay of Bengal (BOB),and South China Sea (SCS),and their roles in the inter-annual variability and decadal changes in the YRB summer rainfall.Using a Lagrangian forward trajectories tracing approach,we create a catalogue of OMEbased diagnosed precipitation fields for three sub-regions spanning the summer seasons (May to August) of 1980−2013 with 6-hour resolution using ERA-Interim data.The results show that the pattern of the diagnosed OME-based precipitation resembles the observed pattern over the YRB in terms of climatology and temporal variation.Climatologically,the SCS region accounts for the largest relative contribution (60.7%),followed by the ARS (30.8%),whereas the contributions of the BOB (8.5%) are almost negligible.On the inter-annual scale,the variation in OME-based rainfall originated from the ARS,and the BOB is out of phase with that of the SCS.In contrast to climatology,the inter-annual changes in rainfall over the YRB are primarily modulated by the variation in the OME from the ARS.However,for the decadal changes,particularly in 1992/1993 over the YRB,the SCS and ARS are equivalent contributors to the summer precipitation over the YRB.These findings thus provide new insights into our understanding of the multi-scale variability of summer precipitation over the YRB region.(Chen Bin,Zhang Wei,Yang Shuai)

2.4 Identifying and contrasting the sources of the water vapor reaching the subregions of the Tibetan Plateau during the wet season

A Lagrangian approach is utilized to identify and compare the sources of water vapor transported to the four subregions of Tibetan Plateau (TP) during the wet season (May−August) of 1980−2016.We focus on the time scale and subseasonal variability of water vapor transport and the relationship between moisture supply and precipitation at the interannual scale.This study finds that:(1) The moisture sources for the four subregions differ significantly in both spatial pattern and magnitude and depend heavily on the combined effects of the summer monsoons,local recycling and the westerlies.(2) The spatial evolution of the moisture sources based on the backward trajectory analysis reveals that,although approximately 80% of the moisture is delivered to the target regions within 1–4 days,the individual subregions feature different transport pathways and associated time scales.(3) The subseasonal migration of the Indian summer monsoon regulates the importance of different moisture sources for the southern TP but not for the northern TP.Additionally,the subseasonal moisture source evolution differs greatly between the southeastern TP and the southwestern TP.(4) The interannual variability of precipitation over the whole TP during summer is negatively correlated with the variation in the moisture transported by the westerlies and is positively related to the moisture conveyed by the Indian summer monsoon for the northern TP and by adjacent moisture transport for the southern TP.(Chen Bin,Zhang Wei,Yang Shuai)

2.5 Evaluation of summer monsoon clouds over the Tibetan Plateau simulated in the ACCESS model using satellite products

Cloud distribution characteristics over the Tibetan Plateau in the summer monsoon period simulated by the Australian Community Climate and Earth System Simulator (ACCESS) model are evaluated using COSP [CFMIP (Cloud Feedback Model Intercomparison Project) Observation Simulator Package].The results show that the ACCESS model simulates less cumulus clouds at atmospheric middle levels when compared with observations from CALIPSO and CloudSat,but more ice clouds at high levels and drizzle drops at low levels.The model also has seasonal biases after the onset of the summer monsoon in May.While observations show that the prevalent high clouds at 9–10 km in spring shift downward to 7–9 km,the modeled maximum cloud fractions move upward to 12–15 km.The reason for this model deficiency is investigated by comparing model dynamical and thermodynamical fields with those of ERA-Interim.It is found that the lifting effect of the Tibetan Plateau in the ACCESS model is stronger than in ERA-Interim,which means that the vertical velocity in the ACCESS model is stronger and more water vapor is transported to the upper levels of the atmosphere,resulting in more high-level ice clouds and less middle-level cumulus clouds over the Tibetan Plateau.The modeled radiation fields and precipitation are also evaluated against the relevant satellite observations.(Hu Liang,Sun Zhian,Deng Difei)

2.6 The large-scale circulation patterns responsible for extreme precipitation over the North China Plain in midsummer

Extreme precipitation events over the North China Plain (NCP) in midsummer during 1979−2016 are classified into two types using objective cluster analysis:a northern pattern with heavy precipitation and a central-southern pattern with relatively moderate precipitation.The large-scale circulation patterns responsible for the midsummer extreme precipitation are then determined.In the northern NCP type,extreme precipitation accompanies a zonal gradient between an anomalous low-pressure system at high latitudes and the westward- and northward-extended western North Pacific subtropical high (WNPSH).Anomalous southwesterlies flow is driven by a trough that extended from the high latitudes to the northern NCP,where it encounters southeasterly wind flow induced by an anomalously northward-extended WNPSH and a southern low-pressure anomaly at low latitudes.Anomalous amounts of moisture are mainly transported from the tropical western Pacific by southeasterlies.In the central-southern NCP type,remarkable anomalous low-pressure systems control all of the northern China with centers over the Sichuan Basin and Northeast China.The westward-extended WNPSH occupies further south than that of the northern NCP type.The southwesterly low-level jet (LLJ) is more prevalent in the central-southern NCP type than in the northern NCP type.This southwesterly LLJ plays an important role in extreme precipitation over the central-southern NCP by transporting moisture primarily from the Bay of Bengal and the South China Sea and generating convergence.In addition,the upper-level anomalous strong divergence that is anchored over the right entrance of the westerly jet makes a greater contribution to extreme precipitation in the northern type than in the central-southern type.(Zhao Yang,Xu Xiangde,Li Jiao)

2.7 Effects of the Tibetan Plateau and its second staircase terrain on rainstorms over North China:From the perspective of water vapour transport

The effects of the Tibetan Plateau (TP) and its second staircase terrain (i.e.,the extension of the TP) on summer rainstorms over North China (NC) from the perspective of large-scale water vapour transport were investigated based on the frequency of summer rainstorms from 70 observational stations in NC and National Centers for Environmental Prediction reanalysis data from 1961 to 2010.Seven rainstorm cases over NC during 1981−2016 acquired from ERA-Interim reanalysis data and observational hourly precipitation data were selected.Two water vapour transport channels provided favourable backgrounds for rainstorms over NC.One channel flowed along the northern edge of the TP via westerlies.The other channel flowed along the eastern edges of the TP and its second staircase terrain accompanied with southerly monsoon airflow,with water vapour sources from the Bay of Bengal,South China Sea,and western Pacific.The abundant water vapour that was transported along the two channels not only offered a favourable moisture background for rainstorms over NC but also resulted in the development of water vapour flux vortexes that supplied convergence conditions for rainstorms in situ.At low levels,water vapour transport mainly flowed along the eastern edge of the TP second staircase terrain.The results from the Weather and Research Forecasting model simulation of a randomly selected rainstorm case originating from the TP to NC confirmed the vital effect of the TP second staircase terrain on rainstorms over NC accompanied with water vapour transport.(Zhao Yang,Xu Xiangde,Zhao Tianliang)

2.8 Trends in observed mean and extreme precipitation within the Yellow River Basin,China

We analyzed spatiotemporal precipitation trends within the Yellow River Basin (YRB) in China and examined the connection between the changes in average and extreme precipitation indices.Data from 423 weather stations recorded from 1961 to 2016 were analyzed using the Mann-Kendall test to explore the linear trends of relationships between various indices,along with a simple linear regression used to detect monotonic positive or negative trends in the annual and seasonal precipitation data.Moreover,we divided the YRB into three distinct topographic regions to better understand the effect of regional geography on precipitation patterns.Our results demonstrated that mean precipitation and extreme precipitation days in different areas of the YRB had different variation trends.Precipitation in the YRB overall showed a negative trend,as did extreme precipitation days in the lower YRB.Mean and extreme precipitation indices were significantly correlated both annually and seasonally.These results may be helpful in preparing for both drought and flood events.(Zhao Yang,Xu Xiangde,Huang Wubin)

2.9 The severity of drought and precipitation prediction in the eastern fringe of the Tibetan Plateau

The trend,severity,and duration of drought in the eastern fringe of the Tibetan Plateau (EFTP) have been investigated using the Mann-Kendall (M-K) trend test,standardized precipitation index (SPI),and generalized extreme value (GEV),using data obtained from 438 rainfall stations and reanalysis datasets for the period 1961−2014.A recent drought trend is evident from a decrease in rainfall,mainly occurring on the eastern slope of the TP (＜3000 m elevation); this is attributed to downward air flows over the eastern slope of the Tibetan Plateau (TP) induced by TP heating.Recent droughts have also been more severe,again mostly occurring on the eastern slopes.The duration of drought illustrates that extreme droughts are becoming more frequent.The study also predicted summer precipitation,due to its crucial role in drought research in the EFTP.Results show that the preceding May-June-July (MJJ) averaged column-integrated meridional water vapor transport (MWVT) from the South China Sea (SCS),Philippine Sea,and tropical western Pacific is a vital predictor of summer precipitation in the EFTP.A partial least squares (PLS) regression prediction model is therefore constructed,using the leading PLS components of preceding MJJ-averaged column-integrated MWVT.Compared to the observed summer rainfall,the PLS prediction model performs an excellent reconstructed skill with a correlation of 0.81 (1961–2006) and exhibits a promising forecast skill with a correlation of 0.67 (2007–2014).Results suggest that southerly moisture transport in early summer would help prevent summer drought in the EFTP.(Zhao Yang,Xu Xiangde,Liao Liufeng)

2.10 Precursory strong-signal characteristics of the convective clouds of the central Tibetan Plateau detected by radar echoes with respect to the evolutionary processes of an eastward-moving heavy rainstorm belt in the Yangtze River Basin

The integrated analysis was carried out of the data from a C-band frequency-modulated continuous-wave (C-FMCW) radar site in Naqu obtained during a rainstorm over the middle and lower reaches of the Yangtze River and the data concerning the three-dimensional structure of the circulation of the precipitation system that occurred over the lower reaches of the Yangtze River Basin during the Third Tibetan Plateau (TP) Atmospheric Experiment from August 15th to 19th,2014.The changes in the echo intensity at the C-FMCW radar site in Naqu were of regional indicative significance for the characteristics of the whole-layer apparent heat source Q1 in local areas and the region of the adjacent river source area,including the Yangtze River,Yellow River,and Lancang River (hereinafter referred to as the source area of three rivers),as well as to the vertical speeds due to the development of convection.This study indicates that the C-FMCW radar echo intensity of the plateau convection zone and the related power structures of the coupled dipole circulations in the middle layer of the atmosphere,as well as in the upper atmospheric level divergence and lower atmospheric level convergence,are important stimuli for convective clouds in this region.Furthermore,these radar data provided a physical image of the development and maintenance mechanisms of an eastward-moving heavy rainstorm belt.This study also shows that changes in the echo intensities at the C-FMCW radar site of Naqu can provide strong signals related to heavy rainstorm processes in the upper reaches of the Yangtze River.(Zhao Yang,Xu Xiangde,Ruan Zheng)

2.11 Diurnal variations in summer precipitation over the Yellow River Basin

The diurnal variations in summer precipitation over the Yellow River Basin (YRB) are investigated based on the National Centers for Environmental Prediction reanalysis dataset and hourly precipitation data from 481 gauge stations over the YRB during the time period 1981−2013.Three stair steps are identified to represent the upper,middle,and lower reaches of the YRB due to complex topography elevations over the different subregions of the YRB.The summer diurnal precipitation over the YRB shows significant spatial and temporal variations.The diurnal peaks in precipitation over the upper and middle reaches of the YRB occur in the evening and late afternoon,respectively.By contrast,double peaks in diurnal precipitation occur in the early morning and late afternoon over the lower reaches of the YRB.The diurnal peaks in summer precipitation along the YRB have an eastward transition,suggested to be associated with the westerlies transporting water vapor.Differing from the increasing tendency of summer hourly precipitation from west to east across the YRB with topography elevations decreased,a distinct reduction in hourly precipitation is observed over the transition region between the first and second stair steps in the YRB.Further analysis attributes this phenomenon to the regional descending airflow induced by the steep terrain along the slope of the Tibetan Plateau (TP).(Zhao Yang,Wang Minzhong,Li Jiao)

2.12 Linkage between moisture transport over the Yangtze River Basin and a critical area of the Tibetan Plateau during the Meiyu

Located downstream of the Tibetan Plateau (TP),the Yangtze River Basin (YRB) in China frequently experiences torrential rainfall and severe rainstorms within the zonally elongate Meiyu belt.Inter-annual variations in drought and flooding in the YRB are shown to be highly correlated with the upstream surface latent heat flux (SLHF) over the TP,suggesting that the SLHF over the TP can be a“strong signal”of Meiyu variability.Results show that the southeastern margin of the TP (SEMTP) is the most“sensitive area”of the“strong signal”.On inter-annual scales,low (high) SLHF over the SEMTP corresponds to high (low) humidity at upper levels,strong (weak) zonal moisture transport and a southerly (northerly) pattern of East Asian summer monsoon (EASM) moisture transport circulation in July.The anomalous moisture flow originating from the mid to low latitude oceans,converges in the east of the SEMTP and turns eastward (northward); thus,the SEMTP acts as the key area of redirection of moisture flow.The ridge of moisture transport is located at about 22.5°N (29.5°N) in East China for low (high) SLHF over the SEMTP.Under low SLHF conditions,the warm,humid airflow from the SEMTP creates favorable conditions for Meiyu rainfall and torrential rainstorms.Numerical simulation and sensitivity analyses using the mesoscale Weather Research and Forecasting (WRF) model further reveal that SLHF over the SEMTP shows pronounced“strong signal”features.The variation of intensity of SLHF over the SEMTP corresponds to the moisture transport structure of the Meiyu rain belt and the location of the ridge of the subtropical high in East China.An increase in the apparent heat source Q1 over the main body of the TP might explain the“strong signal”over the SEMTP.This apparent heat source plays an important role in modulating both the SLHF over the SEMTP and the location of the subtropical high circulation system.(Dong Lili,Xu Xiangde,Zhao Tianliang)

2.13 The causes of“vulnerable regions”to air pollution in winter in the Beijing-Tianjin-Hebei region:A topographic-meteorological impact model based on adaptive emission constraint technique

The Beijing-Tianjin-Hebei (BTH) region,with its complex terrain,has serious issues with air pollution.The northern and western parts of the BTH region are surrounded by the Yan Mountains and Loess Plateau (LP),and the south-central part of that region is a large open plain.Such special geographic configuration is prone to result in a concentrated pollution belt along the north-to-south direction on the eastern margin of the plateau,in addition to the influence of pollutant-emission sources and population distribution.In this study,based on an original adaptive nudging constraint method,we quantitatively analyzed the differences in the influence of emission sources under different dynamic and thermal conditions in the BTH region,which is impacted by a special large-scale leeward slope terrain.The mechanism of air pollution vulnerability and the comprehensive effects of terrain-meteorological conditions on air pollution in the BTH region were also discussed.The results indicated that the atmospheric diffusion conditions on the eastern side of the plateau were poor,and a subsynoptic-scale“vortex sequence”,which was composed of a series of linked vortices,was identified.The corresponding atmospheric pollution convergence line stretched from Beijing to Hebei to northern Henan in the lower atmosphere.On the eastern edge of the plateau,a“warm cover”formed by a temperature anomaly and a downdraft impeded the vertical diffusion of pollutants.Therefore,pollutants tended to converge at the eastern edge of the plateau,and the pollution belts would move longitudinally north and south along the topography of the eastern slope when south-westerly and north-easterly winds alternated.The movement generated a“train”of pollutants that were transported on the eastern edge of the plateau,which then caused air pollution to persist there.Such terrain-meteorological conditions amplified the effects of emissions by an average of 50% to 150% in the region,leading the eastern side of the LP to become a“naturally vulnerable region”to haze pollution region.(Meng Kai,Xu Xiangde,Xu Xiaobin)

2.14 青藏高原能量、水分循环影响效应

青藏高原是世界上总辐射量最高的地区，也是全球超太阳常数的极值区域之一。此处形成了一个“嵌入”对流层中部大气的巨大的热源，可以伸展到自由大气，其超越了世界上任何超级城市群落所产生的中空热岛效应，对全球与区域大气环流系统变化的动力“驱动”产生了难以估计的效应。与地形热力过程季节变化密切相关的亚洲夏季风是世界上范围最广和强度最强的季风；从冬季到早春季节转换过程中，由于太阳辐射的影响造成青藏高原大地形感热的“快速响应”及其相对高值动态移动，在盛夏梅雨及其云降水带前沿线恰好停滞于中国“三阶梯”地形分布山地—平原过渡区。此现象表明，青藏高原可能扮演着夏季风过程陆地—海洋—大气相互作用的关键角色。中国区域低云量与总云量极值区均与青藏高原大江大河的源头（长江、澜沧江、雅鲁藏布江等）、中东部湖泊群与冰川集中区空间分布几乎吻合，这表明“亚洲水塔”形成的关键因素与“世界屋脊”特有的云降水结构不可分割。研究表明，青藏高原大气热源对局地与下游区域云降水过程水汽输送流型等均有显著影响。长江流域降水与全国低云量存在一个明显沿长江流域的带状高相关结构，充分表明长江流域降水与上游“亚洲水塔”“热驱动”以及对流系统具有重要相关关系。从跨赤道经向环流的视角可发现，夏季南、北半球跨赤道气流低层强偏南、高层强偏北气流出现在东亚地区和北美区域两大地形对应的赤道区，这2个跨赤道极值区恰与青藏高原、落基山高原位置相对应。青藏高原纬向与经向环流圈结构与区域-全球大气环流相关机制，印证了“世界屋脊”隆起大地形的“热驱动”及其对流活动在全球能量、水分循环中的作用。青藏高原特殊水汽三维结构分布和跨半球的纬向和经向大气垂直环流图表明青藏高原对全球尺度大气环流变化的贡献显著。文章进一步以东亚、全球水循环的视角，提出了青藏高原作为全球性大气“水塔”的观念，认为高原地区一个水塔的“供水”和“蓄水”循环体系，特别是高原地表冰川、积雪和湖泊作为“蓄水池”系统，使得所有的河流可作为“输水管道”，将“水塔”的水向周边区域输送出去，高层大气也提供向外输送的渠道。青藏高原特殊的跨半球大气水分循环可构建“世界水塔”与其周边地区独特的水文功能概念，综合描绘了青藏高原“世界水塔”及其地球上一个完整的行星尺度陆地-海洋-大气水分循环物理图像。（徐祥德，马耀明，孙婵）

2.15 青藏高原“亚洲水塔”效应和大气水分循环特征

青藏高原是东亚海陆气相互作用最敏感的地区之一。青藏高原大气水分循环结构特征不仅反映了西风气流与“大三角扇形”影响域季风水汽流的相互作用特征，而且凸现出该区域为全球能量、水汽的交换关键区，构成“亚洲水塔”形成的重要背景。隆升的高原地形和强大的表面辐射加热形成了局地上升对流和高耸入对流层中部中空“热源柱”。研究揭示出此热力驱动下青藏高原高、低层互为反环流类似台风的自激反馈机制，其提供了“亚洲水塔”水汽汇流与抽吸动力效应。“亚洲水塔”热源驱动机制有助于“世界屋脊”大气“热岛”“湿岛”的形成和维持，使暖湿气流从低纬海洋向高原输送、汇聚。针对“世界屋脊”高原对流频繁、云降水异常特征，揭示出“世界屋脊”空气低密度条件对高原对流云的触发效应。分析表明，低纬热带海洋成为“亚洲水塔”大气水分循环的重要水汽源区，水汽源区可跨越赤道追踪到南半球。提出了青藏高原热力驱动下大气水分循环结构类似全球性大气水塔的观点，青藏高原特殊的跨半球大气水分循环构建出“亚洲水塔”和其周边地区独特的大气—水文功能体系。给出了西风与季风协同作用背景下青藏高原为核心区的陆地—海洋—大气相互作用的“亚洲水塔”大气水分循环物理图像。（徐祥德，董李丽，赵阳）

2.16 西北太平洋热带气旋路径异常偏折的分类特征

基于中国台风网CMA-STI热带气旋（TC）最佳路径资料，对1949—2016年西北太平洋TC路径发生异常偏折的地理位置进行K-means聚类分析，并将其分为5个区域。对各区TC路径异常偏折的频数、方向变化、周期及时间变率等特征进行分析。结果表明：（1）不同分区TC异常偏折高频月份不同，纬度较高区域主要发生在夏季，纬度较低区域则主要发生在秋季。（2）异常右折TC在发生偏折前移向主要为西北向，偏折后为北向；异常左折TC偏折前主要为北向，偏折后主要转为西北向。（3）西北太平洋TC异常偏折总频数存在准2～4年、准3～6年的年际变化周期，其长期变化趋势表现为20世纪80年代中期之前呈增加趋势，其后呈减少趋势，低纬区域年变化与之最为相似，中高纬区域变化趋势不明显。（4）将研究区域按5°×5°进一步栅格化统计TC异常偏折频数的时间变率，发现其地理分布表现为中国沿海为正、台湾岛以东海域为负的变化特征。其中沿海的增加趋势主要由异常右折的增加引起，台湾岛以东洋面的减弱趋势主要由异常左折的减少引起。（5）异常右折TC强度增强的高频中心主要位于菲律宾半岛以东洋面，次中心位于中国南海中部，而强度减弱位于台湾岛西南区域；异常左折TC强度增强的高频中心位于南海中部，强度减弱中心位于我国东南沿海。（王敏，徐祥德，李英）

2.17 台风Polly（9216）和Matmo（1410）对辽东半岛降水影响的对比分析

台风Matmo（1410）和Polly（9216）影响辽东半岛时的路径近乎重合，但台风Polly造成了大范围的暴雨和大暴雨，而台风Matmo仅造成个别测站出现暴雨。利用中国气象局热带气旋年鉴、FY-2D卫星的黑体亮度温度（TBB）产品（0.1°×0.1°）、日本气象厅TBB资料、大连地区逐时自动气象站降雨量资料、常规观测资料和欧洲中期数值预报中心ERA-Interim全球再分析资料（0.125°×0.125°），对2个台风影响辽东半岛的降水过程进行了对比分析。结果表明：（1）2个台风均进入西风槽区而变性，在其西侧和北侧分别具有冷锋和暖锋锋生，辽东半岛的降水均发生在台风低压环流北侧的锋生区和环境风垂直切变明显增大过程中。但2个变性台风的大尺度环流背景却差异显著，台风Polly与西北侧较强冷空气相互作用，锋区随高度增加向西北倾斜，且与低空东南急流相连获得丰富水汽供应，强降水持续时间长，而台风Matmo与东北部对流层低层冷空气相互作用明显，锋区随高度增加略向东北倾斜，但其低空急流水汽通道被快速隔断，不稳定度和动力抬升条件减弱，强降水持续时间短。（2）台风Polly和Matmo的降水分布非对称明显，均出现在顺垂直切变方向的左侧，但台风相对于高空槽的位置不同，对流活动发展的方位有所差异。台风Polly中尺度对流活动在其北侧发展旺盛，且向西南弯曲，而台风Matmo对流活动仅发生在台风环流东北侧。（3）台风的强降水落区还与其低层环流内冷、暖平流的活动密切相关。台风Polly西北侧的冷平流加强，辽东半岛位于台风北侧低层冷暖平流交汇区，水平辐合加强，深厚的上升运动维持，而台风Matmo东北侧的冷平流加强，辽东半岛逐渐位于台风西侧，低层为冷平流控制的下沉运动区，大气层结趋于稳定。（梁军，张胜军，冯呈呈）

3 暴雨和强对流研究

3 Research on heavy rainfall and strong convection

3.1 How accurate are modern atmospheric reanalyses for the data-sparse Tibetan Plateau region?

More than 6000 independent radiosonde observations from three major Tibetan Plateau experiments during the warm seasons (May−August) of 1998,2008,and 2015−2016 are used to assess the quality of four leading modern atmospheric reanalysis products (CFSR/CFSv2,ERA-Interim,JRA-55,and MERRA-2),and the potential impact of satellite data changes on the quality of these reanalyses in the troposphere over this data-sparse region.Although these reanalyses can reproduce reasonably well the overall mean temperature,specific humidity,and horizontal wind profiles against the benchmark independent sounding observations,they have nonnegligible biases that can be potentially bigger than the analysis-simulated mean regional climate trends over this region.The mean biases and mean root-mean-square errors of winds,temperature,and specific humidity from almost all reanalyses are reduced from 1998 to the two later experiment periods.There are also considerable differences in almost all variables across different reanalysis products,though these differences also become smaller during the 2008 and 2015−2016 experiment periods,in particular for the temperature fields.The enormous increase in the volume and quality of satellite observations assimilated into reanalysis systems is likely the primary reason for the improved quality of the reanalyses during the later field experiment periods.Besides differences in the forecast models and data assimilation methodology,the differences in performance between different reanalyses during different field experiment periods may also be contributed by differences in assimilated information (e.g.,observation input sources,selected channels for a given satellite sensor,quality-control methods).(Bao Xinghua,Zhang Fuqing)

3.2 Shift in the temporal trend of boundary layer height in China using long-term (1979–2016) radiosonde data

The knowledge regarding how the boundary layer height (BLH) changes over time is still poor.Here we analyze the spatial and temporal changes in radiosonde-derived measurements of BLH over China from 1979 to 2016.A qualitatively robust and abrupt change of BLH occurred in 2004.Over the former period (from 1979 to 2003) a spatially uniform increase was found in the BLH,while over the latter period BLH decreased in a spatially nonuniform way.Then,the meteorological influence on the rising BLH was determined to have a negative association with soil moisture,lower tropospheric stability and relative humidity,and a positive association with the near-surface temperature.Yet,there was a different influence of meteorology on the BLH over the latter period,where a negative association was revealed with T-sfc and relative humidity.These findings shed new light on the long-term changes in air pollution across China.(Guo Jianping,Li Yuan,Cohen Jason Blake)

3.3 Declining summertime local-scale precipitation frequency over China and the United States,1981–2012:The disparate roles of aerosols

The local-scale precipitation (LSP) is mainly driven by thermal convection.Here we reveal a decreasing trend in the summertime LSP frequency over both China and the United States (US) by utilizing the hourly rain gauge data from 1981 to 2012.The contrasting aerosol trend likely contributed to this same declining trend of LSP in both countries.As aerosol optical depth (AOD) goes beyond the turning zone of 0.25–0.30,the impact of aerosol on precipitation changes from invigoration to suppression.The mean AOD is generally less and larger than this range and of opposite trends in China and US,respectively,which likely accounts for the same declining trend of LSP hours in the two countries.The observed boomerang shape points to the importance of aerosol loading,which matters much,if not more than the AOD trend,thereby potentially serving as a constraint for climate model evaluation.(Guo Jianping,Su Tianning,Chen Dandan)

3.4 The trend reversal of dust aerosol over East Asia and the North Pacific Ocean attributed to large-scale meteorology,deposition,and soil moisture

The long-term trend in dust loading over East Asia remains under debate and is dependent on the study period chosen.In this study,the long-term trends in springtime dust over East Asia and the North Pacific Ocean (NPO) during 1980−2017 were examined based on the Modern-Era Retrospective Analysis for Research and Applications version 2 reanalysis.Results showed that there was a spatial gradient in dust aerosol loadings,with decreases from the western China eastward toward the NPO.This pattern was corroborated by Cloud-Aerosol Lidar with Orthogonal Polarization observations.Furthermore,the empirical orthogonal function method was used to reveal the leading modes of springtime dust aerosol optical depth (AOD) over East Asia and the NPO.An abrupt shift occurred in the dust AOD trend in 2010 for the empirical orthogonal function 1 mode.The dust AOD increased at a rate of approximately 2×10−4year−1during 1999−2009 and then decreased more sharply (around 5×10−4year−1) afterward.This trend reversal of dust AOD was closely associated with a decrease in 10-m wind velocity,which induces reduced dust emission.Compared with 10-m wind,the soil moisture is less correlated with the trend reversal in dust AOD.Additionally,the trends of dry (wet) deposition were closely associated with the trends of the dust AOD,especially for the period 2010−2016.Overall,our findings add new insights to the long-term nonlinear variability of dust.(Guo Jianping,Xu Hui,Liu Lin)

3.5 Retrieval of aerosol profiles by Raman lidar with dynamic determination of the lidar equation reference height

A reference height that often needs to be assumed in aerosol retrieval from Raman lidar tends to cause high uncertainty in retrieving the vertical distribution of aerosol optical properties.Here,a novel method is proposed to determine the height-revolved reference height,which is then used to retrieve aerosols from Raman lidar.This method can automatically avoid the atmospheric layers with the presence of aerosols,clouds and low signal to noise ratio (SNR).Based on elastic (at 355 nm) and inelastic (at 387 nm) signals collected during the period from 5 December 2016 to 5 March 2017 by a ground-based Raman lidar in Beijing,China,the aerosol optical properties,such as extinction coefficient,backscattering coefficient and lidar ratio have been successfully retrieved.Results show that the averaged nighttime aerosol optic depth (AOD) from Raman lidar is in good agreement with early morning AOD retrieved from a collocated sunphotometer.The AOD exhibits a strong diurnal variation with a peak at 15:00 Beijing Time.On average,the nighttime AOD at 355 nm is 0.32,whereas the daytime AOD is 0.72 over Beijing during the study period.The column averaged lidar ratio is 44 sr at 355 nm,roughly consistent with previous studies.Our findings shed light on the pathways towards improving the retrieval of vertical distribution of aerosols optical properties during nighttime.(Li Jian,Li Chengcai,Guo Jianping)

3.6 Dominant interannual covariations of the East Asian−Australian land precipitation during boreal winter

The present study applies the empirical orthogonal function (EOF) method to investigate the interannual covariations of East Asian−Australian land precipitation (EAALP) during boreal winter based on observational and reanalysis datasets.The first mode of EAALP variations is characterized by opposite-sign anomalies between East Asia (EA) and Australia (AUS).The second mode features an anomaly pattern over EA similar to the first mode,but with a southwest-northeast dipole structure over AUS.El Niño-Southern Oscillation (ENSO) is found to be a primary factor in modulating the interannual variations of land precipitation over EA and western AUS.By comparison,the Indian Ocean subtropical dipole (IOSD) mode plays an important role in the formation of precipitation anomalies over northeastern AUS,mainly through a zonally vertical circulation spanning from the southern Indian Ocean (SIO) to the northern AUS.In addition,the ENSO-independent cold sea surface temperature (SST) anomalies in the western North Pacific (WNP) impact the formation of the second mode.Using the atmospheric general circulation model ECHAM5,three 40-year numerical simulation experiments differing in specified SST forcings verify the impacts of the IOSD and WNP SST anomalies.Further composite analyses indicate that the dominant patterns of EAALP variability are largely determined by the out-of-phase and in-phase combinations of ENSO and IOSD.These results suggest that in addition to ENSO,IOSD should be considered as another crucial factor influencing the EAALP variability during the boreal winter,which has large implications for improved prediction of EAALP land precipitation on the interannual time scale.(Liu Lin,Guo Jianping,Chen Wen)

3.7 Contrasting influence of Gobi and Taklimakan deserts on the dust aerosols in the western North America

The dust generated in East Asia influences the western North America (WNA) through its trans-Pacific transport.This study investigates the distinct contribution between two main dust sources in East Asia on this remote influence based on Modern-Era Retrospective analysis for Research and Applications version 2 data set.Results show that the dust generated in Gobi Desert (GD) exerts a larger influence on the WNA compared to those in Taklimakan Desert (TD).This difference is attributed to the different terrain and background winds in GD and TD.GD is relatively flat and dominated by westerlies throughout the troposphere,which facilitates the trans-Pacific transport of dust to WNA.However,TD is located in the Tarim Basin and dominated by easterly wind in the lower troposphere.The uplifted dust is largely redeposited in TD.Moreover,the influence of GD on dust in WNA experiences decadal change around 1999,which is related to intrinsic change of dust loading in GD.(Liu Lin,Guo Jianping,Gong Hainan)

3.8 A three-dimensional wave activity flux of inertia-gravity waves and its application to a rainstorm event

A three-dimensional transformed Eulerian-mean (3D TEM) equation under a non-hydrostatic and nongeostrophic assumption is deduced in this study.The vertical component of the 3D wave activity flux deduced here is the primary difference from previous studies,which is suitable to mesoscale systems.Using the 3D TEM equation,the energy propagation of the inertia-gravity waves and how the generation and dissipation of the inertia-gravity waves drive the mean flow can be examined.During the mature stage of a heavy precipitation event,the maximum of the Eliassen-Palm (EP) flux divergence is primarily concentrated at the height of 10−14 km,where the energy of the inertia-gravity waves propagates forward (eastward) and upward.Examining the contribution of each term of the 3D TEM equation shows that the EP flux divergence is the primary contributor to the mean flow tendency.The EP flux divergence decelerates the zonal wind above and below the high-level jet at the height of 10 km and 15 km,and accelerates the high-level jet at the height of 12−14 km.This structure enhances the vertical wind shear of the environment and promotes the development of the rainstorm.(Liu Lu,Ran Lingkun,Gao Shouting)

3.9 Science and prediction of monsoon heavy rainfall

With the increasing incidence of heavy rainfall events,particularly over the monsoon regions,the highly dense populations are more vulnerable.Research initiatives on observation,modeling,and prediction of monsoon heavy rainfall have been promoted actively by the World Weather Research Programme’s (WWRP) Working Group on Tropical Meteorology Research (WGTMR) of the World Meteorological Organization (WMO) since 2010.Series of monsoon-heavy-rainfall workshops were held in Beijing (2011),Petaling Jaya (2012),and New Delhi (2015) to benefit scientists worldwide and forecasters from the National Meteorological and Hydrological Services.An international Research and Development Project,namely,the Southern China Monsoon Rainfall Experiment (SCMREX) was established in 2013 to coordinate field campaign experiments and to conduct scientific research on presummer (April−June) heavy rainfall processes in the southern China.The Fourth WMO Monsoon Heavy Rainfall Workshop (MHR-4) was held in Shenzhen in April 2019 to discuss recent advances in analysis,numerical weather prediction (NWP) studies,and development of techniques for observing/forecasting monsoon heavy rainfall,and to review the progresses of SCMREX.Innovated by the discussions in the MHR-4 Workshop,this paper highlights recent progresses on monsoon heavy rainfall research,including topics such as rainfall characteristics and physical mechanisms,field experiment,numerical simulations and model development,forecast methods,and tropical cyclone (TC) related heavy rainfall.Future research directions and some specific research topics are also proposed.(Luo Yali,Li Liye,Johnson Richard H)

3.10 An observational analysis of three extreme rainfall episodes of 19−20 july 2016 along the Taihang Mountains in North China

This study examines the synoptic- and mesoscale processes leading to the generation of three extreme rainfall episodes with hourly rates of greater than 100 mm h−1over the southern,middle,and northern portions of the eastern foothills of Mt.Taihang in North China on 19−20 July 2016.The extreme rainfall episodes took place over the 200−600-m elevation zones in the southern and northern portions but also over the lower elevations in the middle portion of the target region,sequentially during late morning,early evening,and midnight hours.Echo training accounted for the development of a linear convective system in the southern region after the warm and moist air carried by a southeasterly low-level jet (LLJ) was lifted to condense as moving across Mt.Yuntai.In contrast,two isolated circular-shaped convective clusters,with more robust convective cores in its leading segment,developed in the northern region through steep topographical lifting of moist northeasterly airflow,albeit conditionally less unstable.Extreme rainfall in the middle region developed from the convergence of a moist easterly LLJ with a northerly colder airflow associated with an extratropical cyclogenesis.Results reveal that the LLJs and associated moisture transport,the intensifying cyclone interacting with a southwest vortex and its subsequent northeastward movement,and the slope and orientation of local topography with respect to and the stability of the approaching airflows,played different roles in determining the timing and location,the extreme rainfall rates,and convective organizations along the eastern foothills of Mt.Taihang.(Xia Rudi,Zhang Da-Lin)

3.11 Importance of identifying tropical cyclone tornadoes in typhoon warning and defense systems

Tornado hazard produced by tropical cyclones (TCs) is an underestimated threat in China.A recent case concerns a tornado outbreak in Tianjin and Shandong associated with the landfalling typhoon Yagi (2018) on 13–14 August 2018.The cause of this tornado outbreak has aroused curious interest in the society and science community.This study presents first-hand observational facts and preliminary analyses on the vortex patterns from TC scale down to the tornado scale,using data from satellite,Doppler radar and onsite damage survey comprehensively.Damage swaths obtained from aerial photographs laid a solid foundation for the underestimated threat in China of tornado damage exerted by TCs.For weather forecasters and local governments in charge of the defense of severe weather hazards,this precious case strongly suggests that tornadoes may have been neglected based on the current Doppler radar network.Our preliminary findings calls for systematical investigations based on state-of-the-art tornado-resolvable radar observations and numerical simulations techniques,to illustrate the genesis mechanisms and predictability of tornadoes produced by TCs,towards the ultimate goal of effective prediction,warning,and defense against future tornado hazards.(Yao Dan,Liang Xudong,Meng Qing)

3.12 Genesis,maintenance and demise of a simulated tornado and the evolution of its preceding descending reflectivity core (DRC)

This study demonstrates the capability of a cloud model in simulating a real-world tornado using observed radiosonde data that define a homogeneous background.A reasonable simulation of a tornado event in Beijing,China,on 21 July 2012 is obtained.The simulation reveals the evolution of a descending reflectivity core (DRC) that has commonalities with radar observations,which retracts upward right before tornadogenesis.Tornadogenesis can be divided into three steps:the downward development of mesocyclone vortex,the upward development of tornado vortex,and the eventual downward development of condensation funnel cloud.This bottom-up development provides a numerical evidence for the growing support for a bottom-up,rapid tornadogenesis process as revealed by the state-of-the-art mobile X-band phase-array radar observations.The evolution of the simulated tornado features two replacement processes of three near-surface vortices coupled with the same midlevel updraft.The first replacement occurs during the intensification of the tornado before its maturity.The second replacement occurs during the tornado’s demise,when the connection between the midlevel mesocyclone and the near-surface vortex is cut off by a strong downdraft.This work shows the potential of idealized tornado simulations and three-dimensional illustrations in investigating the spiral nature and evolution of tornadoes.(Yao Dan,Meng Zhiyong,Xue Ming)

3.13 Influences of urban building complexes on ambient flows over the Washington-Reston region

This paper examines the collective impacts of urban building complexes on the planetary boundary layer (PBL) winds using both observations and a mesoscale model.Horizontal winds measured on the rooftops of federal buildings over the regions of Washington,D.C.,and a small city nearby (i.e.,Reston,Virginia) show the blocking effects of urban building complexes on the downstream winds during the daytime of 9 July 2007.A modeling study of the case using a coupled version of the Weather Research and Forecasting (WRF)—multilayer urban canopy model in which the observed building height and density information is implemented to advance the calculations of momentum and heat,and reproduces the rooftop-observed wind patterns and the related urban heat island effects,especially the wake flows on the downstream sides of the above-mentioned two cities.Results show that under daytime conditions the building complexes can collectively form a mesoscale wake on the downwind side of each city,about 2−10 km away,horizontally from the edge of the building complexes.The wake flow may extend to much higher levels than the building tops,depending on the incoming flow strength,the static stability in the PBL,the height of the building complexes,and the time of the day because of the strength of surface insolation.(Zhang Da-Lin,Jin Menglin S,Shou Yixuan)

3.14 Mesoscale convective systems in the Asian monsoon region from Advanced Himawari Imager:Algorithms and preliminary results

The knowledge of mesoscale convective system (MCS) in the Asian monsoon region remains still deficient due to the limited available data and less powerful algorithms.Here,by using the data from Advanced Himawari Imager onboard Himawari-8 (HW8),an improved algorithm combining the area overlapping with the Kalman filter is developed,which captures much smaller MCSs that are unavailable otherwise.Several influential factors like the overlapping rate and splitting/merging in the area overlapping method,and the initial state variable in the Kalman filter method,all of which were less appreciated,are handled explicitly.The occurrence frequency,and moving trajectory of two types of MCSs,including the ordinary MCS and superconvective system,has been comprehensively examined in the Asian monsoon region for the warm season (April to September) of 2016.Comparison analyses with ground precipitation and radar measurements confirm the good performance of our algorithm.In particular,the moving direction of MCS strongly depends on latitudes,so does the horizontal velocity.Compared with over ocean,the frequency of MCSs dominates over land or along coasts in the tropics,where strong moisture flux convergence is frequently observed in the low troposphere.In addition,the MCSs detected in the eastern China can roughly capture the meridional propagation over time,which corresponds well to the precipitation belts linked to Meiyu front systems.The superconvective systems dominate over the Bay of Bengal and South China Sea due to the large-scale circulation.Our findings provide new insights to spatiotemporal patterns of MCSs during warm season in the Asian monsoon region.(Chen Dandan,Guo Jianping,Yao Dan)

3.15 Tropopause trend across China from 1979 to 2016:A revisit with updated radiosonde measurements

The long-term trend in tropopause has profound implications for the expansion of tropical zone and the variation of large-scale circulation.However,the changes of tropopause in China have not been explicitly investigated as yet.In this study,the trend of lapse rate tropopause (LRT) height over China has been comprehensively revisited for the period of 1979−2016,using the newly released quality-controlled radiosonde data from China Meteorological Administration.Results show that the LRT height in most parts of China shows a significant upwards trend with a rate of 370 m/decade,most likely due to global warming.The fastest increase occurs in the northwest region,followed by the low-latitude regions (15°−25°N),while the slowest increase occurs in the high-latitude regions (45°−55°N).Overall,the LRT height varies with latitudes,exhibiting a south high and north low pattern.In particular,the high LRT height over low latitudes is found to be expanding rapidly polewards in recent years,in contrast to the almost constant LRT height over mid and high latitudes.In terms of the seasonality,the tropopause height reaches the peak in summer and bottom in winter.The frequency distribution in the vertical direction exhibits a bimodal pattern with the major peak mostly occurring at around 15 km and a secondary peak occurring between 8 and 12 km.This bimodal distribution is similar to the findings revealed in previous studies.Our findings offer important circumstantial observational evidences for the polewards expansion of the Tropics under global warming.(Chen Xinyan,Guo Jianping,Yin Jinfang)

3.16 Layer-wise formation mechanisms of an entire-troposphere-thick extratropical cyclone that induces a record-breaking catastrophic rainstorm in Beijing

A rarely seen entire-troposphere-thick (ETT) extratropical cyclone that induced a record-breaking catastrophic rainstorm in Beijing (the largest since 1963) on 20 July 2016 was reproduced reasonably by a convection-permitting Weather Research and Forecasting Model.This cyclone was the result of a vertical coupling of a lower,a middle,and an upper tropospheric cyclone,which formed at different times and locations.Dry air descending from the stratosphere through a tropopause folding process capped moist air to the middle and lower troposphere,making convection associated with the cyclone relatively shallow.Overall,cyclonic vorticity associated with the ETT cyclone enhanced much rapider than its wind kinetic energy (KE),and formation mechanisms of the ETT cyclone were characterized by layer-wise features:(i) The formation of the middle and upper tropospheric cyclones was dominated by cyclonic vorticity transport,instead of cyclonic vorticity production,whereas the lower tropospheric cyclone was generated through both cyclonic vorticity transport and convergence-related cyclonic vorticity production.(ii) Consistent with the cyclonic vorticity intensification,the formation of the lower and middle tropospheric cyclones featured an enhancement of rotational wind KE (in western section of the cyclone,this was mainly due to the work done by the pressure gradient force,but in eastern section,it was mainly due to the inward transport of wind KE by rotational wind),whereas the formation of the upper tropospheric cyclone featured a decrease in the rotational wind KE (particularly in its eastern section),which was in contrast to its increasing cyclonic vorticity.The export of wind KE by a southwesterly wind within the eastern section of the cyclone governed the rotational wind attenuation.(Li Wanli,Xia Rudi,Sun Jianhua)

3.17 Non-monotonic aerosol effect on precipitation in convective clouds over tropical oceans

Aerosol effects on convective clouds and associated precipitation constitute an important open-ended question in climate research.Previous studies have linked an increase in aerosol concentration to a delay in the onset of rainfall,invigorated clouds and stronger rain rates.Here,using observational data,we show that the aerosol effect on convective clouds shifts from invigoration to suppression with increasing aerosol optical depth.We explain this shift in trend (using a cloud model) as the result of a competition between two types of microphysical processes:cloud-core-based invigorating processes vs.peripheral suppressive processes.We show that the aerosol optical depth value that marks the shift between invigoration and suppression depends on the environmental thermodynamic conditions.These findings can aid in better parameterizing aerosol effects in climate models for the prediction of climate trends.(Liu Huan,Guo Jianping,Koren Ilan)

3.18 Observed link of extreme hourly precipitation changes to urbanization over coastal South China

Understanding changes in subdaily rainfall extremes is critical to urban planners for building more sustainable and resilient cities.In this study,the hourly precipitation data in 1971−2016 from 61 rain gauges are combined with historical land-use change data to investigate changes in extreme hourly precipitation (EXHP) in the Pearl River delta (PRD) region of South China.Also,120 extreme rainfall events (EXREs) during 2011−2016 are analyzed using observations collected at densely distributed automatic weather stations and the radar network.Statistically significant increase of hourly precipitation intensity leads to higher annual amounts of both total and extreme precipitation over the PRD urban cluster in the rapid urbanization period (about 1994−2016) than during the preurbanization era (1971 to about 1993),suggesting a possible link between the enhanced rainfall and the rapid urbanization.Those urbanization-related positive trends are closely related to more frequent occurrence of abrupt rainfall events with short duration (≤6 h) than the continuous or growing rainfall events with longer duration.The 120 EXREs in 2011−2016 are categorized into six types according to the originating location and movement of the extreme-rain-producing storms.Despite the wide range of synoptic backgrounds and seasons,rainfall intensification by the strong urban heat island (UHI) effect is a clear signal in all the six types,especially over the inland urban cluster with prominent UHIs.The UHI thermal perturbation probably plays an important role in the convective initiation and intensification of the locally developed extreme-rain-producing storms during the daytime.(Wu Mengwen,Luo Yali,Chen Fei)

3.19 Declining diurnal temperature range in the North China Plain related to environmental changes

The decreases in diurnal temperature range (DTR) observed in most regions are generally linked to the increase in cloud cover.However,declining clouds and rising aerosols observed over the North China Plain (NCP) of China make it elusive to elucidate the underlying mechanisms behind the declining DTR observed in this region.Here,we analyze the changes in DTR characteristics in the NCP based on 54-year surface temperature observations,in combination with collocated environmental variable measurements.Overall,there is a significant declining trend of DTR from 1960 to 2014 at a rate of −0.12 /decade,largely due to a larger increase in minimum temperature during the night.The cloud effect on DTR is further explored by comparing DTR under clear-sky and overcast conditions,which exhibits a distinct annual cycle with a minimum in summer and a maximum in winter.The decreasing rate of DTR under overcast condition is −0.30 /decade,much faster than the rate of −0.17 /decade under clear-sky condition,indicating steady increases in the nighttime warming effect of middle- or high-clouds.Also,the elevated aerosol concentration could contribute to the declining DTR,due to the cooling effect of aerosols.Moreover,the effect induced by sunshine duration and water vapor on DTR cannot be ignored either.All of the aforementioned environmental variables combine to affect the long-term trend of DTR,despite their different roles in modulating DTR.Our findings call for better understanding of the influence of environmental factors on regional climate system at the diurnal timescale.(Xue Weitao,Guo Jianping,Zhang Yong)

3.20 A case study of the effects of a synoptic situation on the motion and development of warmsector mesoscale convective systems over South China

The effects of a synoptic situation on the motion and development of warm-sector mesoscale convective systems (MCSs) in a heavy rainfall event occurred in South China on 8 May 2014 are investigated using high-resolution observational data and ERA-Interim data.The results show that the blocking of an eastward moving low-level trough over the southwestern China by a stable anticyclone over the eastern China induced strong southerly winds ahead of the trough and an eastward vertical slant of the wind speed maximum.Consequently,a southeastward low-level wind shear formed in the region under the southerly jet,which caused a southeastward turning of the motion of a large inland MCS.Meanwhile,a northeastward/eastward low-level wind shear formed in the region under the east side of the southerly jet,leading to northeastward/eastward movement of some small MCSs in the coastal region.The continuous merging of the southeastward-moving MCS with the northeastward/eastward-moving MCSs resulted in heavy rainfall along the coastal region.On the other hand,the intensification and expansion of associated low and high mean-sea-level pressure (MSLP) centers caused southeastward migration of strong boundary-layer moisture flux convergence and high convective available potential energy zones,which promoted the persistent development of the large MCS and the later development of the small MCSs.Moreover,the strong boundary-layer southeasterly winds associated with the high MSLP center continuously enhanced the frontal ascent of the large MCS.These southeasterly winds also triggered the small MCSs through the convergence induced by differential surface friction between land and sea or the convergence of them with the southwesterly winds from the South China Sea.(Liang Zhaoming,Liu Ying,Yin Jinfang)

3.21 Characteristics and formation of a synoptic situation causing sudden turning of mesoscale convective systems over South China

The characteristics and formation of a synoptic situation that causes a sudden turning motion of warmsector mesoscale convective systems (MCSs) over South China are described,based on the collection and investigation of associated cases during April–June 2011–2017 using high-resolution observational data and ERA (ECMWF Re-Analysis)-Interim data.The results show that the blocking of a marked low-level high over the eastern China (eastern high) on a strengthening low-level trough over the southwestern China (southwestern trough) results in significant enhancement of southerly winds ahead of the trough,which produces a strong southeastward vertical wind shear at low levels near the western Guangdong Province.This low-level vertical wind shear results in sudden southeastward turning motion for the warm-sector MCSs entering into Guangdong Province from Guangxi Province.The formation of the eastern high is mainly attributable to the strong cyclonic wind anomaly over the northwestern Pacific Ocean,which continuously brings cold air from higher latitudes to the eastern China,where high synoptic-scale transient anomaly of geopotential height (SSTA-GH) forms.This cyclonic wind anomaly is induced by a low SSTA-GH,which travels from the north and south sides of the Tibetan Plateau to the northwestern Pacific Ocean and develops significantly as a result of a strong upper-level low SSTA-GH coupling with it or approaching it.On the other hand,the high SSTA-GH over the eastern China blocks the eastward extension of the low SSTA-GH originating from the Tibetan Plateau.Consequently,this low SSTA-GH turns to extend or move southeastward/southward to the southwestern China,leading to intensification of the southwestern trough.(Liang Zhaoming,Liu Ying,Yin Jinfang)

3.22 Observational analysis of characteristics of synoptic situation and evolution of organized warmsector rainfall in coastal region of South China in pre-summer rainy season

The characteristics of the synoptic situation and the evolution of the organized warm-sector rainfalls (OWSRs) in the coastal region of South China in the pre-summer rainy season were investigated,using the high-resolution observational data in the period of 2011–2016 and European Centre for Medium-Range Weather Forecasts Re-Analysis Interim (ERA-Interim) data.The results show that a strong southwesterly lowlevel jet (LLJ) ahead of a trough over the southwestern China with a marked boundary-layer jet (BLJ) over the northern South China Sea (synoptic situation SWLLJ) or a prominent,low-level anticyclone over the Yangtze River Basin (synoptic situation ACR) is present when the OWSRs occur in the coastal region of South China.The OWSRs are prone to initiate on the windward side of the coastal mountains,owing to the convergence enhanced by the colliding of the BLJ with the mountains and the coupling of double LLJs near the coast (for SWLLJ),or due to the convergence between northerly and southeasterly winds near the coastal mountains (for ACR).The OWSRs present a long extension when the LLJ axis is nearby.The translation of the LLJ itself also promotes the long extension of the OWSRs.In contrast,the OWSRs show a short extension when the LLJ axis is farther away or ACR occurs.Meanwhile,the OWSRs are directed northeastward in Guangxi Province and more eastward in Guangdong Province,probably owing to the orientation difference of the LLJ in these two provinces.The rainfall systems in the ACR situation tend to move eastward,whereas those in the SWLLJ situation are prone to move eastward when equivalently strong or much-stronger upper-level winds overlay the LLJ,but move northeastward when much weaker upper-level winds couple with the LLJ.(Liang Zhaoming,Liu Ying)

3.23 BMA probability quantitative precipitation forecasting of land-falling typhoons in Southeast China

The probability of quantitative precipitation forecast (PQPF) of three Bayesian Model Averaging (BMA) models based on three raw super ensemble prediction schemes (i.e.,A,B,and C) are established,through calibration of their parameters using 1–3-day precipitation ensemble prediction systems (EPSs) from the China Meteorological Administration (CMA),the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) and observation during land-falling of three typhoons in Southeast China in 2013.The comparison of PQPF shows that the performance is better in the BMA than that in raw ensemble forecasts.On average,the mean absolute error (MAE) of 1-day lead time forecast is reduced by 12.4%,and its continuous ranked probability score (CRPS) of 1–3-day lead time forecast is reduced by 26.2%,respectively.Although the amount of precipitation prediction by the BMA tends to be underestimated,in view of the perspective of probability prediction,the probability of covering the observed precipitation by the effective forecast ranges of the BMA increased,which is of great significance for the early warning of torrential rain and secondary disasters induced by it.(Zhao Linna,Bai Xuemei,Qi Dan,Cheng Xing)

3.24 1949年以来我国台风暴雨及其预报研究回顾与展望

对1949年以来影响我国的台风暴雨及其预报研究的进展，从台风暴雨形成机理、专题研究、预报研究和研究新动向等4方面进行回顾总结。首先，从环境场、下垫面和内部条件3个角度对台风暴雨形成机理进行简要回顾，并从“75·8”特大暴雨、“莫拉克”极端暴雨、远距离暴雨、台风暴雨的诊断分析和气候特征等5个角度做了专题回顾。同时，简要回顾了台风暴雨预报研究进展，并就台风暴雨研究新动向给出了在云微物理、极端降水和预报方法3个方向上的初步判断。在此基础上，给出了小结与展望。（任福民，杨慧）

3.25 中国极端小时降水2010—2019年研究进展

概述了近10年来关于中国极端小时降水的研究成果，为及时了解和掌握该领域研究进展、开展相关科学研究和进行强降水预报服务提供有价值的科学依据和参考。现有的研究表明：（1）极端小时降水的阈值通常采用百分位和拟合经验函数得到，阈值强度分布在中国的区域差异大，最强阈值位于华南沿海、海南岛、台湾岛和华北平原，次大值位于四川盆地和长江中下游地区。（2）根据中国极端小时降水发生的天气背景特征，主要分为四大类型：热带气旋型、锋面型、低涡或切变线型、弱天气尺度强迫型。每种类型极端小时降水的空间分布、季节变化和日变化特征各不相同。（3）定义“极端小时降水事件”为：站点连续观测到降水大于等于0.1 mm/h的一段时间，其中最多只有1 h的降水间断，且至少发生一次极端小时降水。中国极端小时降水事件维持时间在东南沿海、长江中下游一带较长（超过12 h），而在中国北方则普遍较短（不到6 h）。极端小时降水事件具有不对称性，即从降水开始到出现雨量峰值较为迅速，而从出现峰值至降水结束则变化较为缓慢，该特征在西部地形复杂地区更为明显。（4）过去50多年，中国极端小时降水的变化趋势呈现正负相间的空间分布特征。观测分析表明，上海和珠三角城市地区极端小时降水在城市化迅速发展的近30年间具有区别于周围地区的显著增长趋势，而观测和模拟均表明北京城市强热岛效应有利于增强小时降水。（吴梦雯，罗亚丽）

3.26 不同强度热带气旋对中国降水变化的影响

基于1960—2017年2000多个气象台站逐日降水数据和中国气象局热带气旋（TC）最佳路径资料集，采用客观天气图分析法（OSAT）识别得到TC降水。研究表明，中国TC降水总体呈显著下降趋势，较12年前的研究结果下降趋势变缓；TC盛期（7—9月）降水占到TC总降水的78.5%，TC盛期降水和TC非盛期降水均呈显著下降趋势。TC降水气候趋势在空间分布上以减少为主要特征，并表现出明显的地域差异，自南向北呈“减少—增多—减少”的分布型，减少趋势中心位于广东和海南。按TC影响期最大强度分级（弱TC、中等强度TC和强TC）研究不同强度TC降水变化的结果显示，强TC降水表现出显著减少趋势，主要决定着TC总降水的影响范围和趋势等主要特征。进一步分析发现，影响TC频数在1960—2017年呈显著减少趋势，并在1995年发生突变；对1995年前后2个时期的对比研究显示，与前一时期（1960—1994年）相比，后一时期（1995—2017年）影响TC活动频次在20°N以南的海域呈现出显著的减少趋势，减少大值中心位于南海北部，而且这一特征也主要由影响TC中的强TC所决定；强TC的这一变化趋势导致了华南地区尤其是广东和海南TC降水日数的减少，进而使得TC降水减少。（杨慧，任福民，杨明仁）

3.27 2类不同风灾个例超级单体特征对比分析

采用分钟级加密自动气象站观测资料，盐城、淮安和岳阳、荆州雷达探测数据，以及欧洲中期天气预报中心（ECMWF）高分辨率的ERA-Interim全球再分析数据，对比分析了2016年6月23日江苏阜宁龙卷灾害和2015年6月1日湖北监利下击暴流大风灾害的环境特征与超级单体的结构特征。结果表明：（1）2次强对流大风灾害发生在相似的低空环流背景下。风灾发生在低空急流出口区左侧的暖区内、850 hPa低涡中心东侧6～7个经距的位置；环境大气的对流有效位能大于2000 J/kg。但是风灾的类型不同，江苏阜宁大风灾害主要由超级单体龙卷造成，监利“东方之星”沉船事故主要由超级单体触发的下击暴流造成。短时强降水中心与风灾中心的相对位置不同：阜宁龙卷移动方向的左侧伴随着最强短时降水；湖北监利沉船事件发生期间，风灾中心与短时强降水中心基本重合。鉴于不同性质的对流大风位置与超级单体母体的中心位置对应关系上存在差异，通过比较地面观测的瞬时大风与瞬时强降水中心的相对位置将有助于区分强对流大风的性质。（2）环境风垂直切变强度对对流风暴结构、发展、维持有重要影响。阜宁龙卷发生时，其上空0～6 km风垂直切变达4×10-3/s，超级单体有明显的向前倾斜结构，形成有界弱回波区；而监利强对流沉船位置0～6 km风垂直切变只有2.3×10-3/s左右，风暴单体中的上升气流近乎于垂直。阜宁超级单体中气旋，首先出现在0～1.5 km风垂直切变和0～3 km风暴相对螺旋度带状大值区，在向抬升凝结高度更低的环境移动过程中，其底部不断下降，形成龙卷；而在监利沉船区，中低层风切变和风暴相对螺旋度相对要弱得多，对应风暴单体中的中气旋强度、持续性较弱，中气旋底部高度维持在1.6 km左右。（3）环境湿度垂直结构特征不同可能是风暴单体形成不同类型灾害大风的重要环境因子。监利下击暴流造成的风灾发生时，在地面气温迅速下降过程中，气压变化呈现快速跳升又快速下降的“尖锥”形，气压峰值比降水峰值提前4 min出现。它与对流层中高层环境大气中较为深厚的干空气卷入对流风暴中造成水物质强烈蒸发、冷却过程有关。而阜宁风灾过程中，环境大气中层仅存在非常浅薄的干层，加之低层较为深厚的饱和大气环境，对应的地面冷池效应相对较弱。（杨波，孙继松，刘鑫华）

3.28 湖北3类组织形态强对流系统造成的地面强对流大风特征

利用湖北省2012—2017年区域自动站、天气雷达和周边探空站观测资料，对3类不同组织形态的中尺度对流系统（MCS）（线性MCS、非线性MCS和孤立对流风暴）造成的地面强风（极大风速≥17 m/s）的时空分布、移动与传播、对流环境特征等方面进行了统计对比分析，并结合个例讨论了地面入流大风的成因及其对对流系统发展、组织的影响。结果表明：（1）大量的非线性MCS可能是由更早发生在山区和丘陵的孤立对流风暴向平原地区移动过程中组织形成的，孤立对流风暴造成的地面大风出现的峰值时间在17：00（北京时，下同）前后，非线性MCS地面大风的峰值时间在19：00左右，线性MCS造成的强对流大风主要出现在平原地区。（2）非线性MCS和孤立对流风暴是造成湖北省地面大风的主导系统，其中，非线性MCS造成的地面大风站次数占强对流大风站次总数的41.9%，而39.3%的地面强对流大风站次是由孤立对流风暴造成的。（3）虽然大于17 m/s的地面入流大风占所有强对流大风的比例很小，但存在地面入流大风的强对流系统的影响范围、持续时间均远大于同一类型对流系统的平均值。基于一次长生命史线性MCS（飑线）造成强对流大风事件的分析表明，雷暴系统前侧的地面入流大风是由对流强烈发展造成，这支暖湿入流又进一步增强了对流风暴的发展，同时地面入流大风的形成进一步加强了垂直风切变，因而强的地面入流更有利于对流系统的组织化发展。（4）虽然暖季强对流系统的平均引导气流均以西南风为主，但线性MCS主要自西向东移动、非线性MCS以自西南向东北移动为主、孤立对流风暴的移动方向则更具多样性，也更易出现后向传播现象。孤立对流风暴相对组织化的强对流系统而言，往往发生在更不稳定或更干的层结大气中，且环境垂直风切变更弱、风速更小。（郭英莲，孙继松）

3.29 一次超级单体雹暴的中尺度对流特征和形成条件分析

北京2014年7月16日一次超级单体风暴造成了罕见大冰雹、短时大风和局地暴雨等强天气。利用雷达、自动站、探空、基于雷达观测的四维变分同化系统（VDRAS）等资料针对中尺度对流特征和形成条件进行了分析。结果表明：（1）这次超级单体风暴有穹窿回波和悬垂、中气旋、三体散射等典型大冰雹雹暴云特征。（2）降雹属于西北气流型，雹区出现在低层切变线和地面辐合区（辐合线）附近，低层很好的水汽条件是这种类型下降雹的必要条件。此外，1500 J/kg以上的高CAPE以及上干下湿的不稳定层结、850 hPaθse360 K的高能舌中心均有利于强对流的发生，“喇叭口”形探空、适宜的0 ℃层和-20 ℃层高度以及这两个特性层之间厚度变小是大冰雹出现的典型环境。（3）有利的地形、雷暴下山冷池增强导致前侧辐合增强是超级单体发展并向南移动的有利条件。（雷蕾，孙继松，乔林）

4 台风研究

4 Typhoon research

4.1 Landfalling tropical cyclone research project (LTCRP) in China

Landfalling tropical cyclones (TCs) often experience drastic changes in their motion,intensity,and structure due to complex multiscale interactions among atmospheric processes and among the coastal ocean,land,and atmosphere.Because of the lack of comprehensive data and low capability of numerical models,understanding of and ability to predict landfalling TCs are still limited.A 10-year key research project on landfalling TCs was initiated and launched in 2009 in China.The project has been jointly supported by the China Ministry of Science and Technology,China Meteorological Administration (CMA),Ministry of Education,and Chinese Academy of Sciences.Its mission is to enhance understanding of landfalling TC processes and improve forecasting skills on track,intensity,and distributions of strong winds and precipitation in landfalling TCs.This article provides an overview of the project,together with highlights of some new findings and new technical developments,as well as planned future efforts.(Duan Yihong,Wan Qilin,Huang Jian)

4.2 Impact of different types of ENSO years on intensity changes of landfalling tropical cyclones over China

This study examines whether there are significant differences in intensity and destructiveness of landfalling tropical cyclones (TCs) over China in central Pacific warm (CPW),eastern Pacific warm (EPW) and La Niña (LA) years.By analyzing different seasons and locations of TCs making landfall over China,it was found that TCs in LA years generally had a larger power dissipation index (PDI) and may cause more disasters in China,while TCs in EPW years had a larger PDI over South China in autumn.A larger PDI of TCSC (landing location in Southern China) usually occurred in EPW years and a larger PDI of TCEC (landing location in Eastern China) occurred in LA years,compared with CPW years.The TCs in LA years were generally stronger,more frequent,and of longer duration over China,because of the positive relative humidity (RH) anomalies,the significant anomalous cyclones that occupied the South China Sea (SCS),and the easterly wind anomalies providing a beneficial steering flow for TCs making landfall.In EPW years,although TCs were less frequent,they had stronger intensity when making landfall and a longer lifetime over land which was mainly caused by a broad band of anomalous westerlies over the SCS giving rise to a belt of positive relative vorticity anomalies,as well as the slow translation speed of TCs before landfall supplying more energy for TCs to survive over land.Overall,we conclude that greater caution is warranted when TCs occur in LA and EPW years,as they may result in more serious disasters in China.(Liu Lu)

4.3 Contribution of recycling of surface precipitation to landfalling tropical cyclone rainfall:A modeling study for Typhoon Utor (2013)

Surface evaporation is an important factor in the hydrological cycle.It is unknown however how much the recycling of surface precipitation contributes to rainfall in a landfalling tropical cyclone.In this study,ensemble simulations were performed using the Weather Research and Forecasting model to quantify the contribution of re-evaporation of surface precipitation to rainfall in Typhoon Utor (2013) during and after its landfall over South China.Two sets of ensemble simulations were conducted,one with all default model settings (CTRL) and the other with the surface precipitation rate in the land surface model being set to zero within a radius of 500 km from the storm center so that the re-evaporation of surface precipitation was removed.Results show that the re-evaporation of surface precipitation contributed about 15%−20% to the total rainfall in the inner core within a 100-km radius from the storm center after landfall.It is found that the removal of surface precipitation in the land surface model reduced soil moisture and thus surface latent heat flux,which led to a slightly weaker storm,thus the weaker boundary layer inflow and reduced inward moisture transport into the inner-core region of the storm.The results demonstrate that the reduced surface evaporation and inward moisture transport contributed about 30%−40% and 60%−70%,respectively,to the decrease in precipitation in the inner-core region in the experiment with the re-evaporation of surface precipitation removed.(Liu Lu,Xu Jing,Wang Yuqing)

4.4 Parameterizing sea surface temperature cooling induced by tropical cyclones:1.Theory and an application to Typhoon Matsa (2005)

Sea surface temperature cooling (SSTC) induced by tropical cyclones (TCs) could produce a significant impact on the TC intensity.Although a coupled atmosphere-ocean model could provide such SSTC,various challenges associated with coupled modeling often lead many TC researchers to continue to use atmosphereonly models.Therefore,the main goal of this study is to develop a fast,robust,and effective parameterization scheme for TC-induced SSTC that can be used in atmosphere-only TC models.The following three steps are taken to achieve this goal:(i) results from an idealized ocean simulation,together with theoretical and temperature budget analyses,are analyzed to isolate each major mechanism causing TC-induced SSTC,which is then used as a basis for the parameterization; (ii) building upon the idealized ocean simulation,a new SSTC parameterization scheme including vertical mixing,advection,and SST recovery processes under the influences of sea surface height anomalies and ocean subsurface temperature is developed; and (iii) this SSTC parameterization scheme is evaluated through numerical simulations of Typhoon Matsa (2005) and validated against remote sensing data.Results show significant improvements in the simulated TC intensity and SST changes after applying this parameterization scheme.Although further testing with more TC cases is needed,these results are promising,and the parameterization scheme should be compatible with any TC weather prediction model.It is well known that tropical cyclones (TCs) may induce local cooling in sea surface temperature (SST),through upwelling of cold water from underneath,which would in turn act to slow down TC intensification.So it is highly desirable to incorporate the fine-scale structures of the TC-induced SST cooling into numerical TC models in order to improve the prediction of TC intensity.In this study,we developed an easy-to-use,fast,and effective parameterization scheme for TC-induced SST cooling that can be used in atmosphere-only TC models.Results show significant improvements in the simulated TC intensity and SST changes after applying this parameterization scheme.Although further testing with more TC cases is needed,the parameterization scheme should be compatible with any TC weather prediction model.(Liu Xin,Wei Jun,Zhang Da-Lin)

4.5 Parameterizing sea surface temperature cooling induced by tropical cyclones:2.Verification by ocean drifters

This study analyzes sea surface responses to tropical cyclones (TCs),based on sea surface temperature (SST) and ocean surface current (V-OS) data from 57 buoy drifters,together with satellite SST and sea surface height anomaly data,associated with nine TCs during the year of 2016 in order to validate the TC-induced SST cooling (SSTC) parameterization scheme developed in Part 1.Results show that the drifters-measured SSTs compare favorably to the satellite detected,except near the coastline and at high latitudes,and that SSTCs are negatively correlated with sea surface height anomaly.During TC passage,V-OS usually rotate clockwise on the right-hand side of TC tracks,but their rotating directions cannot be specified on their left-hand side.After TC passage,most drifters,located on both sides,show clockwise rotation.These results help characterize the responses of SST and V-OS to TCs into the following three modes:(i) significant increases in V-OS during TC passage,followed by strong SSTC; (ii) decreases or little changes in V-OS,followed by SSTC; and (iii) little changes in both V-OS and SSTC.All the three response modes are reasonably included in the SSTC parameterization scheme.After optimizing three empirical constants,based on the drifters’ data,the scheme’s performance is evaluated by comparing the parameterized SSTC to the drifters-measured SSTs,showing satisfactory results in terms of the starting time,duration,strength,and rate of SSTCs.An error analysis shows a mean value of about 20% of the observed SSTC with a small standard deviation,confirming the effectiveness of the SSTC parameterization scheme.In this study,we examine sea surface responses to the passage of tropical cyclones (TCs),based on observational data taken from ocean buoy drifters,in order to validate the TC-induced sea surface temperature (SST) cooling parameterization scheme presented in Part 1 of this series of papers.Results show some interesting features in ocean surface currents during TC passage.These features have been more or less included in our TC-induced SST cooling theory.An error analysis shows a mean value of about 20% of the observed SST cooling with a small standard deviation,which confirms the effectiveness of the SST cooling parameterization scheme.(Liu Xin,Zhang Da-Lin,Guan Jian)

4.6 The axisymmetric and asymmetric aspects of the secondary eyewall formation in a numerically simulated tropical cyclone under idealized conditions on an f-Plane

The axisymmetric and asymmetric aspects of the secondary eyewall formation (SEF) in a numerically simulated tropical cyclone (TC) under idealized conditions were analyzed.Consistent with previous findings,prior to the SEF,the tangential wind of the TC experienced an outward expansion both above and within the boundary layer near and outside the region of the SEF later.This outward expansion was found to be closely related to the top-down development and inward propagation of a strong outer rainband,which was characterized by deeper and more intense convection upwind and shallower and weaker convection downwind.In response to diabatic heating in the outer rainband was inflow in the mid-to-lower troposphere,which brought the absolute angular momentum inward and spun up tangential wind in the inflow region and also in the convective region because of vertical advection.As a result,as the outer rainband intensified and spiraled cyclonically inward,perturbation tangential and radial winds also spiraled cyclonically inward and downward along the rainband.As it approached the outer edge of the rapid filamentation zone outside the primary eyewall,the downwind sector of the rainband in the boundary layer was rapidly axisymmetrized.Continuous inward propagation and axisymmetrization and secondarily the merging with inner rainbands led to the spinup of tangential wind in the boundary layer,enhancing surface enthalpy flux and convection and eventually leading to the simulated SEF.Our results demonstrate that the simulated SEF was a top-down process and was mainly triggered by asymmetric dynamics.(Wang Hui,Wang Yuqing,Xu Jing)

4.7 Evolution of the warm-core structure during the eyewall replacement cycle in a numerically simulated tropical cyclone

This study examines the evolution of the warm-core structure during the secondary eyewall formation (SEF) and the subsequent eyewall replacement cycle (ERC) in a numerically simulated tropical cyclone (TC) under idealized conditions.Results show that prior to the SEF,the TC exhibited a double warm-core structure centered in the middle and upper troposphere in the eye region,and as the storm intensified with a rapid outward expansion of tangential winds,the warm core strengthened and a secondary off-center warm ring developed between 8- and 16-km heights near the outer edge of the eye.During the SEF,both the upper-level warm core and the secondary off-center warm ring rapidly strengthened.As the secondary eyewall intensified and contracted and the primary eyewall weakened and dissipated,the off-center warm ring extended inward and merged with the inner warm core to form a warm core typical of a single-eyewall TC.Results from the azimuthal-mean potential temperature budget indicate that the warming in the eye is due to subsidence and the warming above 14-km height outside the eye is largely contributed by radial warm advection in the outflow.The development of the off-center warm ring is largely due to the subsidence warming near the inner edge of the primary eyewall and in the moat area and the warming by diabatic heating in the upper part of the inner eyewall below 14-km height.Further analysis indicates that the eddy advection also played some role in the warming above 12-km height in the upper troposphere.(Wang Hui,Wang Yuqing,Xu Jing)

4.8 Factors affecting the variability of maximum potential intensity (MPI) of tropical cyclones over the North Atlantic

Contributions of atmospheric factors to the variability of the calculated theoretical maximum potential intensity (MPI) of tropical cyclones (TCs) over the North Atlantic are explored using the 6-hour atmospheric reanalysis and TC best track data from 1980 to 2015.The results show that for a given sea surface temperature (SST),the calculated theoretical MPI between the medians of top 10% and bottom 10% samples can vary by as large as 10−15 m s−1,which accounts for 20%−25% of the median of the MPI.It is shown that the drier (moister) and colder (warmer) environment favors higher (lower) MPI,and the TC-MPI is more sensitive to atmospheric temperature at lower SSTs,but more sensitive to atmospheric humidity at higher SSTs.Results from sensitivity experiments show that the tropospheric temperature and humidity profiles and the outflow layer temperature are all responsible for the MPI variability,but their relative importance varies with SST.The atmospheric humidity accounts for 12−13 (7−11) m s−1at SSTs over (below) 28 ,the tropospheric temperature accounts for about 7−12 (5−6) m s−1at SSTs below (above) 28 ,and the outflow temperature accounts for 7−8 m s−1almost independent of SST.These results strongly suggest that the modulation of MPI by synoptic variability needs to be considered when MPI is calculated and used as a predictor/parameter in operational TC intensity prediction schemes,especially for strong TCs.(Xu Jing,Wang Yuqing,Yang Chi)

4.9 Interbasin differences in the median and variability of tropical cyclone MPI in the Northern Hemisphere

Using the 1980−2017 6-hour best-track tropical cyclone data and global reanalysis data,we studied the interbasin differences in the median and variability of tropical cyclone maximum potential intensity (MPI) as a function of sea surface temperature (SST) in the North Atlantic (NA),eastern North Pacific (ENP),western North Pacific (WNP),and North Indian Ocean (NI).Results show that the MPI median increases by 4.8,7.7,6.4,and 4.4 m s−1per degree increase in SST in the NA,ENP,WNP,and NI,respectively.The MPI is the largest in the NI at SST between 27 and 28.5 and in the ENP at SST above 28.5 ,while is the smallest at SST below 29.5 in the WNP.The environmental factors that contribute to such interbasin differences were compared among basins.The greatest MPI in the ENP is largely contributed by the colder troposphere and drier boundary layer.The warmer troposphere and wetter boundary layer are responsible for the smallest MPI in the WNP.In the NA,the warmer outflow layer reduces the thermodynamic efficiency and partly offsets the positive contributions by the colder troposphere and drier boundary layer,resulting in the moderate MPI.In the WNP,the variability in MPI decreases with increasing SST across all four basins and is the largest.The large variability at low SSTs is largely contributed by the variability in air temperature,which includes both the airsea temperature difference and the outflow layer temperature,while the variability at relatively high SSTs is dominantly contributed by boundary layer moisture.(Xu Jing,Wang Yuqing,Yang Chi)

4.10 Microphysical properties of rainwater in Typhoon Usagi (2013):A numerical modeling study

A 2-km resolution simulation using the Weather Research and Forecasting model with Morrison microphysics was employed to investigate the rainwater microphysical properties during different stages of Typhoon Usagi (2013) in the inner-core and outer region.The model reproduced the track,intensity,and overall structure of Usagi (2013) reasonably.The simulated raindrop size distribution showed a rapid increase in small-size raindrop concentration but an oscillated decrease in large-size ones in the inner-core region,corresponding well with the upward motion.It was found that there existed two levels (1.25 and 5.25 km) of maximum number concentration of raindrops.The ice-related microphysics at high levels was stronger than the warm-rain processes at low levels.The larger raindrops formed by self-collection in the inner-core suffered from significant breakup,but the raindrops outside the eyewall did not experience evident breakup.Model results indicated that the dominant terms in the water vapor budget were the horizontal moisture flux convergence (HFC) and local condensation and deposition.The evaporation from the ocean surface (PBL) was similar to 10% of the HFC in the inner core,but up to 40% in the outer region as the air therein was far from saturation.Furthermore,water vapor in the outer region was obtained equally through evaporation from the cloud and inward transportation from the environment.An earlier start of cloud microphysical processes in the inner-core region was evident during the intensification stage,and the continuous decreasing of condensation in both the inner-core and outer regions might imply the beginning of the storm weakening.(Deng Lin,Gao Wenhua,Duan Yihong)

4.11 Effect of unidirectional vertical wind shear on tropical cyclone intensity change:Lower-layer shear versus upper-layer shear

In this study,a quadruply nested,nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower-layer and upper-layer unidirectional environmental vertical wind shears (VWSs).Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions.The TC weakens much more rapidly for a relatively long period in the upper-layer shear than in the lower-layer shear,which is found to be related to the stronger storm-relative asymmetric flow in the middle-upper troposphere and the larger vertical vortex tilt in the former than in the latter.The stronger storm-relative flow in the former imposes a greater ventilation of the warm core in the middle-upper troposphere,leading to a more significant weakening of the storm.The storm in the lower-layer shear only weakens initially after the VWS is imposed but then experiences a quasi-periodic intensity oscillation with a period of about 24 h.This quasi-periodic behavior is found to be closely related to the boundary layer thermodynamic discharge/recharge mechanism associated with the activity of shear-induced outer spiral rainbands.There is no significant intensity oscillation for the storm embedded in the upper-layer shear,even though outer spiral rainbands develop quasi-periodically also.The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.(Fu Hao,Wang Yuqing,Riemer Michael)

4.12 A method for diagnosing the secondary circulation with saturated moist entropy structure in a mature tropical cyclone

Under the adiabatic,axisymmetric and steady assumption,a relationship between the saturated moist entropy structure and the secondary circulation in a tropical cyclone (TC) is derived from the continuity equation.It is found that the isentropic surfaces coincide with the streamlines,and the streamfunction can be expressed with saturated moist entropy.The secondary circulation and the saturated moist entropy structure depend on each other.Thus,a method for diagnosing the secondary circulation with the structure of saturated moist entropy is proposed.The method is verified with a simulated intense idealized TC with a highly axisymmetric structure.The diagnosed secondary circulation reproduces well the moist inflow in the boundary layer and the moist updraft in the eyewall.This method facilitates secondary circulation diagnosis in theoretical or mature TCs that satisfy the adiabatic,axisymmetric and steady approximations.(Huang Yiwu,Duan Yihong,Chan Johnny C L)

4.13 Revisiting the dynamics of eyewall contraction of tropical cyclones

The dynamics of eyewall contraction of tropical cyclones (TCs) has been revisited in this study based on both three-dimensional and axisymmetric simulations and dynamical diagnostics.Because eyewall contraction is closely related to the contraction of the radius of maximum wind (RMW),its dynamics is thus often studied by examining the RMW tendency in previous studies.Recently,Kieu and Stern,et al.,proposed two different frameworks to diagnose the RMW tendency but had different conclusions.In this study,the two frameworks are evaluated first based on theoretical analysis and idealized numerical simulations.It is shown that the framework of Kieu is a special case of the earlier framework of Willoughby,et al.,if the directional derivative is applied.An extension of Stern et al.’s approach not only can reproduce but also can predict the RMW tendency.A budget of the azimuthal-mean tangential wind tendency indicates that the contributions by radial and vertical advections to the RMW tendency vary with height.Namely,radial advection dominates the RMW contraction in the lower boundary layer,and vertical advection favors the RMW contraction in the upper boundary layer and lower troposphere.In addition to the curvature,the increase of the radial gradient of horizontal mixing (including the resolved eddy mixing in three dimensions) near the eyewall prohibits eyewall contraction in the lower boundary layer.Besides,the vertical mixing including surface friction also plays an important role in the cessation of eyewall contraction in the lower boundary layer.(Li Yuanlong,Wang Yuqing,Lin Yanluan)

4.14 Characteristics of tropical cyclone extreme precipitation and its preliminary causes in Southeast China

Extreme precipitation induced by a tropical cyclone (TC) is of great concern to Southeast China.Regional characteristics of daily TC-induced extreme precipitation (TCEP) between 1958 and 2016 and the associated preliminary causes over Southeast China (Zhejiang,Fujian,and Shanghai) were examined by applying the objective synoptic analysis technique,TC track similarity area index,daily precipitation observations,and reanalysis data.The intensity and frequency of high-intensity TCEP (≥100,≥200,≥300 mm) have had an increasing trend over recent decades.Most of TCEP occurs from July to September,with frequency peaking in August for TCEP at all intensity levels,apart from the frequency for TCEP≥300 mm that peaks in September.Regions with high frequency and large TCEP (R-HFLTs) (relatively high frequency for TCEP≥100 mm) were concentrated along the coastline of the southern coastal Fujian (Southern R-HFLT),the regions from northern coastal Fujian to southernmost coastal Zhejiang (Central R-HFLT),and central coastal Zhejiang (Northern R-HFLT),decreasing from the coastline to inland.The Central R-HFLT region had the highest TCEP intensity and frequency for TCEP≥100 mm compared with the other R-HFLTs.Further analysis showed that the special terrain of Southeast China matched the spatial distribution of TCEP,which highlights the significance of the topography of Southeast China.To discover other factors responsible for the heavy TCEP,we compared two TC groups that influence Central R-HFLT.Under a more northerly direction and slow movement combined with the unique terrain,TCs with stronger vortex circulation generated heavier TCEP during landfall in Central R-HFLT.Heavy TCEP occurred with easterly and southeasterly winds interacting with terrain over the eastern coast for Central R-HFLT.Although large changes in the internal and external environment were sensitive to the observed TCEP intensity,the interaction between TC circulation and the complex topography in Southeast China under the northerly track was the dominant factor.(Qiu Wenyu,Ren Fumin,Wu Liguang)

4.15 Contributions of SST anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone genesis frequency over the western North Pacific

This study attempts to evaluate quantitatively the contributions of sea surface temperature (SST) anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP).Three SST factors in the Indo-Pacific Ocean are found to play key roles in modulating the interannual variability of WNP TCGF.They are summer SST anomaly in the east Indian Ocean (EIO),the summer El Niño-Southern Oscillation Modoki index (EMI),and the spring SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool.Results show that the three factors together can explain 72% of the total variance of WNP TCGF in the typhoon season for the period 1980−2015.Among them,the spring SSTG and the summer EIO contribute predominantly to the interannual variability of TCGF,followed by the summer EMI,with respective contributions being 39%,38%,and 23%.Further analysis shows that the summer EMI was affected significantly by the spring SSTG and thus had a relatively lower contribution to the TCGF than the spring SSTG.In addition,a statistical model is constructed to predict the WNP TCGF in the typhoon season by a combination of the May EIO and the spring SSTG.The new model can reproduce well the observed WNP TCGF and shows an overall better skill than the ECMWF Seasonal Forecasting System 5 (SEAS5) hindcasts.This statistical model provides a good tool for seasonal prediction of WNP TCGF.(Zhan Ruifen,Wang Yuqing,Zhao Jiuwei)

4.16 Coastal ocean response and its feedback to Typhoon Hato (2017) over the South China Sea:A numerical study.

The coastal ocean response and feedback to Typhoon Hato (2017) were studied based on high-resolution numerical simulations using both a coupled and an uncoupled cloud-resolving model.As a category 3 landfalling typhoon that moved west-northwestward across the northern South China Sea,Hato (2017) rapidly intensified prior to its landfall and induced significant impacts on the coastal water column,causing warm and cold patches in sea surface temperature (SST) over the continental shelf to the right of the track.This feature was well captured in an air-sea coupled model experiment.The coastal SST warming was found to be related to a two-layer oceanic circulation across the continental shelf forced by the onshore surface wind stress to the right of the storm track.The associated onshore surface currents imposed a warm temperature advection and downwelling,leading to the SST warming in the inner sea shelf,as diagnosed from an ocean temperature budget analysis.A sensitivity experiment,in which the typhoon vortex was removed from the initial conditions,further confirmed that it was the strong onshore wind stress to the right of the storm track that forced the onshore surface currents and the SST warming in the inner sea shelf.Results from an atmosphere-only model experiment with the typhoon-forced coastal warm SST anomalies removed demonstrate that the typhooninduced coastal warm SST anomalies contributed partly to the rapid intensification of Typhoon Hato prior to its landfall over South China and also slowed down the weakening of Hato at and shortly after its landfall.(Zhang Ze,Wang Yuqing,Zhang Weiming)

4.17 An application of the LTP_DSEF model to heavy precipitation forecasts of landfalling tropical cyclones over China in 2018

Recently,a track-similarity-based the Dynamical-Statistical Ensemble Forecast (LTP_DSEF) model has been developed in an attempt to predict heavy rainfall from Landfalling Tropical cyclones (LTCs).In this study,the LTP_DSEF model is applied to predicting heavy precipitation associated with 10 LTCs occurring over China in 2018.The best forecast scheme of the model with optimized parameters is obtained after testing 3452 different schemes for the 10 LTCs.Then,its performance is compared to that of three operational dynamical models.Results show that the LTP_DSEF model has advantages over the three dynamical models in predicting heavy precipitation accumulated after landfall,especially for rainfall amounts greater than 250 mm.The model also provides superior or slightly inferior heavy rainfall forecast performance for individual LTCs compared to the three dynamical models.In particular,the LTP_DSEF model can predict heavy rainfall with valuable threat scores associated with certain LTCs,which is not possible with the three dynamical models.Moreover,the model can reasonably capture the distribution of heavier accumulated rainfall,albeit with widespread coverage compared to observations.The preliminary results suggest that the LTP_DSEF model can provide useful forecast guidance for heavy accumulated rainfall of LTCs despite its limited variables included in the model.(Jia Zuo,Ren Fumin,Zhang Da-lin)

4.18 我国大风机理研究和预报技术进展

大风作为常见且影响严重的天气现象，可在各类天气系统和地形综合影响下产生。全面认识其机理和预报方法，对风灾防御和风能利用都有很大帮助。本文梳理了我国气象观测和业务预报关于大风定义、大风分布特征以及雷暴、台风等主要灾害性系统大风分布情况。之后简述雷暴、台风、冷空气等系统产生大风的机理，以及复杂地形下垫面的热力和动力作用对局地大风的影响，概括了不同系统大风预报技术的研究进展。最后总结大风研究现状和不足。（王黉，李英，吴哲红）

4.19 西北太平洋热带气旋变性过程中的风及降水分布变化特征分析

利用中国气象局上海台风研究所（CMA/STI）整编的热带气旋最佳路径资料、美国飓风联合警报中心（JTWC）最佳路径资料、美国国家海洋与大气管理局（NOAA）的全球多平台热带气旋风场资料（MTCSWA）和CMORPH降水资料、日本卫星云顶黑体辐射温度（TBB）资料等，分析1987—2016年30年间西北太平洋228个变性热带气旋（ETTC）的活动规律、风与降水分布及其演变特征。结果表明：（1）ETTC年均7.6个，除1—2月，各月均有分布，峰值在9月。约90.4%的ETTC变性位置在30°N以北，仅约9.6%在30°N以南较低纬度，且多发生于春夏和秋冬交替季节。（2）TC（热带气旋）变性通常发生在其转向后，半数以上移速加快，大多数中心气压升高或维持，仅10.5%降低。（3）变性过程中ETTC近中心最大风速减小，最大风速半径增大，内核趋于松散。其34节风圈半径北侧明显大于南侧，风场结构非对称性增强。（4）ETTC强风和强降水呈显著非对称性分布，其强风区主要出现在ETTC中心东侧，即路径右后方；强降水区主要出现在北侧，且变性后在东北象限向外扩张。（5）较强的环境水平风垂直切变（VWS）是影响ETTC风及降水分布的重要因子。强降水主要出现在顺风切方向及其左侧，强风（去除TC移速时）出现在切变左侧。（王佳琪，李英）

4.20 高空冷涡影响台风Meranti（1010）北翘路径的集合预报分析

路径突变是台风路径预报中的一个难题。2010年第10号台风Meranti（1010）在台湾岛南部海域西移过程中突然北折，而欧洲中期天气预报中心（ECMWF）集合预报对其北翘路径存在较大分歧。选取预报成功与不成功两组集合成员各8例，对比分析台风Meranti路径变化的主要原因。结果表明：（1）一个来自热带对流层上部槽的切断高空冷涡（UTCL）是该台风路径变化的一个重要影响系统。Meranti北翘路径跟它与UTCL的南北向耦合有关。（2）UTCL通过改变台风上层的环境气流影响台风引导气流。在UTCL移至台风北部过程中，台风的偏南风引导气流明显加强，有利于其路径北翘。（3）UTCL对台风Meranti北翘路径的影响还与其自身结构有关。水平环流宽且气旋性涡旋向下垂直伸展更深的UTCL对台风路径变化影响更明显。（4）位涡倾向方程的诊断分析表明，在TC与UTCL南北向耦合过程中，台风北部的正位涡水平平流项输送显著，有利于台风向北运动，且UTCL影响下产生的非对称风场在其中起主要作用。（温典，李英，魏娜）

4.21 基于路径相似的登陆热带气旋降水之动力-统计集合预报模型

过去几十年，数值天气预报（NWP）在热带气旋（TC）预报方面的最大进步是越来越准确的路径预报。对于登陆TC降水的预报，目前以数值模式为代表的技术手段预报能力还十分有限。围绕动力-统计结合之方法研究，初步发展了登陆热带气旋降水（LTP）预报的一种新方法——基于路径相似的登陆热带气旋降水之动力统计集合预报（LTP_DSEF）模型。该方法主要分为5步：TC路径预报、相似路径TC识别、其他特征相似性的判别、TC降水集合预报和最佳预报方案选择。涉及两个关键技术：TC降水分离的客观天气图分析法（OSAT）和TC路径相似面积指数（TSAI）。LTP DSEF模型对2012—2016年影响华南地区出现最大日降水量≥100 mm的21个TC的定量降水预报（QPF）试验结果显示，该模型对登陆TC过程降水的预报结果优于动力模式。登陆TC过程降水≥50 mm情况下，建模样本和独立样本平均TS评分均高于动力模式（EC、GFS、T639）相应的最好表现。对LTP_DSEF模型3个最佳方案的参数取值分析显示，起报时刻参数设定为最临近影响时刻即TC对陆地产生降水的前一天12：00 UTC、集合参数取最大值时预报效果稳定趋好。（丁晨晨，任福民，邱文玉）

5 雷电研究

5 Lightning research

5.1 雷电野外科学试验

“中国气象局雷电野外科学实验基地”观测条件进一步改善，开展了广州野外雷电综合观测试验以及那曲高原雷电观测试验。（1）针对广州高建筑物雷电观测增加了2个光学观测点，本年度获得的闪电个例数＞200。（2）新建了48 m2的人工引雷控制和观测室，成功引雷次数39次，创历史新高。（3）引进的闪电成像阵列（LMA）实现试运行，连续干涉仪的闪电通道精细化解析能力获得提升，捕获了几十例高质量闪电观测个例。（4）在广东和高原持续开展了基于闪电探测阵列的雷暴闪电活动观测试验，获得了一手的观测数据。（吕伟涛，张阳，郑栋，马颖）

5.2 雷电探测技术研发

提出了一种应用于闪电低频信号的到达时差—时间反转定位算法，其具有对定位最小站数要求低、抗干扰能力强、对时间精度要求不高的优势，并取得良好的定位效果。基于低频电场变化探测阵列的三维定位结果，对一次致人死亡的“晴天霹雳”闪电进行了分析，指出旁络闪击和多回击特征是造成人员死亡的直接原因。基于风云四卫星的闪电成像仪（LMI）对我国登陆台风全闪电的观测，检验了LMI闪电活动观测的连续性、实时性和有效性。（张阳，张文娟，范祥鹏，樊艳峰）

5.3 雷电物理研究

明确了触发闪电中爆发式磁场脉冲的普遍性，并揭示了其辐射来源和放电强度。利用广州高建筑物雷电过程的观测资料，明确了广州塔对下行闪电的影响范围和程度；揭示了闪电反冲先导的确切始发位置和双向传输细节特征；给出了同一次闪电中沿着相同路径的首次和继后回击的光学和电磁波形差异；分析了正地闪触发的并发上行闪电特征，结果表明正地闪回击及之后云内负先导朝建筑物顶端方向快速伸展均会激发上行闪电；基于我们获取的最高时间分辨率的自然下行闪电的光学观测资料，解析了一次近距离闪电的连接过程。采用3D-FDTD电磁场数值算法，研究了海陆混合传播路径对风力发电机上的闪电回击产生的雷电电磁场传播的影响，揭示了海陆界面影响雷电电磁场的主要因素；构建了多个建筑物雷击模型，发现建筑物屏蔽效应对屏蔽体高度的依赖性比屏蔽体的距离、相对距离和长度等参数更强。（张阳，吕伟涛，武斌，齐奇）

5.4 雷暴电学研究

明确了层云区闪电始发和传播位置的反射率特征，发现了层云区闪电的始发与亮带区域之间空间上紧密、实则在因果关系上相对疏远的现象。明确了闪电初始阶段的通道发展规律，揭示了不同类型雷暴以及雷暴不同区域闪电初始先导发展特征的差异，建立了电荷区分布形态和电荷密度不同配置影响闪电初始先导发展的概念模型。揭示了冬季雷暴电荷结构的多样性特征，指出部分冬季雷暴存在较大的下部正电荷区；发现冬季雷暴的闪电尺度和闪电初始击穿能量大于夏季。揭示了东亚以及西太平洋地区闪电的持续时间、空间扩展尺度和辐射能等特征，发现闪电空间扩展与雷暴对流强度之间的反向关联。建立了基于云分档微物理方案的起放电模型，相比于一般电学模型，该模型可以更为精细地模拟云内电过程，是研究起电机制和雷暴电荷结构形成的重要工具。基于模拟研究，揭示高原雷暴中独特的环境条件，如弱上升运动和低冻结层高度，是促使高原雷暴中频繁出现下部大正电荷中心的根本原因。（郑栋，王飞，徐良韬）

5.5 雷电预警预报技术研发与应用

开发了“雷电临近预警系统V2.1版本”，完成与国家级短时临近预报系统（SWAN）系统集成，实现对大区域范围的雷电活动进行临近预警，并通过业务转化认证评审。发展了基于人工智能技术融合应用WRF模式产品和闪电实测数据的雷电短时预报模型，初步完成了“人工智能和WRF模式相结合的闪电短时预报模块”的模块开发工作。为做好雷电监测、预警服务保障工作，2019年对雷电监测预警的各个系统软件进行了更新和完善。在70周年国庆期间，保障了雷电监测产品、雷电预警产品每天24 h实时上传到中国天气网，及时提供了雷电活动服务产品。顺利完成了大兴安岭雷击森林火灾服务系统的运行及维护工作，其监测预警产品实时上传到国家林业局森林防火预警监测信息中心，为各级林业管理部门开展雷击火监测提供了支持。（姚雯，孟青，马颖，陆天舒）

5.6 Characteristics of a multi-stroke“bolt from the blue”lightning-type that caused a fatal disaster

Observational data from the low-frequency electric-field detection array (LFEDA) and radar were used to study the precise location of a fatal accident caused by a lightning in Conghua,Guangzhou,China,2017.Based on a comprehensive analysis,the lightning was concluded to be a negative“bolt from the blue”ground flash with seven return strokes.The first return stroke was on June 3 at 16:36:49 with a peak current intensity of −30.9 kA.The LFEDA system mapped the development of the lightning channel.The horizontal distance from the edge of the cloud anvil area (0 dBz) to the return stroke point was about 0.3 km; the horizontal distance from the edge of the precipitation area (18 dBz) to the return stroke point was about 1.8 km; and the return stroke channel extended as far as 3 km out of the cloud.There was no cloud cover above the head of the victim.The electric field waveform of the return strokes detected by the LFEDA system showed multi-peak characteristics,and all seven return strokes were multiple-termination strokes.The most probable mechanism for the lightning injuries was the side flash.(Fan Xiangpeng,Zhang Yijun,Yin Qiyuan)

5.7 Measurements of magnetic pulse bursts during initial continuous current of negative rockettriggered lightning

During the Shandong Triggering Lightning Experiment (SHATLE) in summer of 2014 and 2015 and the Guangdong Comprehensive Observation Experiment on Lightning Discharge (GCOELD) campaign in 2018,we have conducted the observations of the magnetic pulse bursts (MPBs) during the initial continuous current in negative rocket-triggered lightning.The MPBs are commonly recorded at the main site of SHATLE (970 m from the rocket launch site),but the synchronous magnetic field (B-field) measurements at the close site of SHATLE (78 m from the rocket launch site) show the slow variations with small MPBs superposing on them.Note that both the charge transfer and the relative brightness increase notably during the appearance of the MPBs.After shifting up the operation frequency of the magnetic sensor,the MPBs can be observed at close distance (80 m from the rocket launch site) obviously in GCOELD.Observations show that the radiation sources of MPBs originate from the breakdown in the vicinity of the leader tip,but the sources of the initial magnetic pulses (IMPs) measured at the very initial stage of triggered lightning are from the radiation of the whole steel wire.The continuous current measured at the channel base during the MPBs cannot reflect the characteristics of breakdown current,because the current is attenuated and dispersed when propagating along the high-impedance leader channel.The average peak current associated with the MPBs is estimated to be on the order of kiloamperes.(Fan Yanfeng,Lu Gaopeng,Li Xiao)

5.8 Observations of the magnetic pulses in rocket-triggered lightning with sensitive magnetic sensor

With the observations in SHandong Triggering Lightning Experiment (SHATLE) and Comprehensive Observation Experiment on Lightning Discharge (GCOELD) in China,we investigate the magnetic pulses associated with the upward positive leaders during the initial stage of negative triggered lightning.The initial magnetic pulses (IMPs) measured at the very initial stage are corresponding to the sustained upward positive leaders,and can be divided into two categories (i.e.,impulsive and ripple pulses) according to the discernibility of separation between individual pulses.The IMPs are generated by leader current pulses propagating downward along the steel wire.For the magnetic pulses measured during the stage of initial continuous current or called magnetic pulse bursts (MPBs) since these pulses last longer than IMPs,they are commonly observed both in SHATLE and in GCOELD,even though the associated signals of electrical fields are not obvious.The heights of rocket upon the inception of the MPBs are at least about 268 m in our observations,and the average inter-pulse intervals are affected by the height significantly.The cases with higher height usually have shorter inter-pulse intervals.In comparison with the IMPs,the inter-pulse intervals of the MPBs contain more small time scale (＜20 μs) cases,since the speed of upward leaders shows an accelerating tendency with height and time.MPBs are derived from the breakdown near the leader tip.(Fan Yanfeng,Lu Gaopeng,Li Xiao )

5.9 Development of lightning nowcasting and warning technique and its application

The Chinese Academy of Meteorological Sciences Lightning Nowcasting and Warning System (CAMS_LNWS) was designed to predict lightning within the upcoming 0−1 h and provide lightning activity potential and warning products.Multiple remote sensing data and numerical simulation of an electrification and discharge model were integrated in the system.Two core algorithms were implemented:(1) an area identification,tracking,and extrapolating algorithm; (2) a decision tree algorithm.The system was designed using a framework and modular structure,and integrated warning methods were applied in the warning program.Two new algorithms related to the early warning of the first lightning and thunderstorm dissipation were also introduced into the system during the upgrade process.Thunderstorms occurring in Beijing,Tianjin,and Hebei during 2016−2017 were used to evaluate the CAMS_LNWS by the low-frequency cloud to ground lightning detection data,and the results show that the system has good forecasting and warning ability for local lightning activities.The TS score in 0−1 h ranged from 0.11 to 0.32,with a mean of 0.20.Operational experiments and promotional work for the CAMS_LNWS are now in progress.(Meng Qing,Yao Wen,Xu Liangtao)

5.10 High-speed video observations of natural lightning attachment process with framing rates up to half a million frames per second

Using high-speed video cameras operating with framing rates of 20 and 525 kfps,we imaged the attachment process of a natural negative cloud-to-ground flash,occurring at a distance of 490 m.Nine upward leaders were observed.A total of 12 space stems/leaders in 47 steps of the downward negative stepped leader were captured.The two-dimensional length of them was between 2.0 and 5.9 m,with an average of 3.0 m.The average interstep interval,step length,and two-dimensional speed of the downward negative leader and that of upward positive leader were statistically analyzed.The last step of the downward negative leader making contact with the upward connecting leader was recorded.The two-dimensional length of the final imaged gap between the tips of opposite-polarity leaders was estimated to be about 13 m.(Qi Qi,Lyu Weitao,Ma Ying)

5.11 Numerical simulation of the formation of a large lower positive charge center in a Tibetan Plateau thunderstorm

Numerical modeling is applied to elucidate the formation mechanism of the large lower positive charge centers (LPCCs) observed during thunderstorms over the Tibetan Plateau based on the simulation of a storm at the northeastern boundary of the plateau.Four sensitivity tests were carried out to explore the impacts of inductive charging,reversal temperature,and the choice of noninductive charging scheme.The results show that the unique environmental conditions of the Qinghai-Tibet Plateau,which include weak convection and low freezing level,are fundamental to the formation of large LPCC.A weakened charge density in the upper positive charge center highlights the role of a LPCC in lightning initiation although the charge density of the LPCC has no obvious change compared to that in the LPCC of the typical tripole structure.This accounts for Tibetan Plateau thunderstorms having low frequencies of lightning flashes,which occur mainly in the lower dipole.Inductive electrification,which provided more than 50% of the positive charge on graupel and increased the positive and negative charge on cloud drops by 2 orders of magnitude,is an important complement to the lower dipole of the tripole charge structure originally established by noninductive electrification.The inductive electrification also evidently enhances the LPCC and the middle negative charge center while slightly reducing the upper positive charge center.Subsequently,the lightning activity is strengthened,and lightning flashes are more likely to be initiated at the lower dipole.Varying the reversal temperatures and noninductive charging scheme does not fundamentally affect the formation of the LPCC.(Wang Fei,Deng Xiaohua,Zhang Yijun)

5.12 Radar reflectivity of lightning flashes in stratiform regions of mesoscale convective systems

A total of 254 flashes observed in 14 mesoscale convective systems in Chongqing,China,during the summers of 2014−2015 was used to investigate the characteristics of lightning flashes in stratiform regions (stratiform lightning flashes).The data were analyzed and combined with data from an S-band Doppler radar system.The results show that the reflectivity characteristics of stratiform lightning flashes are different from those of convective lightning flashes.More than 90% of the stratiform lightning flashes had close spatial relationship with the areas in which bright bands were identified in the vertical direction.The reflectivity at the first detected very-high-frequency sources of most stratiform lightning flashes decreased with increasing core reflectivity within the lightning extension area.Further investigations show that only a small proportion (≤10%) of the stratiform lightning flashes were directly initiated by the charge corresponding to the reflectivity cores.It suggests that the reflectivity cores (mostly bright bands) and stratiform lightning flashes are close in space,but different in causality.Some in situ electrifications in stratiform regions,such as nonriming noninductive electrification and melting electrification,are inferred to cause this relationship.The stratiform lightning flashes initiated from lower altitudes tended to have a lower maximum reflectivity within their lightning areas,but a higher maximum reflectivity in the vertical direction of their initiation locations than the flashes initiated at higher altitudes.(Wang Fei,Liu Hengyi,Dong Wansheng)

5.13 High-speed video observations of recoil leaders producing and not producing return strokes in a Canton-Tower upward flash

High-speed video and electric field change data have been used to examine the initiation and propagation of 21 recoil leaders,7 of which evolved into dart (or dart-stepped) leaders (DLs) initiating return strokes and 14 were attempted leaders (ALs),in a Canton-Tower upward flash.Three DLs and two ALs clearly exhibited bidirectional extension.Each DL was preceded by one or more ALs and initiated near the extremity of the positive end of the preceding AL.The positive end of each bidirectional DL generally appeared to be inactive (stationary) or intermittently propagated along the positive part of the preceding AL channel and extended into the virgin air.A sequence of two floating channel segments was formed ahead of the approaching positive end of one DL,causing its abrupt elongation.In this paper,we presented high-speed video observations of recoil leaders (RLs) which were not obscured by the cloud.We examined the initial positions,morphology,and dynamics of RLs in a Canton-Tower upward flash.Out of a total of 21 RLs,7 evolved into dart (or dartstepped) leaders,attaching to the Tower and producing return strokes,and 14 failed to reach the Tower,that is,were attempted dart (or dart-stepped) leaders.We infer that all the RLs were bidirectional with the negative end steadily extending toward the Tower and the positive end generally being stationary or showing intermittent propagation.Each dart (or dart-stepped) leader initiated near the extremity of the positive end of the preceding bidirectional attempted leader.The positive end of one dart (or dart-stepped) leader extended abruptly via connection of at least one floating channel to the positive channel tip.(Wu Bin,Lyu Weitao,Qi Qi)

5.14 Synchronized two-station optical and electric field observations of multiple upward lightning flashes triggered by a 310-kA +CG flash

A positive cloud-to-ground (+CG) lightning flash containing a single stroke with a peak current of approximately +310 kA followed by a long continuing current triggered seven upward lightning flashes from tall structures.The flashes were observed on 4 June 2016 at the Tall Object Lightning Observatory in Guangzhou,Guangdong Province,China.The optical and electric field characteristics of these flashes were analyzed using synchronized two-station data from two high-speed video cameras,one total-sky lightning channel imager,two lightning channel imagers,and two sets of slow and fast electric field measuring systems.Three upward flashes were initiated sequentially in the field of view of high-speed video cameras.One of them was initiated approximately 0.35 ms after the return stroke of +CG flash from the Canton Tower,the tallest structure within a 12-km radius of the +CG flash,while the other two upward flashes were initiated from two other,more distant tall objects,approximately 18 ms after the +CG flash stroke.The initiation of the latter two upward flashes could be caused by the combined effect of the return stroke of +CG flash,its associated continuing current,and K process in the cloud.Each of these three upward flashes contained multiple downward leader/upward return stroke sequences,with the first leader/return stroke sequence of the second and third flashes occurring only after the completion of the last leader/return stroke sequence of the preceding flash.The total number of strokes in the three upward flashes was 13,and they occurred over approximately 1.5 s.(Wu Bin,Lyu Weitao,Qi Qi)

5.15 Simulation of inverted charge structure formation in convective regions of mesoscale convective system

The charge structure evolution of a mesoscale convective system with an anomalous or inverted charge structure,observed in the Severe Thunderstorm Electrification and Precipitation Study,a field project on the Colorado-Kansas border in summer 2000,is simulated using the Weather Research and Forecasting (WRF) model coupled with electrification and discharge processes.Two noninductive electrification schemes are used,based on the liquid water content (LWC) and the graupel rime accretion rate (RAR).The simulation with the LWC-based electrification scheme cannot reproduce the inverted charge structure with a positive charge region sandwiched by two negative charge layers,while the RAR-based electrification scheme produces the evolution process of a normal-inverted-normal charge structure in the convective region,which is consistent with the observations.In the low RAR (＜2 g m−2s−1) region,graupel is mainly negatively charged when it bounces off ice crystals,while the ice crystals take up positive charge.However,in the zone where the inverted charge structure forms,a strong updraft (＞16 m s−1),high LWC (＞2 g m−3),and high RAR (＞4.5 g m−2s−1) region appears above the height of the −20 layer,so that a positive graupel charging region is generated above the −20 layer of the convective region,resulting in a negative dipole charge structure with negatively charged ice crystals above the positively charged graupel.The negative dipole is superposed on the positive dipole (positive above negative) charge structure at the lower position to form an inverted tripole charge structure.(Xu Liangtao,Zhang Yijun,Wang Fei )

5.16 Inner-core lightning outbreaks and convective evolution in super Typhoon Haiyan (2013)

Using lightning data from the World Wide Lightning Location Network,infrared satellite imagery,and micro-wave observations,this study investigates lightning outbreaks and convective evolution in the inner core (0−100 km) of super Typhoon Haiyan (2013),the strongest storm on record to make landfall in the northwest Pacific.This storm was characterized by intense lightning activity with half of the strokes occurring in the inner core.Three major inner-core lightning outbreaks and convective bursts (CBs) were observed during rapid intensification (RI),maximum intensity (MI),and weakening stages.These outbreaks coincided with favorable large-scale environmental conditions for TC development with higher sea surface temperature (29−30 ),higher relative humidity (75%−80%),and weaker deep-layer vertical wind shear (3−8 m s−1),compared to the climatological averages for the month of November in the northwest Pacific.The RI lightning outbreak occurred primarily in the downshear quadrants and CBs were located inside the radius of maximum wind (RMW).The MI lightning outbreak occurred just after the eyewall replacement cycle,inducing marked depression of brightness temperature at 91-GHz.The lightning outbreak during Haiyan’s weakening stage preferred the upshear-left quadrant outside the RMW.In contrast,relative lack of cloud-to-ground lightning in the rainbands was observed during all three main outbreaks.The radial and azimuthal distributions of lightning outbreak within the inner core provided indicative information on the relationships between convective structure and intensity changes of Haiyan.(Zhang Wenjuan,Rutledge Steven A,Xu Weixin)

5.17 Initial leader properties during the preliminary breakdown processes of lightning flashes and their associations with initiation positions

The properties of initial leaders (ILs) of 1056 flashes in two thunderstorms in Guangzhou,China,are analyzed.The median values of IL properties are 11.1 ms for duration,2.2 km for vertical distance,2.7 km for 3-D distance,1.9×105m s−1for vertical displacement speed,2.4×105m s−1for 3-D displacement speed,28° for angle between the IL’s displacement direction and the vertical direction,1.1 per ms for pulse rate,179 m for vertical step length,and 224 m for 3-D step length.All the IL properties follow lognormal distributions.Strong monotonic relationships of IL duration with speed and pulse rate and pulse rate with speed and step length are revealed.With increasing initiation altitude,IL duration and step length tend to increase,and IL speed and pulse rate tend to decrease.The ILs initiated at approximately −20 isotherm are more vertically orientated on average than those initiated at lower or higher levels.The ILs initiated in the strong kinematic areas generally had a greater speed and pulse rate but smaller duration,distance,and step length relative to those initiated in the weak kinematic areas.The possible impacts of the charge distribution patterns and density on the IL properties in different types of thunderstorms or thunderstorm areas are discussed.(Zheng Dong,Shi Dongdong,Zhang Yang)

5.18 Charge regions indicated by LMA lightning flashes in Hokuriku’s winter thunderstorms

The charge distribution of some cells in three winter thunderstorms in the Hokuriku region of Japan is investigated based on lightning mapping array (LMA) flash data.The vertical arrangements of charge regions involved in lightning discharges suggest diverse charge patterns,including quad-polar,tripole,positive dipole,inverted dipole,and inverted tripole.The riming electrification between graupel and ice crystals or their aggregations are thought to be responsible for the electrification of most cells.The charging process between snow/aggregates and ice crystals may be responsible for some inverted charge structure that occurred above 0 isotherm and accompanied with weak radar echoes.Convection indicated by the vertical development of radar reflectivity appears crucial to shaping the diverse charge distribution patterns by determining which charging mechanisms occur and where; it also influences changes in height or even the disappearance of the charge regions.The charged cores are distributed from 0.7 to 5.3 km heights and 2 to −31 temperatures,while the distances between adjacent charged cores with opposite polarities change between 0.2 and 3.4 km,with a mean of 1.3 km.The mean flash duration and horizontal distance are 425.0 ms and 19.8 km,respectively.The average height,temperature,and power of flash initiations are 2.8 km,−11.9 ,and 15.6 dBW,respectively.(Zheng Dong,Wang Daohong,Zhang Yijun)

5.19 Correlated luminosity and magnetic field peaks produced by Canton Tower-strokes

Simultaneously measured luminosity and magnetic field induced by the return strokes in typical upward and downward flashes occurring on the 600-m tall Canton Tower were presented.The correlation between peak relative luminosity and peak relative magnetic field induced by return strokes were examined for 4 upward and 4 downward flashes.All the flashes contained 3 or more return strokes and all the return strokes in a same flash followed the same path.Luminosity of the lightning channel close to the top of the Canton Tower exhibited characteristics of pronounced initial peak with 20%−90% rise time of about 5.2 µs (arithmetic mean value,based on 43 samples) for subsequent strokes,which was inferred to be related to the secondary (typically largest) peak current at the top of the Canton Tower.The magnetic field induced by subsequent strokes exhibited characteristics of pronounced initial peak with 20%−90% rise time of about 1.2 µs (arithmetic mean value,based on 47 samples).The initial peak magnetic field was modified to proportionally represent the largest peak current at the top of the Canton Tower.Roughly linear and quadratic relation between the initial peak luminosity and modified peak magnetic field were found for subsequent strokes in both upward and downward flashes.The quadratic relation fit the data slightly better than the linear relation.The ratios of peaks of luminosity to magnetic field of the downward first stroke were considerably larger than those of the subsequent strokes,and the possible reasons were also discussed.(Chen Lyuwen,Lyu Weitao,Zhang Yijun)

5.20 A method of three-dimensional location for LFEDA combining the time of arrival method and the time reversal technique

Based on fast electric field waveforms of the Low-frequency E-field Detection Array (LFEDA),we introduce the time reversal technique into lightning three-dimensional location for the first time and propose a new algorithm for the three-dimensional location of lightning low-frequency discharges.Without using complex filtering algorithms to remove higher-frequency component,this method obtains similar results to the newly reported LFEDA refinement algorithm.The new algorithm can obtain finer,more continuous,and richer positioning results with a minimum of four stations,5-dB signal-to-noise ratio,and 500-ns time error compared with the low-frequency signal time of arrival three-dimensional positioning method.These results indicate that the new algorithm has the advantages of low requirements on the number of stations,certain antiinterference ability,and low requirements on time accuracy.The standard deviations in theXandYdirections for return strokes of triggered lightning flashes are both approximately 90 m.During the last 20 years,besides the location of return stroke,finer and more accurate positioning of total lightning based on lightning lowfrequency discharge signals can be achieved by using the time of arrival method.The Chinese Academy of Meteorological Sciences developed a lightning Low-frequency E-field Detection Array (LFEDA),consisting of ten substations in Conghua,Guangzhou,since 2014,which is capable of determining the three-dimensional locations of lightning discharge events.With the application of empirical mode decomposition algorithm in the past research of LFEDA,the fine structure of lightning channel can be obtained.This paper proposes a new lightning low-frequency discharge three-dimensional location algorithm based on the multistation waveform data of the LFEDA system.This is the first time that the time reversal method has been applied to total lightning three-dimensional location.This method can obtain accurate lightning three-dimensional location results without using a complex filtering algorithm.Compared with the time of arrival method,the new algorithm not only can yield positioning results similar to those of the fine positioning algorithm (Fan et al.,2018) but also has the advantages of low requirements on the number of stations,certain anti-interference ability,and low requirements on time accuracy.(Chen Zefang,Zhang Yang,Zheng Dong)

5.21 Lightning electromagnetic fields along an ocean-land mixed propagation path generated by return strokes to wind turbines

We evaluate lightning electromagnetic fields along an ocean-land mixed propagation path generated by return strokes to wind turbines (WTs) using a three-dimensional finite-difference time domain method.In these simulations,we consider the following three cases:strikes to onshore WTs,strikes to offshore floating WTs,and strikes to onshore WTs in the presence of horizontally stratified ground.Also,we investigate the effect on lightning electromagnetic fields of the following two parameters:distancedbetween the WT and the observation point,and distancedlbetween the coastline and the observation point.Generally,the lightning electromagnetic fields are affected by the ocean-land interface,anddanddlplay an important role in the effect degree.For onshore WTs,the effect of a mixed path is visible on magnetic field and vertical electric field fordless than 300 m and horizontal electric field fordexceeding 300 m.The effect fordl=10 m is stronger than that fordl=50 m.For offshore WTs,the effect of a mixed path is significant and the effect fordl=10 m is almost the same as that fordl=50 m.(Su Zhiguo,Lyu Weitao,Chen Lyuwen)

5.22 Shielding effect of surrounding buildings on the lightning-generated vertical electric field at the top of a tall building

We have evaluated the vertical electric field E-z on the roof of a 100-m tall building considering the shielding effect of surrounding buildings (the shields) due to nearby lightning return strokes to flat ground by using the 3-D finite-difference time-domain method.Also,we have investigated the influences on the vertical electric field E-z and corresponding enhancement factors of the following parameters:the distancedbetween the strike point and the field point; the lengthlof plan area,the heighth,and the conductivity is of the shields; and the relative position and relative distancerdbetween the field point and those shields in its vicinity.In general,vertical electric fields and enhancement factors at the field point can be reduced because of the effect of the shields in its vicinity.The effect for the shield located in front of the field point is much stronger than that for the shield located in any other positions.Moreover,the dependence of shielding effect on heighthof the shield is stronger than any other parameters such as distanced,relative distancerd,and lengthlof the shield.(Su Zhiguo,Lyu Weitao,Chen Lyuwen)

5.23 Duration,spatial size and radiance of lightning flashes over the Asia-Pacific region based on TRMM/LIS observations

The geographical distributions of flash duration,length,footprint and radiance,as well as their correspondence with thunderstorm structures,are investigated for the first time in the Asia-Pacific region ranging from 70°E to 160°E and from 18°N to 36°N and in six specially chosen regions by employing flash data collected by the Lightning Imaging Sensor (LIS) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and TRMM-based radar precipitation feature (RPF) data from 2002 to 2014.The flash length,footprint and radiance values are,on average,the largest over the deep ocean,followed by offshore waters and land.Flash duration is the longest over the offshore waters near the east coast of China,followed by the deep ocean and land.The Tibet Plateau and the northern part of the Indian Peninsula have the weakest flash properties in the study region.Furthermore,the geographic distributions of the flash properties exhibit evident seasonal changes.The monotonic relationship between flash spatial size and radiance is stronger than the monotonic relationships between flash duration and spatial size or radiance.Based on a comparison of the seasonal and regional changes in flash properties with RPF properties,convective intensity is proposed to play a crucial role in characterizing the flash spatial size and radiance,according to their inverse correlation in most regions.However,the climatological correspondence between flash duration and thunderstorm structures remains poorly constrained.We have launched a discussion of the possible association between thunderstorm structures and flash properties.(You Jin,Zheng Dong,Zhang Yijun)

5.24 Analysis of radiation evolution characteristics of the artificial triggered lightning channel

The spectrum of one triggered lightning with time resolution of 20 s was observed by using the slitless spectrograph and the current intensity at the channel bottom was obtained,based on which the radiation characteristics of the lightning spectrum under different current intensities were analyzed.The duration of the spectral line was analyzed according to the excitation energy of the spectral line and the change of the channel current.As a result,the spectral lines were divided into three types.Furthermore,mechanisms of the continuous background radiation of short wavelength and long wavelength were analyzed,respectively.The influences of two radiation mechanisms on continuous background radiation attenuation were researched.(Zhang Huaming,Lyu Weitao,Zhang Yang)

5.25 Development and assessment of quantitative preciptitation estimation algorithms for S-,C-,and X-band dual-polarization radars based on disdrometer data from three regions of China

The accuracy of quantitative precipitation estimation (QPE) for dual-polarization radars can be improved by using a localized rainfall estimation algorithm derived from the raindrop size distribution (DSD).In the present study,DSDs observed at Suzhou City,Jiangsu Province; Yangjiang City,Guangdong Province; and Naqu City,Tibet are analyzed during the rainy season together with the corresponding polarimetric variables for the above three regions.Most importantly,these DSD data are used to develop optimal“synthetic”QPE algorithms for S-,C-,and X-band dual-polarization radars,which will be built or upgraded in the three regions.Meanwhile,a new piecewise fitting method (PFM) is proposed.It has been found that the number concentrationNDof small raindrops (D＜1 mm) is the highest in Suzhou,while that of larger raindrops (D＞1 mm) is the highest in Yangjiang.The characteristics of the differential reflectivityZDRand specific differential phase (K-DP) are significantly different in the three locations,suggesting that different rainfall estimators are needed for different locations.Further performance assessment of the QPE based on DSD data indicates that the PFM QPE algorithm (LDSD) performs better than the conventional fitting method (CFM),and the localized QPE algorithm can improve the QPE accuracy.Observations from S-band dualpolarization radars and rain gauges in the Southern China Monsoon Rainfall Experiment are implemented to verify the performances of the QPE algorithms proposed in the present study.It is found that compared with non-localized algorithms,the localized LDSD algorithm yields the best results with at least 7.66% and 8.43% reductions in the RMSE and NE,respectively,which implies that while polarimetric variables can reflect DSD characteristics,the localized QPE algorithm remains necessary.(Zhang Yang,Liu Liping,Wen Hao)

5.26 一套双频段三维全闪电定位系统及其初步观测结果

介绍了一套同时工作在甚高频（VHF）和甚低频/低频（VLF/LF）频段的三维闪电定位系统，并给出了初步观测结果。系统使用到达时间差法对2个频段的闪电辐射源进行定位，能连续地给出雷暴过程中闪电活动空间位置，并详细给出单次闪电的发展路径，在使用2个频段辐射波形参数进行定位的同时还可以保存原始波形用于深入研究。VHF、VLF/LF 2个频段闪电定位结果的时间分辨率分别为100 µs和1 ms，定位精度在站网覆盖范围可优于200 m。2个频段的定位结果都能很好地描述雷暴中的闪电活动，但表现出的特征有一定差异。2个频段定位结果对单个闪电发生路径进行描绘时，VLF/LF频段的大部分定位结果能与VHF定位结果重叠。（刘恒毅，董万胜，蔡力）

5.27 一次正地闪触发两个并发上行闪电的光电观测

广州高建筑物雷电观测站光电同步观测系统于2017年6月16日记录到一次峰值电流达+141 kA的单回击正地闪触发两个并发上行闪电过程。利用高速摄像、普通摄像和电场变化数据分析了触发型上行闪电的始发特征和机理。结果表明：正地闪回击后约0.8 ms内，在距正地闪接地点约3.9 km的广州塔（高600 m）和4.1 km的东塔（高530 m）分别有上行闪电始发。正地闪回击过程中和大量正电荷以及之后可能有云内负先导朝高塔方向快速伸展造成塔顶局部区域的电场发生突变是两个上行闪电激发的原因。两个上行闪电在353 ms内发生7次回击，其中6次在广州塔上，仅1次在东塔上，且广州塔回击峰值电流平均值（-21.4 kA）约为东塔回击峰值电流（-7.3 kA）的3倍，表明广州塔上行闪电通道可能比东塔上行闪电通道伸展至分布范围更广、电荷量（或电荷密度）更大的负电荷区。两个上行闪电先导的二维速率变化范围为9.4×104～1.8×106m/s，平均值为6.9×103m/s。（武斌，吕伟涛，齐奇）

5.28 一次“晴天霹雳”致死事件分析

对2017年广州从化地区一次“晴朗”天气下的闪电致死事件进行调查分析发现，这次事件是闪电首先击中一棵大树，然后击中附近人员头顶致死。根据目击者描述的时间和位置，利用闪电低频电场变化探测阵列的定位数据和广东电力等系统的雷电定位数据，结合广州番禺雷达观测资料，确定此事件是由一次含有7 次回击的地闪过程的首次回击造成，其电流峰值强度为-30.9 kA。闪电起始于13.0 km高度的云内，经约600 ms云内发展过程后闪电通道从云体延伸出来，云砧区边缘（0 dBz）到回击点水平距离约300 m，降水区边缘（18 dBz）到回击点水平距离约1.8 km。使用雷击现场等效电路模型，计算旁络闪击空气击穿场强可击穿空气与人头部连接为通路，根据电路分流原理，如果雷电流击中13 m高的大树后流经到“跳点”（树干上方1/4）处后，则有13.2 kA雷电流直接闪击到受害者身上，同时还承受了78.3 kV跨步电压伤害，而距离雷击点10 m远的目击者仅承受1.3 kV跨步电压。（殷启元，范祥鹏，张义军）

5.29 广东省雷电伤亡事故特征分析

通过1995—2016年广东雷电灾害汇编记录的雷灾伤亡事件资料和1999—2016年广东省电力雷电定位系统数据，对广东省雷电伤亡事故特征进行了分析。结果如下：（1）1075起雷灾伤亡事件中，死亡人数约是受伤人数的1.2倍，1起雷灾事件最多伤亡数为17个，2人以上的多人受伤概率要远大于多人死亡；雷灾伤亡高于全省平均水平的农村地区政府更应重视防御工作。（2）每月的雷电灾情特征与闪电活动年变化相关；雷电灾情的时间特征与人们的作息时间相关，尤其对于户外活动较集中的农村地区更应关注雷电频发时段。（3）雷灾伤亡事件发生在农田的最多，造成群死群伤事件主要是发生在未做雷电防护措施的建筑物内和大树旁。（4）雷灾伤亡人员中男女比例相当，但男女同时遭受雷击时女性更易遭受身亡。由于不同年龄段从事不同种类的工作，导致壮年年龄段（25～64岁）男性伤亡高于女性，但是在伤亡最多的31～40岁年龄段中女性伤亡是男性的1.46倍。总之，雷电引起的伤亡事故大部分发生在农村地区，提高农村农民防雷减灾意识和加强乡村振兴防雷减灾服务的工作迫在眉睫。（殷启元，范祥鹏，陈绿文）

5.30 基于光学资料的广州塔附近下行地闪特征

利用2009—2014年广州高建筑物雷电观测站的光学观测资料，结合雷声和电磁场变化波形，对广州塔（高度为600 m）西北部60°扇形区域3 km范围内的119次下行地闪分布特征进行统计分析，结果表明：43.7%（52/119）的地闪发生在区域内4个最高的建筑物上；除了直接击中广州塔的20次地闪（16.8%），距离广州塔附近0～1 km的区域未观测到地闪，观测到的距广州塔最近的地闪离广州塔约1.2 km；距广州塔1～2 km的区域共观测到35次地闪（29.4%），其中每个高度低于300 m的建筑物被击中的次数不超过1次；距离广州塔2～3 km区域共观测到64次地闪（53.8%），其中有些高度低于300 m的建筑物被地闪击中1次以上，最多达5次。广州塔对附近区域下行地闪的吸引作用使其附近1 km左右范围内未观测到地闪，且1～3 km范围内随距离增加下行地闪密度（扣除击中其他高度不低于300 m的建筑物的地闪）有逐渐增加趋势，说明高建筑物对下行地闪的吸引作用随着距离的增加而逐渐减弱。（吴姗姗，吕伟涛，齐奇）

5.31 东亚和西太平洋闪电时空尺度及光辐射能

利用2002—2014年的热带测雨卫星/闪电成像仪TRMM/LIS闪电观测数据分析了18°～36°N和70°～160°E范围内闪电尺度和光辐射能空间分布特征，并选取6个区域（区域1～6），探讨09：00—14：00（地方时，下同）和18：00至次日06：00两个时段闪电上述属性的逐月变化和参数分布特征。研究指出：闪电空间尺度和光辐射能在深海最大，次之为近海和陆地，持续时间在中国东部近海最大，次之为深海和陆地。不同闪电属性大值分布区域差异明显，小值则分布在区域1和区域2。多数区域分析时段内闪电空间尺度和光辐射能的逐月变化趋势较一致，陆地上它们与闪电活动逐月变化的反向对应关系较明显。分析时段内闪电时空尺度和光辐射能均呈对数正态分布，陆地闪电各属性值比海洋闪电更向小值方向集中。在LIS观测性能较高的18：00至次日06：00，各区域内闪电持续时间中值为0.18～0.29 s，通道延展距离中值为12～21 km，光辐射能中值为0.11～0.52 J/(m2·sr·μm）。分析时段内闪电空间尺度与光辐射能的相关性明显优于它们与持续时间的相关性。（尤金，郑栋，姚雯）

6 模式和再分析资料

6 Model and reanalysis data

6.1 Assessment and reduction of the physical parameterization uncertainty for Noah-MP land surface model

The community Noah land surface model with multiparameterization options (Noah-MP) provides a plethora of model configurations with varying complexity for land surface modeling.The practical application of this model requires a basic understanding of the relative abilities of its various parameterization configurations in representing spatiotemporal variability and hydrologic connectivity.We designed an ensemble of 288 experiments from multiparameterization schemes of six physical processes to assess and reduce the structural uncertainty for land surface modeling over 10 hydrologic regions in China for the period 2001−2010.The observed latent heat (LH) was well reproduced by the ensemble.Meanwhile,most experiments underestimated sensible heat (SH) throughout the year and overestimated the cold season but underestimated the warm season terrestrial water storage anomaly (TWSA).The sensitive processes and best-performing schemes varied not only with regions but also among variables.The LH and SH were most sensitive to runoff-groundwater (RUN),surface heat exchange coefficient (SFC),and radiation transfer (RAD).The TWSA was dominated by RUN and RAD while largely influenced by soil moisture factor for stomatal resistance (BTR) and frozen soil permeability (INF) over some limited regions.By contrast,supercooled liquid water (FRZ) had little influence on all variables.Our optimization for individual variables produced high mean Taylor skill scores that ranged from 0.95−0.99 for LH,0.82−0.99 for SH,and 0.63−0.95 for TWSA depending on regions.The simultaneous optimization made trade-off among the three variables,which improved TWSA performance at the cost of reducing the skill for LH and SH over a few regions.(Gan Yanjun,Liang Xinzhong,Duan Qingyun)

6.2 Improving land surface temperature simulation in CoLM over the Tibetan Plateau through fractional vegetation cover derived from a remotely sensed clumping index and model-simulated leaf area index

Parameterizations of fractional vegetation cover (FVC) in land surface models have important effects on simulations of surface energy budget,especially in arid and semiarid regions.This study uses a FVC scheme in which FVC is derived from leaf area index and a remotely sensed clumping index.The performance of the new scheme (SMFVC) is evaluated against Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) and in situ soil temperature observations,together with two other FVC schemes,a general FVC scheme (CTL) based on land cover map and a climatology-based FVC scheme (RSFVC) that uses longterm remotely sensed Normalized Difference Vegetation Index of MODIS.The three FVC schemes were implemented in the Common Land Model (CoLM) and applied in the Tibetan Plateau using the same forcing data and default parameters.Our results demonstrate that FVC schemes have significant influence on the CoLM performance:(1) the RSFVC and SMFVC schemes significantly reduce the LST biases found in CTL,particularly in grassland and during summer; (2) soil temperature evaluation by in situ observations from three networks on the Tibetan Plateau corroborates the LST results; and (3) the improvements are mainly related to representing temporal (seasonal) variability and subgrid heterogeneity of FVC,which improves surface albedo and surface energy balance.In other words,by including more vegetation characteristics,such as using a clumping index,land surface models may better simulate surface vegetation condition and further better represent the land surface energy budget over the Tibetan Plateau.(Li Chengwei,Lu Hui,Leung L Ruby)

6.3 Evaluation of land surface subprocesses and their impacts on model performance with global flux data

Study on the uncertainties in land surface models (LSMs) helps us understand the differences and errors in climate models.Meanwhile,uncertainty in model structure,derived from the many possible parameterization schemes for the same physical subprocess,is a primary source of land model uncertainties.To attribute structural errors and model parameterization scheme uncertainties,it is critical to identify the key subprocesses involved and investigate the interactions of these subprocesses on LSM behavior,which will ultimately help us identify the optimal parameterization schemes for various plant functional types,soil types,and different locations.Here,we conduct physical ensemble simulations for multiple sites from FLUXNET and then apply a variance-based sensitivity analysis method to quantitatively assess the impacts of uncertainties in the parameterization schemes of subprocesses in the Noah with multiparameterization (Noah-MP) LSM on model performance.The results show that three subprocesses—surface exchange coefficient,runoff and groundwater,and surface resistance to evaporation—have the most significant impacts on the performance of the simulated sensible heat flux,latent heat flux,and net absorbed radiation in the Noah-MP LSM.The interaction between two subprocesses could contribute up to 50% of the variation in model performance for some sites,which highlights the need for taking into consideration the interactions of subprocesses to improve LSMs.Finally,a statistical optimal combination of the parameterization schemes is recommended for global land modeling,although it is noticed that the optimal schemes vary with regions and can be different even for neighboring sites.(Li Jianduo,Zhang Guo,Chen Fei)

6.4 An IVAP-based dealiasing method for radar velocity data quality control

Dealiasing is a common procedure in radar radial velocity quality control.Generally,there are two dealiasing steps:a continuity check and a reference check.In this paper,a modified version that uses azimuthal variance of radial velocity is introduced based on the integrating velocity-azimuth process (IVAP) method,referred to as the V-IVAP method.The new method can retrieve the averaged winds within a local area instead of averaged wind within a full range circle by the velocity-azimuth display (VAD) or the modified VAD method.The V-IVAP method is insensitive to the alias of the velocity,and provides a better way to produce reference velocities for a reference check.Instead of a continuity check,we use the IVAP method for a fine reference check because of its high-frequency filtering function.Then a dealiasing procedure with two steps of reference check is developed.The performance of the automatic dealiasing procedure is demonstrated by retrieving the wind field of a tornado.Using the dealiased radar velocities,the retrieved winds reveal a clear mesoscale vortex.A test based on radar network observations has also shown that the two-step dealiasing procedure based on V-IVAP and IVAP methods is reliable.(Liang Xudong,Xie Yanxin,Yin Jinfang)

6.5 Diurnal characteristics of turbulent intermittency in the Taklimakan Desert

A case study is performed to investigate the behavior of turbulent intermittency in the Taklimakan Desert using an intuitive,direct,and adaptive method,the arbitrary-order Hilbert Spectral Analysis (arbitrary-order HSA).Decomposed modes from the vertical wind speed series confirm the dyadic filter-bank essence of the empirical mode decomposition processes.Due to the larger eddies in the CBL,higher energy modes occur during the day.The second-order Hilbert spectra L-2 (omega) delineates the spectral gap separating fine-scale turbulence from large-scale motions.Both the values of kurtosis and the Hilbert-based scaling exponent xi(q) reveal that the turbulence intermittency at night is much stronger than that during the day,and the stronger intermittency is associated with more stable stratification under clear-sky conditions.This study fills the gap in the characteristics of turbulence intermittency in the Taklimakan Desert area using a relatively new method.(Wei Wei,Wang Minzhong,Zhang Hongsheng)

6.6 Generalized Z-grid model for numerical weather prediction

Z-grid finite volume models conserve all-scalar quantities as well as energy and potential enstrophy and yield better dispersion relations for shallow water equations than other finite volume models,such as C-grid and C-D grid models; however,they are more expensive to implement.During each time integration,a Z-grid model must solve Poisson equations to convert its vorticity and divergence to a stream function and velocity potential,respectively.To optimally utilize these conversions,we propose a model in which the stability and possibly accuracy on the sphere are improved by introducing more stencils,such that a generalized Z-grid model can utilize longer time-integration steps and reduce computing time.Further,we analyzed the proposed model’s dispersion relation and compared it to that of the original Z-grid model for a linearly rotating shallow water equation,an important property for numerical models solving primitive equations.The analysis results suggest a means of balancing stability and dispersion.Our numerical results also show that the proposed Z-grid model for a shallow water equation is more stable and efficient than the original Z-grid model,increasing the time steps by more than 1.4 times.(Xie Yuanfu)

6.7 Coupling the common land model to ECHAM5 atmospheric general circulation model

The ECHAM5 model is coupled with the widely used common land model (CoLM).ECHAM5 is a stateof-the-art atmospheric general circulation model incorporated into the integrated weather and climate model of the Chinese Academy of Meteorological Sciences (CAMS-CSM).Land surface schemes in ECHAM5 are simple and do not provide an adequate representation of the vegetation canopy and snow/frozen soil processes.Two AMIP (Atmospheric Model Intercomparison Project)-type experiments using ECHAM5 and ECHAM5-CoLM are run over 30 years and the results are compared with reanalysis and observational data.It is found that the pattern of land surface temperature simulated by ECHAM5-CoLM is significantly improved relative to ECHAM5.Specifically,the cold bias over Eurasia is removed and the root-mean-square error is reduced in most regions.The seasonal variation in the zonal mean land surface temperature and the in situ soil temperature at 20- and 80-cm depths are both better simulated by ECHAM5-CoLM.ECHAM5-CoLM produces a more reasonable spatial pattern in the soil moisture content,whereas ECHAM5 predicts much drier soils.The seasonal cycle of soil moisture content from ECHAM5-CoLM is a better match to the observational data in six specific regions.ECHAM5-CoLM reproduces the observed spatial patterns of both sensible and latent heat fluxes.The strong positive bias in precipitation over land is reduced in ECHAM5-CoLM,especially over the southern Tibetan Plateau and middle-lower reaches of the Yangtze River during the summer monsoon rainy season.(Xin Yufei,Dai Yongjiu,Li Jian)

6.8 Understanding the Asian summer monsoon onset in terms of catastrophe theory

Monsoon has been puzzling that the monsoon occurrence is sudden with the rapid onset observed while the evolution of the differentiating heating is gradual with time of year.Here,focusing on the Asian summer monsoon,we use catastrophe theory to show how gradual changes in the atmospheric heating source with time will cause an abrupt variation of the relevant South Asian High (SAH) to atmospheric circulations and thus lead to the monsoon onset.This argumentation illustrates that the Asian summer monsoon is essentially caused by temporal changes in the differentiating heating owing to surface inhomogeneites such as land-sea contrasts,and the monsoon onset results,in nature,from the nonlinearity of the complex atmospheric system itself.Employing the cusp catastrophe (one of the elementary catastrophes) model,we found that a sudden change like the rapid monsoon onset is an inevitable outcome when the control parameters such as the differentiating heating exceed a threshold though the parameters evolve gradually.(Yin Jinfang,Liu Chongjian)

6.9 The development of an atmospheric aerosol/chemistry-climate model,and simulated effective radiative forcing of nitrate aerosols

This study developed a next-generation atmospheric aerosol/chemistry-climate model,the BCC_AGCM_CUACE2.0.Then,the performance of the model for nitrate was evaluated,and the nitrate direct radiative forcing (DRF) and effective radiative forcing (ERF) due to aerosol-radiation interactions were simulated for the present day (2010),near-term future (2030),and middle-term future (2050) under the Representative Concentration Pathway 4.5,6.0,and 8.5 scenarios relative to the preindustrial era (1850).The model reproduced the distributions and seasonal changes in nitrate loading well,and simulated surface concentrations matched observations in Europe,North America,and China.Current global mean annual loading of nitrates was predicted to increase by 1.50 mg m m−2relative to 1850,with the largest increases occurring in East Asia (9.44 mg m m−2),Europe (4.36 mg m m−2),and South Asia (3.09 mg m−2).The current global mean annual ERF of nitrates was −0.28 W m m−2relative to 1850.Due to global reductions in pollutant emissions,the nitrate ERF values were predicted to decrease to −0.17,−0.20,and −0.24 W m m−2in 2030 and −0.07,−0.18,and −0.19 W m m−2in 2050 for Representative Concentration Pathway 4.5,6.0,and 8.5 relative to 1850,respectively.Although global mean nitrate values showed a declining trend,future nitrate loading remained high in East Asia and South Asia.Nitrate aerosols are effective at scattering solar radiation.Many studies have shown that NOx(NO2and NO) and NH3emissions,the main precursors of nitrates,have continued to increase,resulting in a greater proportion of the total anthropogenic aerosols being composed of nitrates.So,it is very likely that nitrate will become an important climate forcing factor at regional and seasonal scales in the future.Although some studies have investigated the DRF values of nitrate,few have used the optical properties of nitrates in atmospheric chemistry-climate online models.This study developed a next-generation atmospheric aerosol/chemistry-climate model,the BCC_AGCM_CUACE2.0.We evaluated the performance of the model and the nitrate direct radiative forcing and effective radiative forcing due to aerosol-radiation interactions for the present day (2010),near future (2030),and middle future (2050) under different Representative Concentration Pathway scenarios,based on the real nitrate optical properties.Our results are useful for understanding the role of nitrate forcing in future climate change.(An Qi,Zhang Hua,Wang Zhili)

6.10 Half-a-degree matters for reducing and delaying global land exposure to combined daytimenighttime hot extremes

The Paris Agreement has motivated rapid analysis differentiating changes in frequency/intensity of weather and climate extremes in 1.5 versus 2 warmer worlds.However,implications of these global warming levels on locations,spatial scales,and emergence timings of hot spots to extremes are more relevant to policy-making but remain strikingly under-addressed.Based on a bivariate definitional framework,we show that compared to 2 ,the 1.5 target could avoid a transition of prevailing type of summertime hot extremes from daytime-/nighttime-only events to combined daytime-nighttime hot extremes in approximately 18% of global continents and protect 14%−26% of land areas from seeing over threefold-to-tenfold increases in occurrence of combined hot extremes.This half-a-degree reduction also matters for around 21% of global lands,mostly within the tropics,in constraining historically unprecedented combined hot extremes from becoming the new norm within just one to three decades ahead.By contrast,previous analyses based on univariate-defined hot days substantially underestimate the magnitude,areal extent,and emergence rate of 0.5 -caused aggravation of summertime hot extremes.These projected changes of bivariate-classified hot extremes,and therefore,underline not only the imperative but also the urgency of striving for the lower Paris target.Since the milestone Paris Agreement,policymakers are eager to know the extent to which and the time when their own regions will suffer from consequences associated with pertinent global warming levels (1.5 and 2 ).We show that even the aspirational 1.5 target would still be translated into huge threats from summertime hot extremes to the tropics,as manifested by a type transition toward health-damaging daytimenighttime combined heat and threefold-to-tenfold increases in its occurrence.Further rise to 2 would put an extra 20% of global continents at risk from similar transitions and increases.Unexpectedly,despite involving substantial reduction in carbon emissions,the 2 -compatible development pathway would bring about a rapidly emerging novel era of hot extremes for most tropical countries,where historically unprecedented daytime-nighttime combined heat will be registered every other year within the next one to three decades and thereafter.Our results highlight that the extra 0.5 of global warming,from 1.5 to 2 ,would impose the earliest and severest heat-related consequences on the least-developed regions,whose adaptive capacity is unfortunately very limited.(Chen Yang,Zhou Baiquan,Zhai Panmao)

6.11 A modeling study of the effects of vertical wind shear on the raindrop size distribution in Typhoon Nida (2016)

In this study,the effects of vertical wind shear (VWS) on the raindrop size distribution in tropical cyclone are investigated,based on the theoretical analyses that intense VWS,which commonly appears in the lower layers of tropical cyclones,can enhance the collisional breakup of raindrops.This is achieved by comparing the numerical sensitivity experiments of Typhoon Nida (2016) using Weather Research and Forecasting model against the polarimetric radar and disdrometer observations.In the control run with the default Morrison microphysics,unrealistic large raindrops are produced with excessively large differential reflectivity and heavier precipitation.An obvious decrease of raindrop size is present when the constant value of cloud droplet number concentration reduces from 250 to 30 (NC30),leading to more comparable microphysics characteristics with the observations due to the enhancement of autoconversion rate from cloud droplets to raindrops; however,some unrealistic large-sized raindrops still exist.A semiempirical raindrop collection/breakup parameterization is further proposed in NC30_WS run by modifying the threshold diameter of raindrops at which breakup occurs as a function of VWS.The certain improvements of simulating raindrop size distribution and precipitation in Typhoon Nida are present in NC30_WS run,owing to the efficient collisional breakup and the induced stronger raindrop evaporation cooling.Our results indicate that the combined effects of reasonable cloud droplet numbers as well as reliable raindrop breakup parameterization are pronounced and highlight the impacts of VWS on raindrop size distribution,which should be involved in current microphysical schemes to forecast the TCs more accurately.(Deng Lin,Gao Wenhua,Duan Yihong)

6.12 Using multisource satellite data to assess recent snow-cover variability and uncertainty in the Qinghai-Tibet Plateau

Snow cover in the Qinghai-Tibet Plateau (QTP) is a critical component in the water cycle and regional climate of East Asia.Fractional snow cover (FSC) derived from five satellite sources [the three satellites comprising the multisensor synergy of FengYun-3 (FY-3A/B/C),the Moderate Resolution Imaging Spectroradiometer (MODIS),and the Interactive Multisensor Snow and Ice Mapping System (IMS)] were intercompared over the QTP to examine uncertainties in mean snow cover and interannual variability over the last decade.A four-step cloud removal procedure was developed for MODIS and FY-3 data,which effectively reduced the cloud percentage from about 40% to 2%–3% with an error of about 2% estimated by a random sampling method.Compared to in situ snow-depth observations,the cloud-removed FY-3B data have an annual classification accuracy of about 94% for both 0.04 degrees and 0.01 degrees resolutions,which is higher than other datasets and is recommended for use in QTP studies.Among the five datasets analyzed,IMS has the largest snow extent (22% higher than MODIS) and the highest FSC (4.7% higher than MODIS),while the morning-overpass MODIS and FY-3A/C FSC are similar and are around 5% higher than the afternoonoverpass FY-3B FSC.Contrary to MODIS,IMS shows increasing variability in snow cover and snow duration over the last decade (2006–2017).Differences in variabilities of FSC and snow duration between products are greater at 5–6 km than lower elevations,with seasonal snow-cover change showing the largest uncertainty in snowmelt date.(Jiang Yingsha,Chen Fei,Gao Yanhong)

6.13 Pronounced extended duration of tropical cyclone quiescent periods over the western North Pacific in the super El Niño decaying years

This study investigated the relationship between the duration of the western North Pacific (WNP) tropical cyclone (TC) quiescent periods (no TC in the period) and the super El Niño events during 1951–2016.The results show that the average duration of the TC quiescent period (DTCQP) is shorter (90 days) during super El Niño developing (EN) years,compared with the 1951–2016 mean DTCQP (105 days).However,in the four super El Niño decaying (EN+1) years,the DTCQP is about 200 days,which is much longer than the 1951–2016 mean DTCQP.Further study suggests that the delayed super El Niño impacts on the DTCQP in the year following the El Niño peak is due to the persistence of the WNP anomalous anticyclone (WNPAC),which is enhanced during the mature phase of El Niño but maintained through the subsequent spring and summer.The anomalous development and persistence of the WNPAC significantly influences the monsoon trough activity,which leads to delayed and weak development of the monsoon trough in the summer of super EN+1 years.From April to June of the super EN+1 years,there is almost no monsoon trough activity over the WNP and the large-scale environmental conditions are unfavourable for TC genesis.In contrast,from April to June in the super EN years,the WNP is dominated by anomalous cyclonic circulation,which leads to early and strong development of the monsoon trough.The intensified and eastwards extended monsoon trough seems to coincide with favourable environmental conditions for TC genesis.(Wang Qian,Zhai Panmao)

6.14 Observational analyses of topographic effects on convective systems in an extreme rainfall event in North China

An extreme rainfall event occurred in North China between 18 and 21 July 2016,with the strongest precipitation of 783.4 mm occurring within the rainfall zone of Taihang Mountain.In this study,mesoscale convective systems (MCSs) resulting in heavy rainfall are studied using high-resolution surface observations,soundings and radar data.The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall.The synoptic low-pressure environmental circulation promoted mesocyclone and convection development in North China,where a strong and moist low-level jet was observed.The interaction between mesocyclone and mesoscale topography produced two strong,separate precipitation centers,which were clearly the direct results of three MCSs.Orographic blocking played a crucial role in the development of MCSs.Diabatic cooling due to raindrop evaporation in association with MCS1 formed a cold pool on the plain east of the central Taihang Mountain area.Convective cell initiation and development in MCS2 was due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin.Convective cells that were successively generated and shifted along a mesoscale shear line comprised the MCS2 linear distribution and a northwestward rain belt.The northern extreme rainfall center was caused by MCS3 along the southern slopes of an eastward-opening valley.Convective cells were triggered along a shear line,which was affected by a combination of topography,a cold pool in the boundary layer and mesocyclone outer flow.Together,the orographic forcing and cold pool enhanced the Taihang Mountain precipitation event.(Kang Yanzhen,Peng Xindong,Wang Shigong)

6.15 Changes in concentrations of fine and coarse particles under the CO2-induced global warming

Using an aerosol-climate coupled model,we have investigated the climate response to increase in atmospheric carbon dioxide (CO2) levels and the resulting changes in the column concentrations of anthropogenic and natural particulate matters.Special attention has been paid on the fine particulate matters (PM2.5,particulate matters with aerodynamic diameters of 2.5 µm) and coarse particles of non-PM2.5(NPM,particulate matters with aerodynamic diameters of 2.5 µm).Rising CO2levels led to significant effects on surface air temperature,large scale circulation,surface water flux,precipitation,etc.,which considerably affect the column concentrations of anthropogenic and natural particulate matters,for both fine and coarse modes.Changes in burden of fine particles are major contributions to that of total anthropogenic particulate matters by CO2-induced global warming.The anthropogenic PM2.5burden was found to decrease in the regions with latitudes 30°N,whereas increase in the rest areas.The geographical distributions of changes in the column concentrations of fine and coarse natural particles were found in the same order of magnitude in most regions,and the particle size distribution shifted to the coarse mode.The physical mechanisms are analyzed to explain the changes in aerosol local column concentrations due to CO2-induced climate change.(Yang Dongdong,Zhang Hua,Li Jiangnan)

6.16 云微物理过程对台风数值模拟的影响

将中国气象科学研究院（CAMS）混合双参数云微物理方案用于中尺度天气模式WRF，开展了对2013年超强台风“天兔”（1319）的模拟，通过与台风最佳路径、强度及热带降雨测量卫星（TRMM）资料对比，分析CAMS云微物理方案在模拟台风中的适用性及云微物理过程对模拟台风天兔的影响机制。设计了3组敏感性试验：修改雪粒子质量和落速系数（EXP1），采用海洋性云滴参数（EXP2），同时修改雪粒子质量和落速系数并采用海洋性云滴参数（EXP3）。结果表明：EXP1和EXP3由于霰碰并雪速率的增加及减小的雪下落通量，导致雪含量显著降低，同时也减少了整体冰相物的含量；EXP2和EXP3模拟的台风眼区对流有效位能快速减小，再现了前期台风的快速增强过程，路径偏差也最小；各试验模拟的小时降水率总体偏强，EXP3的降水空间分布与实况更接近，明显降低雪粒子含量，并一定程度上改善模拟的台风路径、强度及降水分布等。该结果不但可为改进适用于台风的云微物理参数化方案提供思路，也可加深云微物理过程对台风影响的认识。（常婉婷，高文华，端义宏）

6.17 阴阳网格上通量型非静力模式动力框架的理想试验

为改善球面经纬度网格在高分辨率应用的苛刻限制，提高全球大气动力模式的时间积分效率，选取以阴阳网格为基础构建通量型非静力大气模式动力框架，采用有限体积法和通量型平流显式算法积分方案，保证模式的守恒计算性能。该动力框架在标准三维大气理想试验中进行了中期积分试验，对动力框架的计算效果、性能进行检验。在三维平衡流试验、罗斯贝-豪威茨波试验和山脉罗斯贝波试验中均表现出很好的稳定性和三维计算效果，其中水平2.5°分辨率模式的平衡流垂直速度误差为10-5量级，而经向速度误差在10-2量级，罗斯贝-豪威茨波保持基本波形稳定传播，而地形罗斯贝波试验则给出背风坡激发低槽在发展过程中不断向下游和南半球传播。（黄超，彭新东，李晓涵）

6.18 ECMWF高分辨率模式降水预报能力评估与误差分析

利用2015—2017年6—8月ECMWF高分辨率模式（ECMWF-Hi）的加工产品，结合我国2400多个国家气象站逐小时降水观测资料，对ECMWF-Hi产品24 h降水预报的准确度、集中度和相关性进行了评估，并与ECMWF集合预报模式（ECMWF-EPS）24 h降水预报产品进行了比较。为更好地描述预报的集中度，避免单纯用标准差比或平均值比刻画预报集中度的缺陷，建立了一个综合标准差和平均值的R指数，用之定量描述模式预报的集中度。结果表明：（1）ECMWF-Hi在均方根误差的检验方面并未表现出优势；而分辨率较低的ECMWF-EPS集合平均预报误差最小。（2）ECMWF-Hi对研究区降水预报的集中度的整体描述较为准确，离散度与观测较为相似，预报期望也与观测降水的期望最接近，ECMWF-Hi比ECMWF-EPS的集合控制预报与集合平均对观测降水集中度的刻画较为准确。（3）研究区域内各站点R指数分布表明，ECMWF-Hi与ECMWF-EPS控制预报、平均预报相比，对平均值预报不足的站点较多，且这些站点的预报集中度普遍大于观测，ECMWF-Hi的降水预报更接近观测降水。（4）评估应用结果表明，R指数不仅能定性评估模式的集中度，也可定量描述集中度大小。（曹越，赵琳娜）

7 卫星研究与应用

7 Satellite research and application

7.1 快速辐射传输模式的研发及应用

为了加速风云卫星数据在数值天气预报模式中的应用，中国气象科学研究院灾害天气国家重点实验室卫星团队自主研发了先进辐射传输模拟系统（ARMS：Advanced Radiative transfer Modeling Syste），作为新一代观测算子。ARMS在美国快速辐射传输模式（CRTM：Community Radiative Transfer Model）和欧洲快速辐射传输模式（RTTOV：Radiative Transfer for the TIROS Operational Vertical Sounder TOVS）的基础上优化组合，融入了重要科学进展。

经过一年多的研发，ARMS已初步完成几个关键模块的研发，包括针对风云卫星传感器的大气透过率模块、气溶胶/云和降水的散射/吸收模块、地表发射率模块、矢量辐射传输方程求解模块，以及切线性/伴随模式。不同于CRTM和RTTOV，ARMS使用极化二流近似和矢量离散坐标法作为辐射传输模式的核心求解方案，在散射和发射大气下求解斯托克斯辐射分量。ARMS针对国产气象卫星仪器的光谱响应函数，设计了精确的快速大气透射率计算方案，建立了完整的大气光谱学数据集以用于快速吸收系数的生成。此外，ARMS发展和建立了完整的气溶胶、云粒子散射数据库，实现在全天候条件下，红外及微波大气探测仪的快速高精度的辐射传输计算，并扩展到可见光及紫外波段以实现更多仪器的应用需求。同时，ARMS完善了海洋、陆面发射率理论模型和数据集，并着重发展红外陆面及海冰红外和微波发射率理论模型和数据集以增强“三极”地区复杂地表状况下的应用能力。

2019年4月29日至5月2日，由中国气象科学研究院（灾害天气国家重点实验室/卫星研究与应用联合中心）、欧洲中期天气预报中心（ECMWF）和美国卫星数据同化中心（JCSDA）联合发起的“2019卫星数据同化快速辐射传输模式国际研讨会”在天津顺利召开，本次研讨会有来自美国、英国、德国、日本等国家和地区以及国内多家高校和科研院所的100余名专家出席。本次研讨会通过回顾当前国际前沿快速辐射传输模型的能力，了解卫星数据同化辐射传输模型的新要求，并考虑优先发展大气和地表辐射传输过程中超快速计算能力，将进一步为发展我国快速辐射传输模式提供参考。本次卫星数据同化快速辐射传输模式国际研讨会的顺利召开，体现了中国气象局在卫星数据同化快速辐射传输模式研究中的主动性，推进了国际合作、提升了我国在辐射传输领域的国际影响力。中国快速辐射传输模式ARMS已与美国CRTM和欧洲RTTOV模式形成三足鼎立，共同成为支撑卫星资料数据同化及产品研发和应用的核心技术。

ARMS的研发和应用，将为我国风云卫星发展提供关键理论技术，并为中国气象局数值预报系统提供重要技术支撑。在2019年GRAPES数值预报系统发展专项任务的支持下，卫星团队已在GRAPESMESO框架下，实现快速辐射传输模式ARMS和RTTOV的可选调用并便捷比较。下一步，在对每种仪器类别的ARMS模拟不确定性进行充分表征之后，将对偏差校正方案进行完善，以期在四维变分全球/区域同化预报系统（GRAPES-4DVar）中集成同化更多的微波和红外卫星观测数据。（翁富忠，杨俊，董佩明）

7.2 Verification of Fengyun-3D MWTS and MWHS calibration accuracy using GPS radio occultation data

The newly launched Fengyun-3D (FY-3D) satellite carries microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS),providing the global atmospheric temperature and humidity measurements.It is important to assess the in orbit performance of MWTS and MWHS and understand their calibration accuracy before using them in numerical weather prediction and many other applications such as hurricane monitoring.This study aims at quantifying the biases of MWTS and MWHS observations relative to the simulations from the collocated Global Positioning System (GPS) radio occultation (RO) data.Using the collocated FY-3C Global Navigation Satellite System Occultation Sounder (GNOS) RO data under clearsky conditions as inputs to Community Radiative Transfer Model (CRTM),brightness temperatures and viewing angles are simulated for the upper level sounding channels of MWTS and MWHS.In order to obtain O-B statistics under clear sky conditions,a cloud detection algorithm is developed by using the two MWTS channels with frequencies at 50.3 and 51.76 GHz and the two MWHS channels with frequencies centered at 89 and 150 GHz.The analysis shows that for the upper air sounding channels,the mean biases of the MWTS observations relative to the GPS RO simulations are negative for channels 5−9,with absolute values＜1 K,and positive for channels 4 and 10,with values＜0.5 K.For the MWHS observations,the mean biases in brightness temperature are negative for channels 2−6,with absolute values ＜2.6 K and relatively small standard deviations.The mean biases are also negative for channels 11−13,with absolute values＜1.3 K,but with relatively large standard deviations.The biases of both MWTS and MWHS show scan-angle dependence and are asymmetrical across the scan line.The biases for the upper air MWTS and MWHS sounding channels are larger than those previously derived for the Advanced Technology Microwave Sounder.(Hou Xueyan,Han Yang,Hu Xiuqing)

7.3 Influences of physical processes and parameters on simulations of TOA radiance at UV wavelengths:Implications for satellite UV instrument validation

Numerous factors can influence the radiative transfer simulation of hyper-spectral ultraviolet satellite observation,including the radiative transfer scheme,gaseous absorption coefficients,Rayleigh scattering scheme,surface reflectance,aerosol scattering,band center wavelength shifts of sensor,and accuracy of input profiles.In this study,a Unified Linearized Vector Radiative Transfer Model (UNL-VRTM) is used to understand the influences of various factors on the top of atmosphere (TOA) normalized radiance in the ultraviolet (UV) region.A benchmark test for Rayleigh scattering is first performed to verify the UNL-VRTM accuracy,showing that the model performances agree well with earlier peer-reviewed results.Sensitivity experiments show that a scalar radiative transfer approximation considering only ozone and a constant surface reflectance within the UV region may cause significant errors to the TOA normalized radiance.A comparison of the Ozone Mapping and Profiler Suite (OMPS) radiances between simulations and observations shows that the surface reflectance strongly influences the accuracy for the wavelengths larger than 340 nm.Thus,using the surface reflectivity at 331 nm as a proxy for simulating the whole OMPS hyperspectral ultraviolet radiances is problematic.The impact of rotational Raman scattering on TOA radiance can be simulated through using SCIATRAN,which can also reduce the difference between measurements and simulations to some extent.Overall,the differences between OMPS simulations and observations can be less than 3% for the entire wavelengths.The bias is nearly constant across the cross-track direction.(Ding Shouguo,Weng Fuzhong)

7.4 Hourly PM2.5 estimates from a geostationary satellite based on an ensemble learning algorithm and their spatiotemporal patterns over central East China

Satellite-derived aerosol optical depths (AODs) have been widely used to estimate surface fine particulate matter (PM2.5) concentrations over areas that do not have PM2.5monitoring sites.To date,most studies have focused on estimating daily PM2.5concentrations using polar-orbiting satellite data (e.g.,from the Moderate Resolution Imaging Spectroradiometer),which are inadequate for understanding the evolution of PM2.5distributions.This study estimates hourly PM2.5concentrations from Himawari AOD and meteorological parameters using an ensemble learning model.We analyzed the spatial agglomeration patterns of the estimated PM2.5concentrations over central East China.The estimated PM2.5concentrations agree well with groundbased data with an overall cross-validated coefficient of determination of 0.86 and a root-mean-square error of 17.3 µg m−3.Satellite-estimated PM2.5concentrations over central East China display a north-to-south decreasing gradient with the highest concentration in winter and the lowest concentration in summer.Diurnally,concentrations are higher in the morning and lower in the afternoon.PM2.5concentrations exhibit a significant spatial agglomeration effect in central East China.The errors in AOD do not necessarily affect the retrieval accuracy of PM2.5proportionally,especially if the error is systematic.High-frequency spatiotemporal PM2.5variations can improve our understanding of the formation and transportation processes of regional pollution episodes.(Liu Jianjun,Weng Fuzhong,Li Zhanqing)

7.5 Satellite-based PM2.5 estimation directly from reflectance at the top of the atmosphere using a machine learning algorithm

Atmospheric particulate matter (PM) that have particle diameter less than 2.5 µm PM2.5are hazardous to public health whose concentration has been either measured on the ground or inferred from satellite-retrieved aerosol optical depth (AOD).The latter is subject to numerous sources of errors,making the satellite retrievals of PM2.5highly uncertain.This study developed an ensemble machine-learning (ML) algorithm for estimating PM2.5concentration directly from Advanced Himawari Imager satellite measured top-of-the-atmosphere (TOA) reflectances in 2016 integrated with meteorological parameters.The algorithm is demonstrated to perform well across China with high accuracies at different temporal scales.The model has an overall cross-validation coefficient of determination (R2) of 0.86 and a root-mean-square error (RMSE) of 17.3 µg m−3for hourly PM2.5concentration estimation.Such accuracies of the estimation on PM2.5concentration by using TOA reflectance directly are comparable with those of the common methods on estimating PM2.5concentration by using satellite-derived AODs,but the former has a relatively stronger predictive power relating to spatial-temporal coverages than the latter.Annual and seasonal variations of PM2.5concentration over the three major developed regions in China are estimated using the model and analyzed.The relatively stronger predictive ability of developed model in this study may help provide information about the diurnal cycle of PM2.5concentrations as well as aid in monitoring the processes of regional pollution episodes and the evolution of PM2.5concentration.(Liu Jianjun,Weng Fuzhong,Li Zhanqing)

7.6 基于多时相环境卫星的冬前油菜种植面积估算

油菜是我国重要的油料作物之一，监测油菜种植面积有助于了解油菜生长状况，为油菜病虫害、湿渍害、冻害等灾害损失评估提供数据。文章以湖北省荆州市江陵县为研究区，使用国产HJ-1A/B 30 m分辨率时序多光谱数据，通过地面调查及资料分析确定油菜与其他易混淆作物的主要NDVI时序特征，建立油菜识别决策树，估算了2009—2015年（不包括2011—2012年生长季）冬前油菜种植面积。将基于油菜开花期影像的最大似然法提取的油菜面积作为定性验证数据。以油菜籽面积统计数据和Google Earth高分辨率影像数据对冬前油菜提取的面积和空间位置结果进行定量评价。定性评价结果：2009—2011年生长季的决策树方法提取冬前油菜面积结果与开花期影像最大似然法提取结果基本一致，2012—2015年生长季的油菜提取面积空间分布差异较大。定量评价结果：决策树方法提取冬前油菜面积的用户精度达到80.40%～95.56%，生产者精度达到82.56%～91.43%，相对误差低于15%。基于NDVI时间序列特征的决策树算法估算冬前油菜面积具有可行性，但仍受到云和冬小麦的影响。（魏传文，黄敬峰，杨玲波）

8 软件及大气科学试验与研究平台

8 Software and atmospheric science experiment and research platform

8.1 An open source software suite for multi-dimensional meteorological data computation and visualisation

MeteoInfo Java software tools were developed for multi-dimensional meteorological data analysis and visualisation by integrating a Geographic Information System (GIS) and Scientific Computation Environment (SCE).Included are a Java class library for software developing,a GIS desktop application for spatial data operation and interactive multi-dimensional geoscientific data exploration,and a scientific computation and visualisation environment with Jython scripting.The popular geoscience data formats,such as NetCDF,HDF and GRIB,are supported based on a Unidata NetCDF Java library; also,its multi-dimensional array data model is used for scientific computation.In this paper,the software design framework and its implementation are presented.Furthermore,the software application capabilities are illustrated using several examples.(Wang Yaqiang)

8.2 大气科学试验与研究平台（ASERP2.0）

大气科学试验与研究平台是气科院基本科研业务费资助建立的，经过3年的研发和建设，完成了预期工作目标。

（1）研制了大气科学试验数据管理规范“大气科学试验多源数据分类与编码”和“大气科学试验数据集说明文档格式规范”，开发了“科学试验数据文件名生成系统”和“科学试验数据说明文档填报系统”，在大气科学试验数据的规范化管理中实际应用。

（2）大气科学试验数据和数据产品的收集和规范化处理。包括3次青藏高原大气科学试验、JICA青藏高原大气科学试验、华南季风/台风强降水观测试验以及外场试验期间试验区相关业务数据等，整合生成大气科学试验规范化数据集。

（3）开发了大气科学试验场景展示系统。

（4）建立了大气科学试验与研究综合共享平台。

（5）开展了华南暴雨试验强降水个例观测数据的质量控制及数值同化分析。

大气科学试验与研究平台提供了5个重大科学试验项目的规范化数据集、58个科研常用基础数据集、多种实时数据及图像产品的在线共享，以及数据下载服务。至2019年底平台注册用户155人，全年数据下载量达37.8 TB，日均使用110 GB，用户满意率为93.76%。（高梅）