气候与气候变化

气候与气候变化研究进展

Progress in Research on Climate and Climate Change

1 次季节至季节变化特征与机理

1 Features and mechanism of the subseasonal to seasonal variability

1.1 Diversity of the coupling wheels in the East Asian summer monsoon on the interannual time scale: Challenge of summer rainfall forecasting in China

Two types of three-dimensional circulation of the East Asian summer monsoon (EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979‒2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high (SAH)anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upperlevel SAH via the meridional flow over Southeast Asia.These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years,respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel’s interannual variability, illustrating the challenges in forecasting summer rainfall over China. (Zhu Congwen, Liu Boqi, Xu Kang, Jiang Ning, Liu Kai)

1.2 Seasonal evolution of anomalous rainband over East China regulated by sea surface temperature anomalies in the Northern Hemisphere

A seasonal evolution of rainbands over East China is evident and shows remarkable year-to-year variations. The present study identifies two dominant interannual modes of the seasonal evolution of rainbands over East China from 1981 to 2018: 1) the sudden change pattern, in which the anomalous rainfall changes abruptly from boreal spring to summer, especially over South China; and 2) the northward migration pattern,which shows a gradual poleward migration of the anomalous rainband over East China with the East Asian summer monsoon (EASM). Both of them are regulated by the sea surface temperature anomalies (SSTAs)in the Northern Hemisphere from spring to summer. In the sudden change pattern, the SSTAs in the Pacific modulate spring rainfall over South China via the ENSO-EASM teleconnection. By contrast, the North Atlantic SSTAs change the midlatitude wave train and modify summer rainfall over South and North China, in conjunction with the anomalous tropical circulation due to the Indian Ocean SSTAs. In the northward migration pattern, the North Pacific SSTAs alter spring rainfall over South China by varying the low-level western North Pacific subtropical high and the zonal land-sea thermal contrast over East Asia. Afterward, the ENSO-like SSTAs induce a Pacific-Japan teleconnection and shift the anomalous rainband northward to the Yangtze-Huai River and North China in summer. The seasonal switch of the SSTAs regulating these two modes is physically linked from boreal spring to summer. This mechanism provides potential seasonal predictability of the seasonal evolution of the anomalous rainband over East China. (Liu Boqi, Zhu Congwen, Jiang Ning, Guo Li)

1.3 Subseasonal-to-seasonal predictability of onset dates of South China sea summer monsoon: A perspective of meridional temperature gradient

The onset of the South China Sea summer monsoon (SCSSM) has traditionally been ascribed to El Niñosouthern oscillation (ENSO) on an interannual time scale, but the two do not correspond in some years. The present study applies harmonic analysis on the meridional temperature gradient (MTG) in the mid-upper troposphere over the South China Sea (SCS) and decomposes the onset process to be a slow-varying seasonal cycle and transient subseasonal component. The ENSO-related air temperature anomaly in the southern SCS provides seasonal predictability of SCSSM onset by a stable and robust relationship between ENSO and MTG seasonal cycle. However, in the northern SCS, the MTG is regulated by an intraseasonal oscillation (ISO) of extratropical air temperature with a significant 10‒30-day period. This ISO originates over the western Tibetan Plateau (TP) and then propagates eastward and gets enhanced by anomalous diabatic heating due to spring rainfall anomaly over South China as a result of subseasonal thermal forcing of TP. When the ISO arrives to the north of the SCS, it directly changes the tropospheric temperature to modulate the MTG. Meanwhile, the upper-level circulation associated with the ISO alters themeridional potential vorticity advection and pumping effect, followed by the anomalous low-level westerly wind andmonsoon convection over the SCS. The SCSSM onset is evidently disrupted from its seasonal cycle when this ISO is more active. Since the independence of its intensity from ENSO, this extratropical ISO over TP and South China provides additional subseasonal predictability of the onset dates of the SCSSM. (Liu Boqi, Zhu Congwen)

1.4 Regulation of the subseasonal variability of winter rainfall in South China by the diversity of El Niño-southern oscillation

Winter precipitation over South China tends to increase with enhancement of the 10–30-day intraseasonal oscillation (ISO) during El Niño-southern oscillation (ENSO) events from 1981 to 2017. This study shows that,in contrast with central Pacific El Niño and La Niña events, the 10‒30-day ISO of rainfall particularly intensify during eastern Pacifc (EP) El Niño events. The seasonal evolution of the anomalous circulation, indicated as the annual cycle (AC), bridges the slow-varying ENSO and the transient 10–30-day ISO of winter rainfall over South China. As to the AC component, the EP El Niño events not only enhance the Philippine Sea anticyclone to provide a wetter low-level background over South China, but also develop the mid-latitude cyclone and westerly winds in the upper-level over East Asia by changing tropical convection. The moisture over South China further increases due to the low-level wind convergence on a 10–30-day timescale. In the upper troposphere, the AC component of the anomalous westerly redistributes the 10–30-day relative vorticity by modulating the subseasonal mid-latitude wave train to strengthen the positive vorticity advection and ascending motion over South China. Their collaboration amplifes the subseasonal variance of winter precipitation in situ.By contrast, the AC component of the upper level circulation at mid-latitudes is not well organized in other ENSO subsets, corresponding to their lack of infuence on the ISO variability of rainfall over South China. (Guo Li, Zhu Congwen, Liu Boqi)

1.5 Combined impacts of sea surface temperature in tropical Pacific and North Atlantic oceans on the winter rainfall in southern China under decadal background

The sea surface temperature anomalies (SSTAs) in the tropical Pacific (TP) and North Atlantic (NA)are both important factors regulating the winter rainfall over the southern China (SC). Our results show the combined impact of SSTAs exhibitd an offset effect before 1985, after a decadal transition period, the two SSTAs patterns formed an additive effect on the interannual timescale after 1995. These distinct impacts of the TP and NA result in the southward shift of the winter rainfall variability, which is associated with the decadal change of the TP and NA. Both the background modes in TP and NA show a phase transition during the period 1985–1995, accompanied by the interannual variations of the SSTAs in two ocean basins before 1985 and after 1995. Before 1985, the canonical El Niño-related southerly winds of the anticyclone over the western North Pacific transport sufficient water vapour and enhance rainfall in SC. While the concurrent dipole NA SSTArelated wave train propagates to Eurasia, causing a cyclone and divergence of water vapour fluxes suppressing the local rainfall. After 1995, the El Niño-related warm SSTAs move westward and strengthen the rainfall anomalies in the Yangtze River basin. Meanwhile, a tripole mode of SSTAs replaces the dipole counterpart in NA, resulting in the anomalous cyclonic circulation over South Asia with more rainfall in the south edge of SC.The decadal changes of the TP and NA mode combination could affect the seasonal distribution and prediction of China winter rainfall. (Yu Minjie, Jiang Ning, Zhu Congwen, Su Jingzhi)

1.6 Atmospheric circulation regime causing winter temperature whiplash events in North China

Temperature whiplash events, which are characterized by a rapid transition between persistent and extreme warm and cold conditions, usually damage natural systems and human communities and can even have catastrophic impacts. To more deeply understand the severe winter temperature whiplash events in North China, in this study, the atmospheric circulation regime that is primarily responsible for such events is investigated based on Japanese 55-year reanalysis data. The results show that the drastic temperature shifts during winter temperature whiplash events are closely correlated with the southeastward propagation of the dipolar anomalous temperature in the middle and lower troposphere over the Eurasian continent. The dipolar structure of the temperature signals features an initial cold anomaly over the West Siberian Plain and a warm anomaly over East Asia during negative (extremely warm to extremely cold) events and the opposite pattern during positive (extremely cold to extremely warm) events. This dipolar temperature anomaly is tightly coupled with the southeastward development of an upper-level wave-like anomalous circulation pattern over the Eurasian continent, which is associated with a Rossby wave originating near the Kara Sea during negative events and in the northern Europe during positive events. Based on the calculation of the temperature budget,the drastic temperature drop during negative events is mainly dominated by anomalous meridional temperature advection, and adiabatic heating due to vertical motion is negligible. In contrast, adiabatic heating due to sinking plays a dominant role in the drastic temperature increase during positive events, while anomalous meridional temperature advection also makes a substantial contribution to the temperature increase. (Ma Shuangmei, Zhu Congwen)

1.7 Subseasonal forecast barrier of the North Atlantic oscillation in S2S models during the extreme Meiyu rainfall event in 2020

Enhanced predictability of high-impact weather events is a Subseasonal to Seasonal (S2S) Prediction Project priority. In early summer 2020, a record-breaking heavy rainfall event occurred over the Yangtze River valley during the Meiyu season (June and July). Here we evaluate the S2S model forecast performance concerning the summer 2020 extreme Meiyu event over the Yangtze River valley. Our results show all operational S2S models exhibit fluctuating high-low-high forecast skill patterns during this three-stage Meiyu event, determined by different dominant circulations. In particular, the poor midlatitude circulation forecast(low trough over the northern China) in the North Atlantic oscillation (NAO) negative phase, with relative more contribution than that of the low-latitude circulation (western North Pacific subtropical high and South Asian high), decreases the model forecast skill for Meiyu rainfall in early-mid July. The prediction barrier of the NAO pattern and its downstream response, via the eastward-propagating wave train, jointly decrease the rainfall forecast skill during the NAO active negative period. Therefore, the skillful model prediction of the active summer NAO could help predict mei-yu events in the East Asian summer monsoon, suggesting further model improvements to the S2S forecasting of summertime mid-latitude variations are urgently needed. (Yan Yuhan, Liu Boqi, Zhu Congwen, Lu Riyu, Jiang Ning, Ma Shuangmei)

1.8 Subseasonal predictability of South China Sea summer monsoon onset with the ECMWF S2S forecasting system

Accurate forecasting of the South China Sea summer monsoon (SCSSM) onset is critical for water resource management in the rainy season across East Asia. The present work evaluates the skill of the ECMWF subseasonal-to-seasonal (S2S) forecasting system in predicting SCSSM onset dates using a hindcast experiment. The model provides accurate predictions of the SCSSM onset date 10 days in advance. The predictability mainly arises from the sea surface temperature anomaly (SSTA) in the northern Indian Ocean and central-eastern tropical Pacific, which usually coincides with moderate and extreme decaying El Niño,thus creating windows of opportunity for SCSSM onset forecasting. The model can reasonably reproduce the SCSSM-related SSTA and its induced atmospheric teleconnections. However, when anomalous Ural blocking influences the mid-high latitudes, the ECMWF model forecasts of the SCSSM onset are trapped in the synoptic timescale because of the limited ability of the model to consider blocking in forecasting. (Yan Yuhan, Liu Boqi,Zhu Congwen)

1.9 Annual cycle of East Asian precipitation simulated by CMIP6 models

Annual cycle is fundamental in the East Asian monsoon (EAM) systems, profoundly governing the spatiotemporal distribution of the East Asian rainfall. The present study identified the dominant modes of the annual cycle in the East Asian rainfall based on the fourier harmonic analysis and the empirical orthogonal function (EOF) decomposition. We evaluated the performance of the first two leading modes (i.e.,EOF-1 and EOF-2) in historical experiments (1979–2014) of the 21 released climate models of the Coupled Model Intercomparison Project phase 6 (CMIP6). Comparing with the observation, although the CMIP6 models yield the essential fidelity, they still show considerable systematic biases in the amplitude and phase of the annual cycle, especially in East and South China. Most models exhibit substantial phase delays in the EOF-2 mode of the annual cycle. Some specific models (BCC-ESM1, CanESM5, and GFDL-CM4) exhibiting better performance could capture the observed annual cycle and the underlying physics in climatology and interannual variability. The limited fidelity of the EOF-2 mode of the EAM annual cycle primarily hinders the monsoon variability simulation and thus the reliable future projection. Therefore, the dominant modes of the EAM annual cycle act as the evaluate benchmark in the EAM modelling framework. Their improvement could be one possible bias correction strategy for decreasing the uncertainty in the CMIP6 simulation of the EAM.(Yan Yuhan, Zhu Congwen, Liu Boqi, Jiang Song)

1.10 Relations between the quasi-biweekly oscillation over the East Asian monsoon region and the East Asian tropical monsoon depressions

An empirical orthogonal function analysis was applied to 34 years of anomalous daily 10‒20 day filtered outgoing longwave radiation data to determine the north-westward propagating mode of quasi-biweekly oscillations (QBWOs) over tropical East Asia during boreal summer. The centres of active QBWO convection coincided with positive potential vorticity (PV) anomalies at 500 hPa and cyclonic gyres at 850 hPa, whereas the centres of suppressed convection were superimposed upon negative PV anomalies and anticyclonic gyres. These circulations formed an inclined northwest-southeast wave train. The vertical dynamic and thermodynamic structures of the QBWO mode evince two centres of PV at 500 and 750 hPa, respectively,and an almost upright structure in the troposphere up to 300 hPa. The QBWO’s convective cell has a warmover-cold structure in the troposphere. These features have several similarities to East Asian tropical monsoon depressions (EAMDs), implying a close relationship between the north-westward propagating mode of QBWOs and EAMDs. Further investigation indicates that EAMDs are prone to appear in the region of deep convection and move north-westward with the QBWO circulation. The westernmost position of the western Pacific subtropical high (WPSH), which is linked to the QBWO phase, strongly influences the route of EAMDs. An EAMD generated in the easterly winds to the south of WPSH moves westward under the influence of the horizontal adiabatic advection of PV. More eastward-moving and turning EAMDs appear when WPSH weakens and retreats eastward due to enhancement of QBWO convection over the South China Sea (SCS)and western North Pacific. Most EAMDs reach peak intensity when moving over the SCS, and show a strong relation to the vigorous convection. However, there is no significant relation between the QBWO convection and the points of disappearance of the EAMDs. (Li Jingyi, Wen Min, Wang Zunya, Hu Yi)

1.11 Trends in the differences between homogenized ground surface temperature and surface air temperature in China during 1961–2016 and its possible causes

Based on the latest series of homogenized ground surface temperature (GST) and surface air temperature(SAT) data for China, this study performed a detailed analysis of the trend of the differences between the two homogenized series during 1961–2016. The differences, referred to as surface–air temperature differences(SATDs) in this study, were separately averaged by month, season, and year. The long-term and spatial changes in the trends of SATDs were investigated. Moreover, interdecadal trend breakpoints were identified to understand the characteristics of trends in fluctuation. The possible influences of precipitation, Pacific decadal oscillation (PDO) and global warming on SATDs were also analyzed. The results showed that during the 12 months of a year, only three months, March, April, and May, exhibited increasing trends of stationaveraged monthly mean SATDs while the other nine months exhibited decreasing trends. In addition, the station-averaged annual and seasonal mean SATDs of summer, autumn, and winter all showed significant decreasing trends, while the spring mean SATDs showed a significant increasing trend. The spatial distribution pattern of the linear trends of monthly, seasonal and annual SATDs in meteorological stations indicated that SATDs had more obviously increasing trends in the northern regions than in the southern regions of China.The trends of station-averaged monthly mean SATDs (except for April) and station-averaged seasonal and annual mean SATDs experienced interdecadal breakpoints, fully indicative of obvious interdecadal fluctuations with temporal complexity among China’s SATD trends. By the regression analysis of monthly STADs against simultaneous precipitation, as well as the comparative analysis of their linear trends, we found that both amount of precipitation and the change of precipitation type have important influences on SATDs. The results of convergent cross mapping analysis also revealed the causal effect of precipitation on SATDs. (Shi Xiaohui,Chen Jinqiu)

1.12 Remote forcing effect of sea surface temperatures in the northern tropical Atlantic on tropical cyclone genesis over the western North Pacific in July

In July 2020, neither genesis nor landfall of typhoons occurred in the western North Pacific (WNP).This was the first time that the “no-typhoon” phenomenon had occurred since 1949. This study analyzed the variation in the tropical cyclone genesis frequency (TCGF) in the WNP in July over the past 40 years,including the corresponding atmospheric background circulation and the possible influence of tropical sea surface temperature (SST). The results revealed that the eastward and northward location of the western Pacific subtropical high (WPSH), the deepened and eastward expanded tropical monsoon trough (TMT) in the WNP,the active convection in the main body of TMT, the weakened southwest water vapor transport of the East Asian summer monsoon, as well as the strengthened and eastward expanded equatorial westerly wind were conducive to the tropical cyclone genesis in the WNP in July. Furthermore, we used regression analysis to explore the possible impact of tropical SSTs. We found that the relatively high SSTs in the northern tropical Atlantic (NTA) in June and July triggered a cyclonic circulation over the tropical East Pacific and NTA,resulting in strong upward motion. The airflow subsidence in the tropical central Pacific then caused the WPSH to extend westward and the easterly airflow on the south side of WPSH to strengthen, which weakened the WNP monsoon trough. Besides, the influence of warming NTA SSTs could be conveyed to the Indian Ocean and indirectly induce the enhancement of WPSH. Such a series of the large-scale circulation variations were not favorable for tropical cyclone genesis in the WNP. As a result, the TCGF was relatively low in July. Thus,one of the causes of the July 2020 “no-typhoon” phenomenon in the WNP could be the remote forcing effect of SST in NTA. (Shi Xiaohui, Fang Yin)

1.13 Diverse inter-annual variations of winter Siberian high and link with Eurasian snow in observation and BCC-CSM2-MR coupled model simulation

An observational study illustrates that three distinct modes of winter Siberian high variability exist in observations at the inter-annual time scale. In this paper, we compare the connection between these diverse Siberian high variation modes with pre-autumn and simultaneous Eurasian snow cover in an observation and BCC-CSM2-MR coupled climate model run under pre-industrial conditions from the CMIP6 project. Our analyses indicate that the inter-annual variation of observed Siberian high modes do have a connection with pre-autumn and simultaneous Eurasian snow cover anomalies, but the BCC-CSM2-MR coupled climate model does not capture the observed diverse Eurasian snow–Siberian high relationships well. The BCC-CSM2-MR coupled climate model can partly reproduce the observed Siberian high variation modes, but fail to capture the spatial distribution and statistics of boreal fall and winter Eurasian snowpack, which is a key facet of simulated diverse Siberian high variability irrespective of the influence of Eurasian snow cover. (Sun Chenghu, Zuo Zhiyan, Shi Xiaohui)

1.14 Effect of upper-level air temperature changes over the Tibetan Plateau on the genesis frequency of Tibetan Plateau vortices at interannual timescales

Tibetan Plateau vortices (TPVs) are important rainfall producers over the Tibetan Plateau in summer and can also influence wide areas east of the Tibetan Plateau when emigrating from the plateau. The effects of the variations in air temperature over the Tibetan Plateau on the genesis frequency of TPVs at interannual timescales are explored in this work to understand the interannual variations in TPVs and the resultant precipitation. The results indicate that the interannual variations in the genesis frequency of TPVs are significantly related to that in the air temperature at 250 hPa over the Tibetan Plateau. The upper-level air temperature affects the genesis of TPVs by modulating the large-scale circulations at 200 hPa. In warm (cold)years, an anomalous high (low) at 200 hPa is observed over the eastern Tibetan Plateau and central China, and a strong (weak) westerly jet is found north of the Tibetan Plateau. The upper-level westerly jet is considered to have a direct influence on TPVs. Impact of 250-hPa air temperature over the Tibetan Plateau on the westerly jet is further explained from two perspectives. First, the air temperature at 250 hPa regulates the thermal wind between 250 and 200 hPa, thereby affecting the wind at 200 hPa. Second, the air temperature at 250 hPa changes the geopotential height gradient at 200 hPa north of the Tibetan Plateau, which results in variations in the westerly jet there. Consequently, more TPVs are generated over the Tibetan Plateau in warm years, and vice versa for cold years. (Li Lun, Zhang Renhe)

1.15 Regionally different precipitation trends over the Tibetan Plateau in the warming context: A perspective of Tibetan Plateau vortices

Precipitation over the Tibetan Plateau (TP) significantly affects both the local ecology and the downstream water resources. In this study, it is found that there exists an increasing precipitation trend over the northern TP but not in the southern TP, which is interpreted from the perspective of the activities of Tibetan Plateau vortices(TPVs). In the context of recent warming over the TP, the increasing rate of genesis frequency of TPVs over the northern TP is remarkably larger than that over the southern TP, which is responsible for different precipitation trends in the northern and southern TP, respectively. That is, the most significant warming over the TP appears in the upper troposphere, which intensifies the 200 hPa westerly jet north of the TP, hereby produces more favorable conditions for the genesis of TPVs over the northern TP than over southern TP, and contributes to regional difference in precipitation trends. (Li Lun, Zhang Renhe, Wen Min, Lyu Junmei)

1.16 Roles of the Tibetan Plateau vortices in the record Meiyu rainfall in 2020

Meiyu rainfall in June–July of 2020 hit the Yangtze-Huaihe River basin, and the precipitation at lots of observational stations broke the records, inducing severe disasters there. Tibetan Plateau vortices (TPVs)generated over the Tibetan Plateau are local major rainfall triggers. In previous knowledge, TPVs can affect the rainfall in the eastern China, only after emigrating from the Tibetan Plateau. In this work, roles of TPVs in the record Meiyu are revealed, implying that the effects of TPVs may be underestimated previously. Firstly,TPVs dying out over the Tibetan Plateau may be transformed into troughs and continue to move eastward,influencing the rainfall in Yangtze-Huaihe River basin. Secondly, southwest vortices, the important rainfall producers in China, tend to be generated and maintained when TPVs are located over eastern plateau. The results give extended knowledge on the effects of TPVs, which are beneficial for the rainfall prediction in the eastern China. (Li Lun, Zhu Congwen, Zhang Renhe, Liu Boqi)

1.17 Climatological intraseasonal oscillation in the middle-upper troposphere and its effect on the northward migration of the East Asian westerly jet and rain belt over eastern China

Using daily atmospheric circulation data for 1981–2010, we identify the climatological intraseasonal oscillation (CISO) in the mid-upper troposphere and reveal its critical role in anchoring the Meiyu season to the period from mid-June to mid-July and the rainfall episode of northeastern China to begin in late July. Wavelet analysis shows a significant 20–90-day period in the climatological 200- and 500-hPa zonal winds during boreal summer, indicating the existence of the zonal wind CISO in the mid-upper troposphere. The leading mode of zonal wind CISO features a dipole pattern along the coast of East Asia controlled by convection over the southeastern China-western North Pacific (SC-WNP). In mid-June, suppressed convection over the SCWNP induces the negative phase of intraseasonal Pacific-Japan pattern, leading to the strong anticyclone with a poleward tilt with height to dominate over the WNP. Consequently, more moisture are transported to the middle and lower reaches of the Yangtze River Valley (YRV), and the mid-tropospheric (upper-tropospheric) East Asian westerly jet (EAWJ) is shifted north of 32°N (36°N), which further increases the YRV rainfall through enhancing warm advection and upper-tropospheric divergence, thus facilitating the initiation of Meiyu season.In mid-July, active convection occupies the SC-WNP instead, with the reversed circulation pattern terminating the Meiyu season. In late July, active convection shifts from the equator to the tropical western Pacific, leading to the second mode of zonal wind CISO, which is characterized by a quasi-barotropic Mongolian anticyclone in the mid-upper troposphere. The upper-tropospheric EAWJ is further shifted to 45°N, displacing the induced upper-tropospheric divergence northward, thus initiating the rainfall episode of the northeastern China. (Li Jianying, Liu Boqi, Mao Jiangyu)

1.18 The influence of atmospheric intraseasonal oscillations on terrestrial biospheric CO2 fluxes in Southeast China forest

With its strong carbon sequestration capacity and significantly expanding trend, forest over Southeast China (SEC) is an important biospheric carbon sink. As intraseasonal oscillation (ISO) is a remarkable variability of the East Asian summer monsoon—the major climate system influencing the carbon exchange of SEC forest, the objective of this study is to examine the impact of atmospheric intraseasonal oscillation on CO2fluxes in SEC forest based on daily CO2fluxes and meteorological data. Significant 15‒60-day ISO of CO2fluxes is identified in SEC forest during May‒August of 1980‒2013. The 15‒60-day ISO of net ecosystem exchange between atmosphere and biosphere mainly results from that of gross primary productivity (GPP),with the explained variances greater than 50% over most SEC forest areas. Due to the high drought-resistance of SEC forest, the 15‒60-day ISO of its GPP is controlled primarily by local solar radiation, which is in turn associated with the northward-propagating atmospheric ISO. Suppressed (active) convection anomaly and the thermally induced anomalous anticyclone (cyclone) on its northwestern side initiate from the southern South China Sea and then propagate northward to SEC coherently in the form of a Rossby wave-like pattern.Decreased (increased) cloud cover associated with the suppressed (active) convection anomaly leads to more(less) solar radiation reaching the SEC forest canopy, thus promoting (impairing) forest productivity. (Li Jianying, Jong-Seong Kug, Mao Jiangyu)

1.19 Synergistic effect of the 25–60-day tropical and midlatitude intraseasonal oscillations on the persistently severe Yangtze floods

Significant 25‒60-day intraseasonal oscillations (ISOs) in rainfall over the middle-lower reaches of the Yangtze River (MLYR) are found for eight summers during 1979–2020. These 25–60-day ISOs of MLYR rainfall account for 66.7% of persistently severe floods (PSFs) during 1979‒2020, indicating their major role in inducing PSFs and the potential for extended-range forecasting of PSFs. The 25–60-day ISO in MLYR rainfall results from the synergistic effect of tropical and midlatitude ISOs. During the eight identified summers, more barotropic energy is transferred from summer-mean to 25–60-day circulation, thus forming a midlatitude ISO wave train, which cooperates with the tropical ISO in triggering twin vertical cells. Phase locking of ascending branch from the vertical cell induced by the tropical ISO with that from the reversed vertical cell triggered by the midlatitude ISO brings about the wet phase of 25–60-day ISO of MLYR rainfall. (Li Jianying, Zhai Panmao, Mao Jiangyu, Song Lili)

1.20 2019年4—6月云南持续性高温天气的大气环流异常成因

2019年4—6月云南省发生了历史罕见的持续性极端高温天气，并引发了严重气象干旱。本文利用1961—2019年逐日温度和大气再分析等资料以及CESM-LE（Community Earth System Model Large Ensemble Project）计划模式模拟结果，分析了历史同期云南极端高温天气发生的环流特征，探讨了2019年云南破纪录持续性高温的成因。历史极端高温日的合成分析表明，云南地区对流层上层显著异常反气旋伴随的强下沉异常和到达地表太阳辐射增加，是引发该区域极端高温天气的主要成因。该异常反气旋的形成主要源自北大西洋经东欧平原、西西伯利亚平原向东亚传播的高纬度罗斯贝波和经北非、黑海、伊朗高原向东亚传播的中纬度罗斯贝波之间的相互作用。2019年极端高温的强度和与之相应异常反气旋出现自1961年以来的最强。外强迫导致的增暖对2019年极端暖异常强度的贡献约为37.51%，同时对类似2019年以及更强极端暖事件发生概率的贡献为56.32%，内部变率对该事件也具有重要贡献。2019年4—6月北极涛动（AO）和ENSO事件分别处于历史极端负位相和暖位相。一方面，在AO强负位相影响下，极地上空深厚的位势高度正异常向南伸至东欧平原，有利于高纬度波列和云南上空的反气旋异常增强。另一方面，ENSO事件暖位相加强了西北太平洋异常反气旋环流，令西北太平洋副热带高压增强西伸至我国内陆地区，维持了云南上空反气旋异常。两者的共同作用，造成了2019年4—6月云南上空持续的深厚异常反气旋，云南地区继而出现持续性极端高温事件。（马双梅，祝从文，刘伯奇）

1.21 北太平洋异常高压对京津冀地区PM2.5污染影响分析

以2021年3月2—5日京津冀地区持续4 d的PM2.5重污染过程为例，使用PM2.5质量浓度格点资料、城市空气质量指数（AQI）资料、NCEP再分析资料、海温资料以及HYSPLIT轨迹追踪模式，重点分析了北太平洋异常高压对京津冀地区PM2.5污染的影响。结果表明：（1）500 hPa北太平洋异常暖性高压系统的稳定维持是京津冀地区污染发生的有利环流形势，当该地区处于此异常高压后部并受偏南风控制时，不利于该地区污染物扩散。（2）低层850 hPa异常高压系统引导来自西北太平洋暖湿气流向京津冀地区输入使低层增温增湿，不利于污染物垂直扩散，并使污染物快速吸湿增长。（3）北太平洋异常高压与北太平洋中西部海温呈正相关，与赤道中东太平洋海温呈现负相关。北太平洋异常高压配合京津冀地面南风对该地区空气污染的发生具有一定的指示意义。（陈运，蒋宁，朱莉莉）

1.22 次季节至季节气候预测业务支撑与决策服务

气候所次季节至季节（S2S）研究团队先后参加国家气候中心、东北区域气候中心、国家海洋局海洋环境预报中心、水利部黄河委员会和北京冬奥气象中心等部门组织的全国和区域/流域气候预测会商共计17次，并提供预测意见，服务效果得到相关部门认可。基于自主研发的统计预测方法（CAMSASPS）和气科院动力预测系统（CAMS-CSM），在3月份不仅准确给出今年夏季我国北方地区和长江流域降水明显偏多、黄河流域防洪压力大的预测结果，还准确预测出初夏东北地区降水异常偏多、气温偏低的气候格局，为国家和区域汛期防灾减灾工作提供了重要参考。（刘伯奇、鄢钰函、祝从文、蒋宁、马双梅、苏京志）

在决策气象服务方面，S2S团队针对2021年国内、外高影响天气气候事件，先后开展了多次复盘工作，包括：春季北方沙尘暴天气、2020/2021年冬季气温冷暖急转、6月东北冷涡异常活跃、“7·21”郑州暴雨、盛夏北美极端高温热浪、华北超长汛期和降水异常以及美国超级龙卷事件。以院专报形式上报决策服务材料4份。气候与气候变化研究所获得2021年度中国气象局“重大气象服务先进集体”称号。（刘伯奇、蒋宁、马双梅、鄢钰函、鲁萌萌、祝从文、孙丞虎、齐艳军）

2 气候特征与气候变化

2 Climate characteristics and climate change

2.1 Anthropogenic emissions and urbanization increase risk of compound hot extremes in cities

Urban areas are experiencing strongly increasing hot temperature extremes. However, these urban heat events have seldom been the focus of traditional detection and attribution analysis designed for regional to global changes. Here we show that compound (day-night sustained) hot extremes are more dangerous than solely daytime or nighttime heat, especially to female and older urban residents. Urban compound hot extremes across the eastern China have increased by 1.76 days per decade from 1961 to 2014 with fingerprints of urban expansion and anthropogenic emissions detected by a stepwise detection and attribution method.Their attributable fractions are estimated as 0.51 (urbanization), 1.63 (greenhouse gases) and −0.54 (other anthropogenic forcings) days per decade. Future emissions and urbanization would make these compound events two to five times more frequent (the 2090s versus 2010s), leading to a threefold-to-sixfold growth in urban population exposure. Our findings call for tailored adaptation planning against rapidly growing health threats from compound heat in cities. (Wang Jun, Chen Yang, Zhai Panmao)

2.2 Detectable anthropogenic changes in daily-scale circulations driving summer rainfall shifts over eastern China

Wetting in the south while drying in the north during the last few decades constitutes the well-known“southern flood-northern drought” (SFND) precipitation pattern over the eastern China. The fingerprint of anthropogenic influence on this dipole pattern of regional precipitation trends has not been confirmed,especially for forced changes in relevant dynamics at the synoptic scale. Using a process-based approach involving model experiments both with and without anthropogenic inputs, it is demonstrated that the occurrences of daily circulation patterns (CPs) governing precipitation over the eastern China during 1961−2013 have been altered by human influence. Due to anthropogenic forcing, CPs favoring SFND have become more likely to occur at the expense of CPs unfavorable to SFND. Regression analysis shows that changes recorded in the occurrence of CPs from the factual simulations could explain a large part of the precipitation trends over the eastern China. CP frequencies driven by purely natural forcing do not reproduce this dipole pattern nor the inferred magnitude of precipitation trends over the eastern China. These results suggest that human influence has played a critical role in shaping the contrasting north-south precipitation trends. (Zhou Baiquan, Zhai Panmao)

2.3 Detectable intensification of hourly and daily scale precipitation extremes across East China

Detecting long-term changes in precipitation extremes over monsoon regions remains challenging due to large observational uncertainty, high internal variability at the regional scale, and climate models’ deficiency in simulating monsoon physics. This is particularly true for East China, as illustrated by limited yet controversial detection results for daily scale precipitation extremes and the lack of detection analysis on hourly scale extremes there. Relying on high-quality gauge observations, two complementary techniques are used to detect the footprint of anthropogenic forcings in observed changes in both hourly and daily scale precipitation extremes across East China. Results show that, scaled with global mean surface temperature during 1970‒2017, the regional-scale intensification nearly doubles the Clausius-Clapeyron rate (C-C; 6.5% 8C21)for the wettest 10 h in the period and almost triples the C-C rate for the top 10 heaviest daily precipitation extremes. The intensification at both time scales, as well as the resulting increase in frequency, is discernibly stronger and more widespread than expected due to random internal variability. This not only lends supports to the model-based detection of forced trends for daily scale precipitation extremes, but it also suggests that anthropogenic warming has already been intensifying hourly scale precipitation extremes in this monsoon region. The magnitude and detectability of observed changes arise primarily from systematic intensification of non-tropical-cyclone-related precipitation extremes in response to the past warming. (Chen Yang, Li Wei, Jiang Xiaoling, Zhai Panmao, Luo Yali)

2.4 Hourly extreme precipitation changes under the influences of regional and urbanization effects in Beijing

Short duration extreme precipitation has devastating impacts on city area. Local urbanization effects,superimposed upon regional climate change, complicate examination of long-term changes in short duration precipitation extremes in urban areas. Based on high-quality rain gauge observations of summer time hourly precipitation in Beijing region over 1977–2013, this study reveals that despite the general drying tendency for North China, the urban area of Beijing has experienced more hourly precipitation extremes (HPE) than the suburban area since 2004, coinciding with the surge in the growth of urban built-up areas. These hourly urban precipitation extremes are increasingly inclined to occurring during night-time, especially during 18:00 LST to 02:00 LST (UCT+8). On the one hand, the amplified urban heat inland effect, which was more significant at night-time, seems to have facilitated formation of more intense small scale thermal-low and resultant ascending branch; on the other hand, it has favoured in establishing unstable stratification in the lower level. This possible mechanism explains the preference of HPE in urban areas during night-time and climate change diversity under the influence of megacity superposition. (Yuan Yufeng, Zhai Panmao, Chen Yang, Li Jian)

2.5 Detectable increases in sequential flood-heatwave events across China during 1961−2018

Traditional univariate analysis on weather and climate extremes failed to consider temporally compounding events and the resulting cascading impacts. A case in point is a sequence of flood and heatwave within a week, which slows recovery and amplifies damages. We show that across China, floods and heatwaves seldom occurred serially within seven days in the past, but after 2000 the probability is five-to-ten times higher in the southern, northwestern and northeastern sectors. It is the significant increase in heatwaves that alters the clustering of independent extremes, and facilitates the emergence of sequential extremes. Typhoonparticipating sequential extremes have also increased significantly in frequency in both inland and coastal areas, with the fastest rate at around 200% per decade registered within the 30°‒35°N latitudinal band. The observed increases in sequential flood-heatwave events are discernibly stronger and more widespread than what would be expected from pure random variability, implying a detectable role of anthropogenic forcings.(Chen Yang, Liao Zhen, Shi Yan, Tian Yangmei, Zhai Panmao)

2.6 Fine-scale characteristics of summer precipitation over Cang Mountain

Fine-scale characteristics of summer precipitation over Cang Mountain, a long and narrow mountain with a quasi-north‒south orientation in Southwest China, are studied using station and radar data. Three kinds of rainfall processes are classified according to the initial stations of regional rainfall events (RREs) by utilizing minute-scale rain gauge data. RREs initiating in the western part of Cang Mountain exhibit eastward evolution and tend to reach their maximum rainfall intensity on the mountaintop. The results indicate differences in the precipitation evolution characteristics between short-duration (1–3 h) and long-duration (at least 6 h) events.Short-duration events begin farther from the mountaintop and then propagate eastward, whereas long-duration events remain longer around the mountaintop. RREs that initiate from the eastern part of Cang Mountain display westward propagation and frequently reach their maximum rainfall intensity over the eastern slope of the mountain. Among them, short-duration events tend to propagate farther west of Cang Mountain at high speeds, but the westward evolution of long-duration events is mainly confined to the eastern part of Cang Mountain. For mountaintop-originated RREs, precipitation quickly reaches its maximum intensity after it starts and then continues for a long time around the mountaintop during the period from late afternoon to early morning. These findings provide references for the fine-scale prediction of precipitation evolution in smallscale mountainous areas. (Zhang Mengke, Li Jian, Li Nina)

2.7 Application of an improved analog-based heavy precipitation forecast model to the Yangtze–Huai River Valley and its performance in June–July 2020

Precipitation extremes, such as the record-breaking Meiyu characterized by frequent occurrences of rainstorms that resulted in severe flooding over the Yangtze-Huai River Valley (YHRV) in June‒July 2020, are always attracting considerable interest, highlighting the importance of improving the forecast accuracy at the medium-to-long range. To elevate the skill in forecasting heavy precipitation events (HPEs) with both long and short durations, the key influential systems based analog model (KISAM) was further improved and brought into operational application in 2020. Verification and comparison of this newly adapted analog model and ensemble mean forecasts from the ECMWF at lead times of up to 15 days were carried out for the identified 16 HPEs over the YHRV in June‒July 2020. The results demonstrate that KISAM is advantageous over ECMWF ensemble mean for forecasts of heavy precipitation≥25 mm day−1at the medium-to-long (6–15-day) lead times, based on the traditional dichotomous metrics. At short lead times, ECMWF ensemble mean outperforms KISAM due largely to the low false alarm rates (FARs) benefited from an underestimation of the frequency of heavy precipitation. However, at the medium-to-long forecast range, the large fraction of misses induced by the high degree of underforecasting overwhelms the fairly good FARs in the ECMWF ensemble mean, which partly explains its inferiority to KISAM in terms of the threat score. Further assessment on forecasts of the latitudinal location of accumulated heavy precipitation indicates that smaller displacement errors also account for a part of the better performance of KISAM at lead times of 8‒12 days. (Zhou Baiquan, Zhai Panmao, Niu Ruoyun)

2.8 Investigation of the effects of dynamic vegetation processes on global climate simulation using the NCEP GFS and SSiB4/TRIFFID

To investigate the effects of dynamic vegetation processes on numerical climate simulation, two experiments are conducted globally by integrating the National Centers for Environmental Prediction global forecast system (GFS) coupled with a biophysical model, simplified simple biosphere model (SSiB) version 2 (referred to as GFS/SSiB2) and with a biophysical and dynamic vegetation model, SSiB version 4/TRIFFID(referred to as GFS/SSiB4/TRIFFID) from 1948 to 2008. By assessing GFS/SSiB4T/TRIFFID and GFS/SSiB2 results against satellite-derived leaf area index (LAI) and albedo, as well as observed land surface temperature and precipitation, we identify the effects of dynamic vegetation processes on the simulations of precipitation,near-surface temperature and the surface energy budget at monthly and annual scales. The results show that compared to the GFS/SSiB2, the temporal correlation coefficients between globally averaged monthly simulated LAI and the GIMMS/GLASS LAI in the GFS/SSiB4/TRIFFID increase from 0.31/0.29 (SSiB2) to 0.47/0.46 (SSiB4). Meanwhile, the correlation coefficients between simulated and observed monthly mean near-surface air temperature increase from 0.58 (over Africa), 0.47 (over Southeast Asia) and 0.50 (over South America) to 0.66, 0.55 and 0.58 respectively. While the correlation coefficients between model-simulated and observed monthly mean precipitation increase from 0.31 (over Africa), 0.41 (over East Asia) and 0.21 (over Australia) to 0.38, 0.47 and 0.24 respectively. The most improvement occurs over arid and semi-arid areas.The spatial-temporal variability and changes in vegetation and ground surface albedo modeled by GFS with dynamic vegetation model are more consistent with observations, which contribute to the surface energy and water balances, and in turn improve the annual variations in simulated regional temperature and precipitation.The dynamic vegetation processes have the greatest influences on the temporal and spatial changes of latent heat fluxes. This study demonstrates that the dynamic vegetation processes in the Earth system model are capable to improve the climate mean status simulation significantly. (Zhang Zhengqiu, Xue Yongkang, Zhai Panmao)

2.9 Changes in summer persistent precipitation over the middle-lower reaches of the Yangtze River and associated atmospheric circulation patterns

Persistence is an important property of precipitation and its related impacts. However, changes in persistent precipitation and the possible underlying mechanisms in the context of global warming have not yet been discussed in sufficient depth. In this study, the changes in persistent precipitation in summer and related atmospheric circulation patterns over the middle-lower reaches of the Yangtze River (MLRYZR)—a typical monsoon region frequently hit by consecutive rainfall events—are analyzed based on observed daily precipitation and NCEP/NCAR reanalysis data from 1961 to 2019. The results reveal that persistent precipitation events (PPs) tend to happen in a more persistent way, with increased frequency and intensity in the MLRYZR region. Mechanism analyses show that persistent precipitation has happened along with simultaneous enhancement of some large-scale atmospheric circulation patterns, including the Lake Baikal blocking (BB), the Okhotsk Sea blocking (OB), and the western Pacific subtropical high (WPSH). Such enhanced anomalous circulation patterns could persistently reinforce the convergence and supply of water vapor in the MLRYZR region, contributing to the increase in PPs in this region. Based on the above results,we are able to offer some new insights into the long-term changes in precipitation structure and the possible causes. This study is also expected to support attribution studies on regional precipitation changes in the future.(Yu Rong, Zhai Panmao)

2.10 Divergent responses of ecosystem water use efficiency to drought timing over Northern Eurasia

Warming has resulted in increases in frequency, intensity and/or duration of droughts in most land regions over the globe. Nevertheless, knowledge on how ecosystem water use efficiency (WUE) responds to extreme drought stress and whether the responses are affected by drought timing is still limited. In this study,we examined the changes in ecosystem WUE under extreme drought years over Northern Eurasia during 1982–2011 and further assessed WUE responses to droughts with separate groupings designed to characterize the timing of extreme drought stress. We found that drought timing indeed influenced the responses of ecosystem WUE under extreme drought years. Negative impacts of extreme drought stress during the dry season on ecosystem WUE were more remarkable than those from extreme drought stress during the wet season. Particularly, impacts of droughts on ecosystem carbon-water interactions differed among ecosystem types due to the specific hydrothermal condition of each biome. The information provided by our analyses plays an importance role in identifying water use strategies of terrestrial vegetation in response to drought stress and will help improve our understanding and predictions of the response of ecosystem WUE to global environmental change. (Huang Mengtian, Zhai Panmao, Piao Shilong)

2.11 Performance of the CRA-40/Land, CMFD, and ERA-Interim datasets in reflecting changes in surface air temperature over the Tibetan Plateau

We analyzed the spatiotemporal variations in surface air temperature and key climate change indicators over the Tibetan Plateau during a common valid period from 1979 to 2018 to evaluate the performance of different datasets on various timescales. We used observations from 22 in-situ observation sites, the CRA-40/Land (CRA) reanalysis dataset, the China meteorological forcing dataset (CMFD), and the ERA-Interim(ERA) reanalysis dataset. The three datasets are spatially consistent with the in-situ observations, but slightly underestimate the annual mean surface air temperature. The daily mean surface air temperature estimated by the CRA, CMFD, and ERA datasets is closer to the in-situ observations after correction for elevation. The CMFD shows the best performance in simulating the annual mean surface air temperature over the Tibetan Plateau, followed by the CRA and ERA datasets with comparable performances. The CMFD is relatively accurate in simulating the daily mean surface air temperature over the Tibetan Plateau on an annual scale,whereas both the CRA and ERA datasets perform better in summer than in winter. The increasing trends in the annual mean surface air temperature over the Tibetan Plateau from 1979 to 2018 reflected by the CRA dataset and the CMFD are 0.5 (10year)−1, similar to the in-situ observations, whereas the warming rate in the ERA dataset is only 0.3 (10year)−1. The trends in the length of the growing season derived from the in-situ observations, the CRA, CMFD, and ERA datasets are 5.3, 4.8, 6.1, and 3.2 day (10year)−1, respectively. Our analyses suggest that both the CRA dataset and the CMFD perform better than the ERA dataset in modeling the changes in surface air temperature over the Tibetan Plateau. (Yang Jiaxi, Huang Mengtian, Zhai Panmao)

2.12 Achieving Paris Agreement temperature goals requires carbon neutrality by middle century with far-reaching transitions in the whole society

The concept of carbon neutrality is much emphasized in IPCC Spatial Report on Global Warming of 1.5℃ in order to achieve the long-term temperature goals as reflected in Paris Agreement. To keep these goals within reach, peaking the global carbon emissions as soon as possible and achieving carbon neutrality are urgently needed. However, global CO2emissions continued to grow up to a record high of 43.1 Gt (CO2)during 2019, with fossil CO2emissions of 36.5 Gt (CO2) and land-use change emissions of 6.6 Gt (CO2) . In such case, the global carbon emissions must drop 32 Gt (CO2) (7.6% per year) from 2020 to 2030 for the 1.5 ℃ warming limit, which is even larger than the COVID-induced reduction (6.4%) in global CO2emissions during 2020. Recently, China has announced scaling up its national commitments, aiming to peak its CO2emissions before 2030 and achieve carbon neutrality before 2060. Achieving these goals requires rapid and farreaching transitions in the whole society. On the one hand, deeper emissions reduction in all sectors includes decarbonization of energy, electrification, increasing share of renewables, energy efficiency, sustainable land management, decarbonization of transport, reducing food loss and waste, as well as behavior and lifestyles changes. On the other hand, possible actions by removing CO2from the atmosphere involves enlarging land and ocean net carbon sink, CO2removal technologies (such as bioenergy with carbon capture and storage), and CO2capture, utilization and storage technologies, but should be caution for their scales and tradeoffs. (Huang Mengtian, Zhai Panmao)

2.13 Change in drought conditions and its impacts on vegetation growth over the Tibetan Plateau

Understanding climate change as well as its impacts on vegetation growth over the Tibetan Plateau has important implication for ecosystems. This study investigated changes in drought conditions and their impacts on vegetation growth over the Tibetan Plateau. The results reveal that the precipitation in growing season(May−September) shows a significant increase over most parts of the central and eastern Tibetan Plateau during 1961−2019. Consequently, drought conditions have generally relieved except in the south and northeast where precipitation has decreased. Combining analyses of gridded-dataset-derived drought indices with vegetation indices during 1982−2015, vegetation improvement in most regions of the Plateau is mainly due to lessened drought conditions. Noticeably, vegetation degradation is also found in part of the southern Tibetan Plateau mainly resulting from drought enhancement. This study is expected to provide scientific basis for understanding of change in drought condition and its impacts on vegetation over different regions of TP under global warming background. (Wang Chenpeng, Huang Mengtian, Zhai Panmao)

2.14 Growing threats from unprecedented sequential flood-hot extremes across China

When multiple extremes occur in rapid sequence, their impacts cascade to cause disproportionate damages. However, the prevalence of univariate definitions and inability to identify low-likelihood events in short observations/simulations leave the knowledge on sequential extremes sparse. Leveraging two initialcondition large ensembles, we project future changes in historically unprecedented sequential flood-hot extremes in China. Results show that despite dozens of 1 in 50-year floods and hot extremes in more than 2000 years of historical simulations, their sequenced occurrence within a week has no historical precedent. This out-of-ordinary configuration is projected to be increasingly possible across China, with earlier emergence and larger frequency increases expected in Southwest and Southeast China. The direction, spatial extent,and magnitude of projected changes in unprecedented sequential extremes cannot be explained by internal variability alone, though it has the potential to modulate human-caused changes in emergence timing and magnitude. (Liao Zhen, Chen Yang, Li Wei, Zhai Panmao)

2.15 Investigating the influence of synoptic circulation patterns on regional dry and moist heat waves in North China

Summer (June–August) heat waves in North China are found to be either primarily dry or moist, based on surface meteorological observations. This study characterizes synoptic circulation patterns (i.e., 500 hPa geopotential height) using the self-organizing map (SOM) method and investigates the influence of synoptic circulation patterns on these two types of heat waves. Results show that regional dry and moist heat waves are associated with different circulation patterns, which significantly modulate the advection of water vapor within the low-level atmosphere, and soil moisture and evaporation conditions at the surface. Regional dry heat waves are associated with times when a continental high pressure ridge is situated to the northwest of North China,and when the northern edge of the western North Pacific subtropical high (WNPSH) is south of 30°N. Regional moist heat waves are associated with a northward shift of the WNPSH. Long term variations of dry and moist heat wave occurrences correlated significantly with the occurrences of their associated circulation patterns at 0.38 (p = 0.02) and 0.71 (p = 0.00), respectively. On sub-seasonal time scales, the dominant heat wave type transforms from dry in June to moist in late July, which is in accordance with summer north–south WNPSH shifts. In addition, training the SOM with absolute geopotential height results in representative circulation patterns that are closely related to surface heat wave conditions in North China rather than the anomaly field,which mixes different circulation regimes. (An Ning, Zuo Zhiyan)

2.16 Changing structures of summertime heatwaves over China during 1961–2017

Despite the prevalence of artificial separation of daytime and nighttime hot extremes, they may actually co-occur or occur sequentially. Considering their potential lead-lag configuration, this study identified an entire heatwave period as consecutive days with either daytime or nighttime hot extremes and investigated the changes of the prevalence and sequence of daytime and nighttime hot extremes during heatwaves over China from 1961 to 2017. It was found that the majority (82%) of heatwaves were compound heatwaves that had both daytime and nighttime hot extremes exceeding the 90th percentile-based thresholds, while only 7% (11%)were purely daytime (nighttime) heatwaves that contained only daytime (nighttime) hot extremes. During the entire periods of compound heatwaves, daytime hot extremes usually occurred one day or a few days before nighttime hot extremes, which was in accordance with the daily variations in radiation and meteorological conditions, such as the increasing surface humidity and cloud cover, and decreasing solar radiation during the entire heatwave periods. From 1961 to 2017, compound heatwave numbers exhibited the sharpest increase with a statistically significant trend of 0.44 times per decade, in contrast to an insignificant trend of 0.00 times per decade for purely daytime heatwaves and a significant trend of 0.09 times per decade for purely nighttime heatwaves. Within the compound heatwave periods, hot nights were starting earlier and ending later, and numbers of concurrent daytime-nighttime hot extremes increased significantly at 0.20 days per decade. In particular, urban areas were not only subject to increasingly more frequent and longer compound heatwaves,but also to more occurrences of concurrent daytime-nighttime hot extremes with more serious impact. This study provides instructions for researchers to customize and select appropriate heatwave indices. (An Ning,Zuo Zhiyan)

2.17 Evaluating boreal summer circulation patterns of CMIP6 climate models over the Asian region

Our confidence in future climate projection depends on the ability of climate models to simulate the current climate, and model performance in simulating atmospheric circulation affects its ability of simulating extreme events. In this study, the self-organizing map (SOM) method is used to evaluate the frequency,persistence, and transition characteristics of models in the Coupled Model Intercomparison Project phase 6(CMIP6) for different ensembles of daily 500 hPa geopotential height (Z500) in Asia, and then all ensembles are ranked according to a comprehensive ranking metric (MR). Our results show that the SOM method is a powerful tool for assessing the daily-scale circulation simulation skills in Asia, and the results will not be significantly affected by different map sizes. Positive associations between each two of the performance in frequency, persistence and transition were found, indicating that a good ensemble of simulation for one metric is good for the others. The r10i1p1f1 ensemble of CanESM5 best simulates Z500 in Asia comprehensively, and it is also the best of simulating frequency characteristics. The MR simulation of the highest 10 ensembles for the western North Pacific subtropical high (WNPSH) and the South Asia high (SAH) are far better than those of the lowest 10. Such differences may lead to errors in the simulation of extreme events. This study will help future studies in the choice of ensembles with better circulation simulation skills to improve the credibility of their conclusions. (Bu Lulei, Zuo Zhiyan, An ning)

2.18 气候变化科学方面的几个最新认知

IPCC第六次评估报告（AR6）第一工作组报告主要从以下几个方面的进展提升了我们对气候系统变化、气候变化原因以及预估未来气候系统变化等方面的认知，对过去气候变化及其与人类活动的关系有了更加清晰、可靠的认识。综合多重证据评估指出，全球气候正经历着前所未有的变化；包括极端事件在内的归因进展已把人类活动对气候系统影响的认识从大气圈扩展到水圈、冰冻圈和生物圈，进一步强化了人类活动影响全球和区域气候的认识；有关区域气候变化信息的内容更加丰富，与各行业和敏感地区的气候变化影响联系更加紧密，使这些信息能更好地为气候变化风险评估和气候变化区域适应提供支持；气候模式和约束预估方法的发展以及对气候敏感度认识的深化，减少了未来不同排放情景下全球地表温度（GST）、海平面上升和海洋热含量的变化预估的不确定性。这份最新报告对我国提升气候变化研究水平和防灾减灾应对能力具有十分重要的指导意义。（翟盘茂，周佰铨，陈阳，余荣）

2.19 IPCC第六次气候变化评估中的气候约束预估方法

得益于第五次评估报告（AR5）以来约束预估研究的迅速发展，观测约束成为政府间气候变化专门委员会（IPCC）第一工作组（WGI）第六次评估报告（AR6）提升对未来预估约束的证据链中的重要一环。IPCC 第一工作组第六次评估报告首次利用包括根据历史模拟温度升高幅度得到的观测约束、多模式预估以及第六次评估报告中更新的气候敏感度在内的多条证据链来约束全球地表温度未来变化的预估，减小了多模式预估的不确定性。文中回顾并介绍了IPCC 第一工作组第六次评估报告中涉及的几种主要观测约束方法（多模式加权方法、基于归因结论的约束方法（ASK方法、萌现约束方法）及其应用情况。在IPCC 第一工作组第六次评估报告以及很多针对不同区域不同变量的预估研究中，观测约束方法均显示出了订正模式误差、改善模式预估的潜力。相比而言，目前中国在观测约束预估方面的研究还不多，亟待加强观测约束方法研究以及在中国区域气候变化预估中的应用，为中国应对气候变化的政策制定和适应规划提供更丰富、不确定性更小的未来气候信息。（周佰铨，翟盘茂）

2.20 IPCC第六次评估报告中的《图集》

决策者和公众正在越来越多地关注气候变化带来的影响，而这需要更加丰富的、区域尺度上的当前和未来气候状况的精细信息。《图集》与IPCC第六次评估报告第一工作组报告中其他章节相协调，评估区域气候变化的观测、归因、预估的基本信息，并建立了在线交互图集系统。《图集》包含图集章节和交互图集两部分，图集章节基于新的区域划分，评估了各区域的气候变化，重点关注地表温度和降水的观测趋势、归因以及预估的未来变化。交互图集是AR6 WGI报告的一个新组成部分，基于观测、全球（CMIP5和CMIP6）和区域（CORDEX）模式数据，以互动地图的形式提供观测和预估时间段的气候变化和归因的综合信息。（李建，陈昊明）

2.21 关于复合型极端事件的新认识和启示

近年来，极端天气气候事件频繁发生，且常常是由多种事件交织形成的复合型极端事件。为了更好地认识复合型极端事件，IPCC AR6 基于现有的新证据评估了复合型极端事件的最新研究成果，并取得一些新认识：扩展了有关复合型极端事件的定义，重点围绕高温干旱复合型极端事件、复合洪水和野火，评估了复合型极端事件的变化特征，探讨了复合型极端事件多因子之间的依赖性，对人类活动的影响进行了归因分析并给出了未来可能的变化。这些评估结果丰富了对复合型极端事件的基本认识，但根据现有的评估可以看出目前在复合型极端事件发生发展机理认识方面还存在不足；同时，未来仍需进一步完善跨学科跨部门跨区域研究，加强对复合型极端事件形成机理、预估及其对生态系统、经济社会影响风险的评估，提高对区域气候变化的适应能力。（余荣，翟盘茂）

2.22 基于CMIP6模式优化集合平均预估21世纪全球陆地生态系统总初级生产力变化

利用国际耦合模式比较计划第六阶段（CMIP6）中18个地球系统模式总初级生产力（GPP）模拟数据，基于传统的多模式集合平均（MME）和可靠集合平均方法（REA），在4个未来情景（SSP1-2.6，SSP2-4.5，SSP3-7.0和SSP5-8.5）下预估了21世纪全球陆地生态系统GPP的变化量，并分析了GPP变化的驱动因子。研究结果表明：在4个未来情景下，基于REA方法预估的全球陆地生态系统年GPP在未来时期（2068—2100年）比历史时期（1982—2014年）分别增长了（14.85±3.32）、（28.43±4.97）、（37.66±7.61）和（45.89±9.21）Pg C，其增量大小和不确定性都明显低于MME方法。在4个情景下，大气CO2浓度增长对GPP变化的贡献最大，基于REA方法计算的贡献占比分别为140%、137%、115%和75%；除了SSP5-8.5 （24%）外，其他情景下升温均导致全球陆地生态系统GPP降低（-42%、-37%、-16%），部分抵消了CO2施肥效应的正面贡献。温度的影响存在纬度差异：升温在低纬度地区对GPP有负向贡献，在中高纬度地区为正向贡献。降水和辐射变化对GPP变化的贡献相对较小。（黄禄丰，朱再春，黄萌田）

2.23 气候动力诊断和分析系统设计与应用

气候动力诊断和数值模拟是认识气候变化规律、提高短期气候预测与科学决策服务水平的重要手段。但基于气候模拟的动力诊断技术在气候预测业务中还未得到广泛应用，缺乏支撑科研成果转化为业务应用的中试平台。为此，通过集成多种现代计算机通信协议、可视化编辑和气象数值计算等技术，研发可视化交互气候动力诊断和分析系统（CDDAS），以促进气候模拟的动力诊断技术在气候业务中的广泛应用。该系统具有结构开放、诊断方法集成度高、方便易用等特点，包括数据更新备份、气候动力诊断、多模式数值模拟、结果分析4个功能模块，并设计了一种远程交互控制脚本语言，为用户二次开发提供语言环境，可实现本地客户端、服务器端和超级计算机三者交互通信控制可视化管理。该系统使用便捷，目前已在国家级业务和科研单位获得应用，在气候异常成因分析、气候预测和气候决策服务中可显著提高工作效率。（张正秋，祝从文，苏京志，刘伯奇，蒋宁，陈昊明）

2.24 IPCC工作支撑

气候变化团队成员圆满完成本年度IPCC第一工作组（WGI）联合主席技术支撑工作任务，通过网络会议、亲自出访等多种方式，为IPCC WGI第六次评估报告（AR6）的撰写、评审、批准、发布及后续科学传播活动以及IPCC综合报告的准备、撰写等相关工作提供技术支持。团队成员作为副主编、主要作者、技术支持等角色参与《中国气候与生态环境演变：2021》《青藏高原气候变化、影响与应对》《新疆气候变化科学评估报告》等国内气候变化评估报告的编写工作中。（翟盘茂，黄萌田，陈阳，余荣）

2.25 气候变化决策服务

气候变化团队分别围绕气候变化的影响与应对、极端天气气候事件的变化及风险等方面展开专题研究，并报送多份决策服务材料，受到高度重视，获得良好的决策支撑效果。（翟盘茂，陈阳，余荣，黄萌田，廖圳）

3 气候模式研发

3 Climate model development

3.1 The CMIP6 historical simulation datasets produced by the climate system model CAMS-CSM

This paper describes the historical simulations produced by the Chinese Academy of Meteorological Sciences (CAMS) climate system model (CAMS-CSM), which are contributing to the Coupled Model Intercomparison Project phase 6 (CMIP6). The model description, experiment design and model outputs are presented. Three members’ historical experiments are conducted by CAMS-CSM, with two members starting from different initial conditions, and one excluding the stratospheric aerosol to identify the effect of volcanic eruptions. The outputs of the historical experiments are also validated using observational data. It is found that the model can reproduce the climatological mean states and seasonal cycle of the major climate system quantities, including the surface air temperature, precipitation, and the equatorial thermocline. The long-term trend of air temperature and precipitation is also reasonably captured by CAMS-CSM. There are still some biases in the model that need further improvement. This paper can help the users to better understand the performance and the datasets of CAMS-CSM. (Rong Xinyao, Li Jian, Chen Haoming, Su Jingzhi, Hua Lijuan,Zhang Zhengqiu, Xin Yufei)

3.2 Seasonal prediction skills in the CAMS-CSM climate forecast system

The seasonal prediction skills in the CAMS-CSM (the acronym stands for the Chinese Academy of Meteorological Sciences climate system model) climate forecast system is evaluated with a set of retrospective forecast experiments during the period of 1981−2019. The CAMS-CSM, which has been registered for the Coupled Model Intercomparison Project phase 6 (CMIP6), is an atmosphere-ocean-land-sea ice fully coupled general circulation model. The assimilation scheme used in the forecast system is the 3-dimentional nudging,including both the atmospheric and oceanic components. The analyses mainly focus on the seasonal predictable skill of sea surface temperature, 2-m air temperature, and precipitation anomalies. The analyses revealed that the model shows a good prediction skill for the SST anomalies, especially in the tropical Pacific, in association with El Niño-southern oscillation (ENSO) events. The anomaly correlation coefficient (ACC) score for ENSO can reach 0.75 at 6-month lead time. Furthermore, the extreme warm/cold Indian Ocean dipole (IOD) events are successfully predicted at 3- and even 6-month lead times. The whole ACC of IOD events between the observation and the prediction can reach 0.51 at 2-month lead time. There are reliable seasonal prediction skills for 2-m air temperature anomalies over most of the Northern Hemisphere, where the correlation is mainly above 0.4 at 2-month lead time, especially over the East Asia, North America and South America. However,the seasonal prediction for precipitation still faces a big challenge. The source of precipitation predictability over the East Asia can be partly related to strong ENSO events. Additionally, the anomalous anticyclone over the western North Pacific (WPAC) which connects the ENSO events and the East Asian summer monsoon(EASM) can be well predicted at 6-month lead time. (Liu Bo, Su Jingzhi, Ma Libin, Tang Yanli, Rong Xinyao,Li Jian, Chen Haoming, Liu Boqi, Hua Lijuan)

3.3 AMIP simulations of a global model for unified weather-climate forecast: Understanding precipitation characteristics and sensitivity over East Asia

A global model formulation for unified weather-climate forecast is evaluated, with emphasis on the climate simulations at typical hydrostatic resolutions. The internal sensitivity is explored by considering different dynamical configurations (resolution, solver type, transport scheme). After a basic assessment of the global mean climate, a detailed analysis of precipitation characteristics is extended to East Asia. The model shows a reasonable mean state, seasonal variation, frequency-intensity structure, and diurnal phase time.The artificial rainfall around the steep slopes of the Tibetan Plateau can be improved through choices in the dynamical configuration. The regional features characterized by “afternoon versus nocturnal-to-early-morning peaks” are properly distinguished. The hourly climatic features are comparable to super-parameterized CAM5.Different dynamical configurations demonstrate unique sensitivities related to underling physical mechanisms,which are studied from the perspective of the diurnal cycle for three representative regions. Over South China,the higher-resolution models decrease the weak-precipitation while increase intense rainfall, thus reducing the dry biases. This is contributed by enhanced grid and sub-grid scale motions associated with daytime convection progression. Over the central western China, the variable-resolution model better simulates the eastward propagating episodes characterized by a transition from convective to stratiform rainfall along the eastern slope of the Plateau. This reduces the positive biases at the high topography of the Plateau and alleviates the negative biases at the lower foot. Over the central eastern China, the model replicates the dominant role of large-scale governing factors in regulating the early morning rainfall peaks, and produces stratiform heating patterns.(Zhang Yi, Yu Rucong, Li Jian, Li Xiaohan, Rong Xinyao, Peng Xindong)

3.4 Evaluation of CMIP6 HighResMIP models in simulating precipitation over Central Asia

The High Resolution Model Intercomparison Project (HighResMIP) experiment within the Coupled Model Intercomparison Project phase 6 (CMIP6), for the first time, has provided an opportunity to evaluate the performance of climate models over complex topographies. Based on the HighResMIP’s historical simulations of atmospheric general circulation models, the performances of global high-resolution models,with a horizontal resolution finer than 50 km, in representing precipitation over Central Asia were evaluated using rain gauge observation datasets. All the models successfully reproduce the large precipitation regions that are located over the mountainous areas and the northern Central Asia. However, nearly all the models overestimate precipitation frequency over Central Asia and large overestimations of precipitation amount and frequency are located over the mountainous areas. Although the HighResMIP multi-model ensemble mean performs better than all individual models at simulating the spatial pattern of precipitation frequency,it is inferior to HadGEM3-GC31-HM and ECMWF-IFS-HR at simulating that of precipitation amount. The simulation performance exhibits remarkable regional differences. Over the Qilian Mountains, the relationship between precipitation and elevation is totally captured by climate models. In contrast, over the Tianshan Mountains, the models fail to simulate the decrease in precipitation frequency after elevation higher than the maximum precipitation elevation. Most models successfully reproduce the annual cycle shape of precipitation amount over the southern Central Asia, Qilian Mountains and Tianshan Mountains, but fail to reproduce it over the northern Central Asia. More than half of the high-resolution GCMs have a reduced bias relative to the corresponding low-resolution GCMs. The performances of most high-resolution GCMs in simulating precipitation pattern are well over the Tianshan Mountains. (Li Liangliang, Li Jian, Yu Rucong)

3.5 Convection-permitting modelling improves simulated precipitation over the central and eastern Tibetan Plateau

The Tibetan Plateau (TP) plays an essential role in influencing the global climate, and precipitation is one of its most important water-cycle components. However, accurately simulating precipitation over the TP is a long-standing challenge. In this study, a convection-permitting model (CPM, with 4 km grid spacing) that covers the entire TP was conducted and compared to two mesoscale models (MSMs, with model horizontal resolutions of 13 and 35 km) over the course of a summer. The results showed that the two MSM shave notable wet biases over the TP and can overestimate the summer precipitation by more than 4.0 mm day−1in some parts of the Three Rivers Source region. Moreover, both MSMs have more frequent light rainfall;increasing horizontal resolution of the MSMs alone does not reduce the excessive precipitation. Further investigation reveals that the MSMs have a spurious early-afternoon rainfall peak, which can be linked to a strong dependence on convective available potential energy (CAPE) that dominates the wet biases. Herein,we highlight that the sensitivity of CAPE to surface temperatures may cause the MSMs to have a spurious hydrological response to surface warming. Users of climate projections should be aware of this potential model uncertainty when investigating future hydrological changes over the TP. In comparison, the CPM removes the spurious afternoon rainfall and thus significantly reduces the wet bias simulated by the MSMs. In addition, the CPM also better depicts the precipitation frequency and intensity, and is therefore a promising tool for dynamic downscaling over the TP. (Li Puxi, Kalli Furtado, Zhou Tianjun, Chen Haoming, Li Jian)

3.6 Reevaluating the impacts of oceanic vertical resolution on the simulation of Madden-Julian oscillation eastward propagation in a climate system model

The upper ocean plays a critical role in determining the Madden-Julian oscillation (MJO) characteristics through modulating the tropical atmosphere-ocean interaction. By increasing the oceanic vertical resolution, its impacts on the MJO eastward propagation are discussed in this study by using a climate system model. With a refined vertical resolution in the upper ocean, warmer surface ocean and shallower mixed layer depth are produced in the tropics, which induces associated atmospheric changes as the response to the ocean feedbacks.Enhanced November‒April-mean vertically-integrated specific humidity is found around the equatorial region with the increased vertical resolution, which strengthens the zonal and meridional moisture gradients. The leadlag correlation of MJO precipitation demonstrates that the simulated MJO eastward propagation is improved with increased oceanic vertical resolution by improving the simulations of convective instability at the east of the MJO convective center, the boundary layer moisture convergence, the low- and upper-level circulation, and the vertical structure of equivalent potential temperature and diabatic heating. Moreover, the zonal asymmetry of the tendency of specific humidity is also improved by increasing the oceanic vertical resolution. The vertically-integrated moisture budget analysis is applied to further investigate the dominance of the moistening and drying processes. Results reveal that the drying processes are successfully reproduced over the central Indian Ocean in the case of increased oceanic vertical resolution, whilst the moistening processes are not well captured over the Maritime Continent and the MJO “detour” region. It suggests that additional modifications are needed to further improve the MJO simulation. (Ma Libin, Jiang Zijun)

3.7 Sensitivity of ENSO simulation to the convection schemes in the NESM3 climate system model:Atmospheric processes

The El Niño-southern oscillation (ENSO) is the most prominent climate system in the tropical Pacific.However, its simulation, including the amplitude, phase locking, and asymmetry of its two phases, is not well reproduced by the current climate system models. In this study, the sensitivity of the ENSO simulation to the convection schemes is discussed using the Nanjing University of Information Science and Technology Earth System version 3.0 (NESM3) model. Three convection schemes, including the default, the default coupled with the stochastic multicloud model (SMCM), and the default used in the Coupled Model Intercomparison Project phase 6 (CMIP6), are implemented. The model results reveal that the low-level cloud cover and surface net shortwave radiation are best represented over the tropical Pacific in the model containing the SMCM. The simulations of the ENSO behavior’s response to changes in the convection scheme are not uniform. The model results reveal that the model containing the SMCM performs best in terms of simulating the seasonal cycle of the sea surface temperature anomaly along the equatorial Pacific, the phase locking, and the power spectrum of ENSO but with a modest ENSO amplitude. Compared to the model containing the default convection scheme,the coupling of the default scheme and the SMCM provides a good simulation of the ENSO’s asymmetry,while the model containing the CMIP6 convection scheme outperforms the others in terms of the simulation of the ENSO’s amplitude. Two atmospheric feedback processes were further discussed to investigate the factors controlling the ENSO’s amplitude. The analyses revealed that the strongest positive atmospheric Bjerknes feedback and the thermodynamic damping of the surface net heat flux occurred in the model containing the CMIP6 convection scheme, suggesting that the atmospheric Bjerknes feedback may overwhelm the heat flux damping feedback when determining the ENSO’s amplitude. The results of this study demonstrate that perfectly modeling and predicting the ENSO is not simple, and it is still a large challenge and issue for the entire model community in the future. (Ma Libin, Jiang Zijun)

3.8 基于GWR模型的中国中东部降水与海拔高度关系特征分析

利用中国国家级地面气象站逐时降水资料，采用地理加权回归（GWR）模型系统分析了中国中东部暖季降水与海拔高度的关系，并将二者关系作为一种客观标准，评估了ECMWF-IFS 模式对2017年暖季降水的预报能力。主要结论如下：（1） 总体来看，中国中东部降水频率（强度）随海拔高度升高而增加（减小），二者在不同地区的贡献程度不同导致降水量与海拔高度关系的区域差异显著。（2） 通过对比午后短时和夜间长时降水事件与海拔高度关系的差异，发现午后短时降水事件的降水量主要随海拔高度升高而增加，且以降水频率与海拔高度关系的贡献为主。而夜间长时降水事件的降水量与海拔高度关系的区域一致性较差。相较于午后短时降水事件，夜间长时降水事件中有更多站点表现出降水量随海拔高度升高而减小的特征，在大地形周边陡峭地形处的站点所表现出的此种差异较东部孤立地形处更加显著。（3） 根据ECMWF-IFS 模式的评估结果，模式能够较好地刻画出中国中东部2017年暖季降水气候态的空间分布特征，且与观测具有较大的空间相关系数。但从降水与海拔高度关系来看，观测与模式的空间相关性偏弱。此外，模式能够表现出降水强度（频率）主要随海拔高度升高而减小（增加）的特征，但绝大多数站点在模式中的降水强度（频率）与海拔高度的负（正）回归关系要弱（强）于观测结果。（张铭明，李建，甘玉婷，李妮娜）

3.9 千米尺度数值预报模式对泰山地区暖季降水预报性能的评估

为深入认识对流可分辨模式对小尺度孤立地形区降水的预报性能，使用2017 年暖季（5—9 月）台站逐时降水观测数据，以小时尺度降水特征为指标，细致评估了千米尺度分辨率（3 km）的北京“睿图”短期数值预报子系统（RMAPS-ST）对泰山及其周边地区降水特征的预报能力，并对比了不同起报时次（北京时08:00和20:00）的预报差异。评估发现，RMAPS-ST可以再现泰山站的局地降水中心，但区域西南侧降水预报小于观测，而泰山站及其东北侧则相反。清晨和午后时段的降水预报与观测相比存在较大偏差。以泰山站为例，RMAPS-ST易于低估夜间至清晨时段的降水频率，这可能与模式对降水系统发展演变过程的预报偏差以及清晨泰山站弱降水事件的漏报有关；清晨泰山站降水强度的预报在不同起报时次的结果中存在差异，20:00起报存在大幅度高估的问题，进而导致其暖季平均降水量预报大于观测，而08:00起报对于清晨降水强度的高估不明显；08:00起报易高估泰山站午后的降水频率，这与其午后短历时降水事件数预报偏多有关，模式对山区热动力场的预报偏差是午后降水空报的可能原因。小时尺度降水特征已应用于中国气象局区域数值预报模式的业务评估体系中，本研究结果也表明，此类评估有助于深入认识千米尺度数值预报模式对降水日内变化的预报能力，从而为精细化降水产品的订正提供更详实的科学依据。（甘玉婷，陈昊明，李建）

3.10 2020年夏季江淮流域降水ECMWF模式预报偏差分析

基于CMPAS 多源融合降水和ERA5 再分析产品，评估全球高分辨率确定性预报产品（ECMWF）对2020 年梅汛期（6月10日至7月20日）极端强降水过程的预报性能。同时，基于面向对象的诊断评估方法（MODE），揭示ECMWF 模式对强降水落区的质心经纬度、面积、长度、宽度、轴角等空间特征的预报性能。结果表明：ECMWF 模式对于梅雨期的日降水量预报，在雨带的空间位置上，模式预报偏北、偏西的偏差较多；在落区形态上，模式预报的雨带面积偏大，轴角倾斜度更大。观测中江淮流域区域平均降水的日变化主峰值出现在清晨至上午，ECMWF 预报能够再现降水日变化特征。针对模式对主雨带南北落区质心位置预报偏差的评估表明，模式预报主雨带位置偏北的频次呈现出双峰分布的日变化特征，峰值出现在夜间和午后。雨带位置预报偏南的频次为单峰分布，峰值在上午。低空急流的日变化特征明显，且峰值时刻超前降水峰值时刻3 h，而ECMWF 预报急流峰值时刻则较观测早3 h。ECMWF 预报降水落区位置偏差与预报低层南风分量的强弱偏差相关，当对流层低层南风分量偏强时，雨带位置预报易偏北；南风分量较弱时雨带位置预报易偏南。针对ECMWF 预报位置偏北和偏差较小的两次典型强降水事件的对比分析，结果表明在小时尺度上急流与降水的日变化一致，ECMWF预报降水落区的偏北与前3 h 内强度更强的急流有关。（卜文惠，陈昊明，李普曦）

3.11 千米尺度模式降水的检验评估进展及展望

数值模式检验评估是理解模式偏差、完善数值模式和提升预报服务产品质量的有效方法，是现代数值模式发展的基础环节之一。传统的降水业务检验主要基于累积降水量的评估，较少考虑降水的发展演变过程。随着模式分辨率和降水精细化预报需求的提升，仅基于降水量的检验评估已不能满足定时、定点、定量的降水预报服务需求。在回顾已有降水检验评估方法的基础上，重点介绍基于降水频次、强度等小时尺度特征量及其日变化峰值时间的新评估方法，并示例给出此类评估结果对于深入认识日内降水预报偏差的应用价值。初步结果表明，相关方法和结论为丰富和深化数值模式评估，改进客观预报方法具有有益的启发作用。随着千米尺度模式在降水精细化预报业务中的广泛应用，未来仍需进一步发展和完善针对不同区域降水过程分类、分型的细致评估方法，并进一步研发面向快速更新循环系统和对流可分辨集合预报模式应用的检验评估指标，为千米尺度数值模式的改进和模式产品的合理应用提供科技支撑。（陈昊明，李普曦，赵妍）

3.12 基于GRIST模式的多年气候积分

确认了模式动力求解器的多尺度计算和物理性能的合理性。进而通过耦合物理过程，实现了全球模式准均匀网格（120 km）和变分辨率网格（120～30 km）的长期稳定气候积分，验证了前期所开发的模式动力框架在真实下垫面条件下的计算性能和稳定性。在小时尺度降水特征方面，GRIST模式具有和超级参数化模式相当的效果，准确区分了东亚地区以“午后v.s.午夜至清晨”为特点的的降水气候模拟。（张祎，李晓涵，王蕾，李建）

3.13 基于非结构网格的海洋模式动力框架研发和球面三维海洋平坦地形基准测试

在GRIST浅水框架的基础上研发了并行的球面海洋模式的三维动力框架。完成了三维正压海洋的无地形Rossby Wave测试和理想地形平流测试，确认了三维海洋模式离散的正确性。同时，开展了三维理想地形的斜压的Baroclinic Channel测试。（刘波，容新尧）

3.14 CAMS-CSM的大气分量更新为GRIST模式及多年稳定积分

基于ESMF底层耦合软件自主开发了适用于全球公里级分辨率以及多种网格结构的耦合框架，并基于新一代非结构网格大气模式GRIST和海洋模式MOM6建立了不同分辨率的新一代海陆气冰多圈层耦合模式，实现不同分辨率的稳定积分。其中，G6（120 km）分辨率大气+50 km海洋模式，实现稳定积分超过40年（20年spinup积分+20年freerun积分）；G5B3×4变分辨率（120～30 km）大气+25 km海洋稳定积分20年（spinup积分）；G8分辨率（15 km）大气+10 km海洋稳定积分40天。（容新尧，华莉娟，唐彦丽，马利斌，苏京志，刘波）

3.15 CAMS-CSM的集合资料同化系统研发及资料同化和回报试验

大气分量采用nudging方法，海洋分量采用考虑偏差订正的集合卡尔曼滤波方法，建立了CAMS-CSM的集合资料同化系统。大气同化CRA40再分析资料，海洋同化海表温度、卫星高度计、海表流速和温盐剖面数据，共20个集合成员，完成了10年的资料同化和回报试验。（马利斌，容新尧）

3.16 区域数值预报检验评估系统的完善及业务运行

面向区域数值预报系统发展和灾害天气精细化预报服务的业务需求，持续完善面向精细化预报的模式检验评估方法和指标体系，针对区域模式特点研发了降水时空演变过程的检验评估方法。基于此建立的区域高分辨率数值预报检验评估系统，通过局预报与网络司组织的业务化评审，实现业务运行。承担区域模式检验评估业务，针对“7·21”河南暴雨等多次强降水过程进行了系统评估，发布区域模式检验评估报告4期，评估结果得到数值预报中心、华北、华东和华南等模式研发团队以及国家气象中心和上海中心气象台等国家级和省级业务单位的认可。（陈昊明，李普曦，陆天舒，杨琳韵，周佰铨）

4 其他

4 Others

4.1 Wind characteristic in the complex underlying terrain as studied with CALMET system

In the present study, the wind data during two synoptic processes observed by the surface wind tower in the Tibetan Plateau and the high spatial resolution model data simulated by CALMET were be evaluated,the horizontal and vertical distribution characteristics of the model wind data around the region of the wind tower point and the reference point were be analysed. Results showed that the wind observation data during the two processes in the near-surface boundary were a well consistency. The horizontal distribution of the wind fields were consistent at the moment of the maximum wind speed during the two processes. The trend of the ten minute mean wind speed was upward with time variations during the two strong wind weather systems.The air was flowed from the mountain ridge to the mountain valley over the complex region. The vertical distribution of the wind speed had more relevant that the correlation coefficients were 0.6811 (α = 0.05) in the observation tower point and 0.8304 (α = 0.05) in the reference point between 16:00 pm Jan 30th and 16:00 pm Jan 31st. The advantages of using the advanced numerical models will be reflected over the extremely complex underlying terrain where the lack of observation stations. (Liao Rongwei, Fang Xiaoyi, Liu Huaiyu, Zhou Rongwei, Zhang Lei, Zhu Yuzhou, Zhang Dongbin, Meng Fanchao)

4.2 Study of wind characteristic in Tibet with numerical simulation

In order to analyse the model simulation results and study the impact of the horizontal resolution in model during different weather process in complicated topography areas in Tibet, the mean wind observation data from wind tower and high-resolution numerical model data were used, the characteristics of the mean wind and the turbulent kinetic energy (TKE) in the near-surface boundary layer around the region of the wind tower point and comparison point were analyzed. The results showed that the mean wind observation data were a well consistency. With time variations, the mean values of ten minute wind speed had an upward trend over time during different strong wind weather systems. The air flow and the spatial distribution of TKE were consistent around the region of the wind tower point and comparison point. The air flowed from high elevation areas (mountain ridge) to the low elevation areas (mountain valley) during different resolution numerical model data. Comparative analysis of different resolutions simulation results, the higher-resolution simulation result showed more clear in the characteristics of the air flow. The correlation had increased considerably between the different times of the mean value of the ten minute wind speed or TKE on the vertical layers, and the bias was decreased. (Liao Rongwe, Fang Xiaoyi, Zhang Dongbin, Zhu Yuzhou, Zhang Lei, Meng Fanchao)

4.3 A view for atmospheric unpredictability

Based on chaotic dynamics, this paper illustrated the necessity of research and the objective existence of atmospheric unpredictability. Actually, inaccurate forecast happens all the time in both operational weather forecasting and climate prediction in which atmospheric unpredictability hides. By means of discrete mathematics, this paper also defined the degree of hesitation and the predictable days with which to discuss and compare the relationship between the predictability and unpredictability of several different forecast objects. In addition, this paper discussed the approaches of evaluating the atmospheric predictability and unpredictability,emphatically showed the experience assessment method. At the last, this paper also proved the existence of atmospheric unpredictability by an example. (Cai Xiuhua, Cao Hongxing, Fang Xiaoyi, Sun Jingli, Yu Ying)

4.4 Identification of regional drought processes in North China using MCI analysis

Comprehensive identification of drought events is of great significance for monitoring and evaluating drought processes. Based on the data of daily precipitation, temperature and drought-affected areas of 403 meteorological stations in North China from 1960 to 2019, the comprehensive drought process intensity index(CDPII) has been developed by using the meteorological-drought composite index (MCI) and regional drought process identification method, as well as the EIDR theory method. The regional drought processes in the past 60 years in North China, including Beijing, Tianjin, Hebei, Shanxi and middle Inner Mongolia, were analyzed and identified. The result shows that the distribution characteristics of droughts with different intensities are as follows: The number of days of all annual-average mild droughts, moderate droughts and severe droughts was the highest in Tianjin and that of extreme droughts was the highest in Shanxi. The number of days of mild droughts was the highest in May and the lowest in January. The number of days of moderate droughts was highest in June. The number of days with mild and moderate droughts showed an overall increasing trend, while the number of days with severe droughts and above showed an overall decreasing trend (through a 0.05 significance test). The number of drought days was the highest in the 1990s. The annual frequency of drought was between 66.7% and 86.7%; the drought frequency in Hebei was the highest at 86.7%, followed by Beijing at 80%. There were 75 regional drought processes in North China from 1960 to 2019, and the correlation coefficient between process intensity and the drought-affected area was 0.55, which passed the 0.01 significance test. The comprehensive intensity of drought process from 27 April to 1 September 1972 was the strongest. From 18 May to 31 October 1965, the drought lasted 167 days. The overall drought intensity had a slight weakening trend in the past 60 years. A total of 75 regional drought processes occurred in North China,and the process intensity showed a trend of wavy decline with a determination coefficient (R2) of 0.079 (0.05 significance test). Overall, the regional drought process identification method and strength assessment result tally with the drought disaster, which can better identify the regional drought process. Furthermore, including the last days, the average intensity, and average scope comprehensive strength, there are many angles to monitor and evaluate the droughts and drought processes. These provide a reference for drought control and decision-making. (Cai Xiuhua, Zhang Wenqian, Fang Xiaoyi, Zhang Qiang, Zhang Cunjie, Chen Dong, Chen Cheng, Wen Jiefan, Yu Ying)

4.5 我国城市通风廊道研究与实践进展

城市通风廊道作为协同解决城市气候问题的一种有效手段被纳入国土空间规划中。本文回顾了城市通风廊道规划的由来，总结了其功能与机理，重点梳理了我国通风廊道规划研究与实践进展，旨在为国土空间规划中科学、合理地开展廊道划定工作提供参考。通风廊道的主要功能是为气流进入城市建成区内部提供通道，引导城市空间形态优化以减少风的损耗，结合科学保护城市开敞空间，促进局地空气流通，对缓解城市热岛效应、减轻空气污染、降低建筑物能耗和提高城市宜居性也有积极作用；而城市内部的功能布局、用地类型、空间形态、建筑高度和密度等因素会很大程度上对这种功能的发挥造成影响。我国通风廊道规划研究与实践经历了4个发展阶段，分别为古代风水与营城、新中国成立至20世纪90年代末的恢复阶段、21世纪初期的发展阶段和2012年至今生态优先的新阶段。全国超过2/3的省开展了相关工作，在技术方法、应用策略和政策法规3个方面取得了重要进展。通风廊道规划体现了快速城镇化进程背景下中国城市对发展质量的追求，但在规划过程中的介入时间、跨领域融合、与城市设计衔接及廊道绩效评估机制等工作途径层面仍存在问题，在气象观测分析、仿真模拟、效果评估等技术层面也有诸多提升空间。（房小怡，李磊，刘宛，任超，王佳文，程宸，于瀛，张硕，杜吴鹏）

4.6 福清核电厂厂址区域龙卷风设计基准参数的估算

基于1959—2017年福清核电厂区龙卷风的调查资料，采用Rankine 涡模型估算该区域超过某一特定风速的概率分布，通过概率值导出设计基准龙卷风和基准设计风速，按照压降模型计算出龙卷风的压降，研究结果表明：福清核电评价区域龙卷风的总压降为4.29 kPa；平移速度为13.8 m/s，最大旋转风速为57.6 m/s，最大压降速率为1.18 kPa/s，基准设计风速为71.4 m/s，属于F3 级别的龙卷风；在125 kg 下落的穿甲弹类和2.5 cm 实心钢球两种不同情景下计算出的龙卷风产生的飞射物的最大水平碰撞速度均为24.99 m/s、碰撞动量依次为3123.75 kg/ （m/s）和1.615 kg/ （m/s）。这些计算结果，从龙卷风的角度，为政府相关部门在规划和建设福清核电厂时提供了可靠的理论依据。（蔡秀华，吕文忠，陈龙泉）

4.7 影响福清厂址区域热带气旋工程参数估算

统计和分析了福清核电厂区热带气旋的分布特征及极值气压和风速。以1949—2017年影响福清核电厂区的台风资料为基础，利用Gumbel-Ⅰ型极值分布概率方法和Pearsonl-Ⅲ型分布函数方法及梯度风公式对影响福清核电厂区最低中心气压和最大风速进行经验拟合，估算出可能最大热带气旋的强度参数，并经比较后得出更加合理的数值，百年一遇的可能最低气压估值为887 hPa。在此基础上估算出的10 m和200 m高度百年一遇的10 min平均最大风速和3 s阵风的极大风速依次为52.7、68.4、87.7和113.8 m/s。（蔡秀华，吕文忠，陈龙泉，范雯杰）

4.8 1961—2019年河南秋季连阴雨气候特征分析

利用河南省103个地面气象站1961—2019年逐日资料，采用滑动平均、小波分析、EOF分解等方法分析了河南秋季区域性连阴雨降水的时空分布特征，采用自组织神经网络方法对河南秋季区域性连阴雨环流进行客观分型。结果表明：（1）河南区域性秋季连阴雨以7～9 天的过程居多，其次为10天以上过程。20世纪60年代和80年代是河南区域性秋季连阴雨多发期，90年代为连阴雨少发期，70年代和21世纪10年代发生次数与近59年年均频次基本持平，21世纪10年代河南区域性秋季连阴雨比其它年代强，20世纪90年代强度最弱。20世纪70年代区域性秋季连阴雨年平均过程降水量存在准5年周期，21世纪00年代前半期及10年代区域性秋季连阴雨年平均过程降水量存在2～3年周期。（2）近59年河南区域性秋季连阴雨平均降水量分布上，黄河以南地区比黄河以北地区降水多，山区比平原地区多，降水大值区与山区分布基本一致。河南区域性秋季连阴雨降水空间分布型上，最主要的特征是全省一致变化型，其次是南北反相型。（3）SOM方法不仅可以区分出环流形态上的差别，还可以区分出环流型的发生时间和连阴雨期间环流的阶段性演变特征。SOM分型得到8类区域性连阴雨环流型，从天气学意义上可归结为阻塞型、低槽型和平直环流型。大部分河南秋季区域性连阴雨过程为2～3种环流型的组合，不同环流型之间存在转换关系。（杜明哲，鲁坦，朱玉周，廖荣伟，房小怡）

4.9 界壳论探讨生态环境安全

从界壳论来探讨生态环境安全问题，介绍了界壳的概念及其理论，指出界壳论在该安全问题中的可应用性。就古文明衰落、全球增暖、生物入侵等问题进行了论述。界壳现象广泛存在于自然界和人类社会中，如栅栏、国界、防火墙、人的衣服等，例如蜗牛长有一个坚硬的壳用以保护自己。界壳被定义为处在系统外围能卫护系统且与环境进行交换的中介体，它是系统的一部分，又和环境相毗邻。界壳论研究界壳的普适性规律，从一般意义上研究界壳的结构、功能和行为。生物入侵是典型的界壳论问题，一个物种离开原来的栖息地，进入到另一个原不是它生息的地方，即从一个该物种生存的界壳进入到另一个新界壳中生活。若大量繁殖，会造成生态环境危害。（蔡秀华，于瀛 ，曹鸿兴 ，彭誉葆 ，范雯杰 ，吕文忠）