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 Record-breaking Meiyu rainfall around the Yangtze River in 2020 regulated by the subseasonal phase transition of the North Atlantic Oscillation

In 2020,the Yangtze River (YR) suffered a long-persisting Meiyu season.The accumulated rainfall broke its record since 1961 and caused severe flooding and death in China.Our results show the sequential warm and cold Meiyu front regulated by the North Atlantic Oscillation (NAO) was responsible for this unexpected extreme Meiyu event.From 11 to 25 June with the positive NAO,the interaction between the South Asian high (SAH) and the western Pacific subtropical high maintained a warm front to strengthen the rainband north of the YR.Afterward,the coupling between SAH and midlatitude Mongolian Cyclone induced a cold front,which retreated the rainband to the south of YR from 30 June to 13 July with the negative NAO.Although the ECMWF S2S model successfully predicted the warm-front-related Meiyu rainband,it failed to forecast the Meiyu rainband in the cold-front period,suggesting a great challenge of S2S forecasting on Meiyu rainfall.(Liu Boqi,Yan Yuhan,Zhu Congwen,Ma Shuangmei,Li Jianying)

1.2 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 the Yangtze-Huaihe River basin.Secondly,southwest vortices,the important rainfall producers in China,tend to be generated and maintained when TPVs are located over the 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)

1.3 Boosting effect of tropical cyclone “Fani” on the onset of the South China Sea summer monsoon in 2019

The late onset of the South China Sea (SCS) summer monsoon (SCSSM) has traditionally been attributed to El Niño events.However,the SCSSM built up around Pentad 26 in 2019,which was 10 days earlier than expected following the 2018/2019 El Niño event.The present study showed that the early onset of the SCSSM in 2019 could be ascribed to tropical cyclone (TC) Fani over the Bay of Bengal (BOB) from 27 April to 3 May 2019.Before the landfall in Pentad 24,the anomalous condensation heating released by the TC not only shifted the South Asian high (SAH) northward but also reinforced the upper-level barotropic trough to the west of the Tibetan Plateau (TP) at midlatitudes.This phenomenon facilitated the early establishment of monsoon convection by intensifying the upper-level pumping over the SCS.In addition,the TC carried abundant moisture to the eastern TP and the SCS when it landed in the eastern India in Pentad 25 and strengthened the local rainfall in front of the deeper midlatitudinal trough in the upper troposphere over the TP,which in turn released more condensation heating to warm the tropospheric atmosphere.Afterwards,this warmer air advected downstream to the north of the SCS by the basic flow,resulting in the further northward shift of the SAH and the stronger monsoon onset convection over the SCS.Such an upscaling effect of the TC prevented the onset time of the 2019 SCSSM from being accurately forecasted by the ECMWF S2S model.(Liu Boqi,Zhu Congwen)

1.4 Diagnostics of westward propagating East Asian monsoon low-pressure systems that reach the Indian monsoon region

The East Asian monsoon low-pressure systems (MLPSs) are important rain-producing weather disturbances in East Asia during the summer (June,July and August).After formation,they primarily move westward and impact the southern China and Indochina Peninsula.We analyzed the climate distribution characteristics of generation locations and frequency of different intensity events in westward East Asian MLPSs during the summers of 1979–2012.The East Asian and the Indian MLPSs are relatively independent weather systems and the Indochina Peninsula area separates them.The westward East Asian monsoon lowpressure system (MLPS) rarely moves to the Indian monsoon region (IMR).When an East Asian MLPS crosses the boundary between the East Asian monsoon region (EAMR) and the IMR,the potential vorticity (PV) axis near the low-pressure center remains vertical with height,while the PV axis near the center of the East Asian MLPSs that do not cross the boundary shifts to the northeast with height.The East Asian MLPSs that reach the IMR are stronger before crossing the boundary than the MLPSs that do not cross the boundary.The strength of the MLPS that approach the boundary gradually weakens during the westward movement.The middle and high-level PV center shifts to the northeast of the low-pressure center,which causes the MLPS to turn and fail to cross the boundary between EAMR and IMR.Variation of the PV budget at 500 hPa over time shows that a strong positive PV tendency forms on the north side of low-pressure center mainly due to horizontal adiabatic PV advection.The PV center gradually shifts to the north and the MLPS turns.The PV budget at 500 hPa for the East Asian MLPS that reach the IMR shows that the horizontal and vertical adiabatic advection PV transport has a strong positive tendency on the northwest side of the low-pressure center,causing the MLPS to continue moving northwestward and eventually cross the boundary to reach the IMR.(Shi Xiaohui,George Kiladis,Wen Min)

1.5 Diverse impacts of the Siberian high on surface air temperature in Northeast China during boreal winter

The robust negative correlation between the Siberian high (SH) intensity and winter surface air temperature (SAT) in Northeast China has been widely used for winter climate seasonal forecasting.However,this traditional viewpoint varies when considering the changes in SH spatial extension during boreal winter.Here,a newly defined SH index representing both SH intensity and spatial extension shows two distinct influences of a stronger SH on the SAT in Northeast China.The stronger SH with a decreasing eastern edge,which features a meridional see-saw dipole of abnormal circulations centred over the Arctic and Siberia,respectively,results in a warmer winter over Northeast China.In this situation,the decreasing SH is linked to the enhanced Arctic Polar vortex through a tropospheric–stratospheric interaction.However,a stronger SH with an expanding eastern edge is coupled with the East Asian trough (EAT) and results in a colder winter in Northeast China.The SH and EAT anomalies are embedded in a zonally oriented wave-train originating from the North Atlantic in the troposphere.Because of the diversity of impacts,the SH intensity and zonal extension should be jointly considered for seasonal forecasting of winter SAT anomalies in Northeast China.(Liu Boqi,Zhu Congwen)

1.6 Seasonal forecast of South China Sea summer monsoon onset disturbed by cold tongue La Niña in the past decade

It has been suggested that a warm (cold) ENSO event in winter is mostly followed by a late (early)onset of the South China Sea (SCS) summer monsoon (SCSSM) in spring.Our results show this positive relationship,which is mainly determined by their phase correlation,has been broken under recent rapid global warming since 2011,due to the disturbance of cold tongue (CT) La Niña events.Different from its canonical counterpart,a CT La Niña event is characterized by surface meridional wind divergences in the central-eastern equatorial Pacific,which can delay the SCSSM onset by enhanced convections in the warming Indian Ocean and the western subtropical Pacific.Owing to the increased Indian-western Pacific warming and the prevalent CT La Niña events,empirical seasonal forecasting of SCSSM onset based on ENSO may be challenged in the future.(Jiang Ning,Zhu Congwen)

1.7 Increased European heat waves in recent decades in response to shrinking Arctic sea ice and Eurasian snow cover

In recent decades,unprecedented extreme summer heat waves have occurred in Europe,and they have exhibited an increasing trend since the 1970s.Although previous studies have suggested that these recent hot European summers could have been instigated by the underlying surface thermal conditions,the possible influence of shrinking Arctic sea ice and Eurasian snow cover on heat waves are not well understood.Herein,we present evidences obtained via observational analyses and numerical experiments indicating that the interdecadal increase in European heat waves is closely linked to the reductions in Arctic sea ice concentration(ASIC) and Eurasian snow cover fraction (EASC) across mid-high latitudes via the excitation of the anomalous Eurasian wave train.The combined effects of declined ASIC and EASC,accompanied by the drier soil and the stronger heat flux,tend to weaken the poleward temperature gradient at mid-high latitudes and affect the midlatitude jet stream and transient eddy activities.These dynamic and thermodynamic circulations increase the likelihood of more persistent European blocking events that favor frequent and strengthened heat waves.Further projection analysis of simulations from 13 CMIP5 climate models suggests that Europe may experience more hot summers as the ASIC and EASC continue to decline over the next century.(Zhang Ruonan,Sun Chenghu,Zhu Jieshun,Zhang Renhe,Li Weijing)

1.8 Opposing trends of winter cold extremes over Eastern Eurasia and North America under recent Arctic warming

Under recent Arctic warming,boreal winters have witnessed severe cold surges over both Eurasia and North America,bringing about serious social and economic impacts.Here,we investigated the changes in daily surface air temperature (SAT) variability during the rapid Arctic warming period of 1988/1989–2015/2016,and found the daily SAT variance,mainly contributed by the sub-seasonal component,which shows an increasing and decreasing trend over the eastern Eurasia and North America,respectively.Increasing cold extremes(defined as days with daily SAT anomalies below 1.5 standard deviations) dominated the increase of the daily SAT variability over the eastern Eurasia,while decreasing cold extremes dominated the decrease of the daily SAT variability over North America.The circulation regime of cold extremes over the eastern Eurasia (North America) is characterized by an enhanced high-pressure ridge over the Urals (Alaska) and surface Siberian(Canadian) high.The data analyses and model simulations show the recent strengthening of the high-pressure ridge over the Urals was associated with warming of the Barents-Kara seas in the Arctic region,while the high-pressure ridge over Alaska was influenced by the offset effect of Arctic warming over the East Siberian-Chukchi seas and the PDO (Pacific decadal oscillation)-like sea surface temperature (SST) anomalies over the North Pacific.The transition of the PDO-like SST anomalies from a positive to negative phase cancelled the impact of Arctic warming,reduced the occurrence of extreme cold days,and possibly resulted in the decreasing trend of daily SAT variability in North America.The multi-ensemble simulations of climate models confirmed the regional Arctic warming as the driver of the increasing SAT variance over the eastern Eurasia and North America and the overwhelming effect of SST forcing on the decreasing SAT variance over North America.Therefore,the regional response of winter cold extremes at midlatitudes to the Arctic warming could be different due to the distinct impact of decadal SST anomalies.(Ma Shuangmei,Zhu Congwen)

1.9 Combined Impacts of warm central equatorial Pacific Sea surface temperatures and anthropogenic warming on the 2019 severe drought in East China

A severe drought occurred in East China (EC) from August to October 2019 against a background of longterm significant warming and caused widespread impacts on agriculture and society,emphasizing the urgent need to understand the mechanism responsible for this drought and its linkage to global warming.Our results show that the warm central equatorial Pacific (CEP) sea surface temperature (SST) and anthropogenic warming were possibly responsible for this drought event.The warm CEP SST anomaly resulted in an anomalous cyclone over the western North Pacific,where enhanced northerly winds in the northwestern sector led to decreased water vapor transport from the South China Sea and enhanced descending air motion,preventing local convection and favoring a precipitation deficiency over EC.Model simulations in the Community Earth System Model Large Ensemble Project confirmed the physical connection between the warm CEP SST anomaly and the drought in EC.The extremely warm CEP SST from August to October 2019,which was largely the result of natural internal variability,played a crucial role in the simultaneous severe drought in EC.The model simulations showed that anthropogenic warming has greatly increased the frequency of extreme droughts in EC.They indicated an approximate twofold increase in extremely low rainfall events,high temperature events,and concurrently dry and hot events analogous to the event in 2019.Therefore,the persistent severe drought over EC in 2019 can be attributed to the combined impacts of warm CEP SST and anthropogenic warming.(Ma Shuangmei,Zhu Congwen,Liu Juan)

1.10 Variations in the annual cycle of the East Asian monsoon and its phase-induced interseasonal rainfall anomalies in China

The East Asian monsoon (EAM) exhibits a robust annual cycle with significant interannual variability.Here,the authors find that the EAM annual cycle can be decomposed into the equinoctial and solstitial modes in the combined sea level pressure,850-hPa low-level wind,and rainfall fields.The solstitial mode shows a zonal pressure contrast between the continental thermal low and the western Pacific subtropical high,reaching its peak in July and dominating the East Asian summer monsoon.The equinoctial mode shows an approximate zonal contrast between the low-level cyclone over the east of the Tibetan Plateau and the western Pacific anticyclone over the east of the Philippines.It prevails during the spring rainy season in South China and reaches its peak in April.The interannual variations of the lead-lag phase of the two modes may result in the negative correlation of rainfall anomalies in North China between spring and fall and in South China between winter and summer,which provides a potential basis for the across-seasonal prediction of rainfall.The warm phase of ENSO in winter could give rise to the reverse interseasonal rainfall anomalies in South China,while the SST anomaly in the Northwest Pacific Ocean may regulate the rainfall anomaly in North China.(Jiang Song,Zhu Congwen,Jiang Ning)

1.11 Roles of Tibetan Plateau vortices in the heavy rainfall over southwestern China in early July 2018

Tibetan Plateau vortices (TPVs),generated over the Tibetan Plateau,are important rainfall triggers in the southwestern and eastern China when moving off the plateau.The heavy rainfall event that occurred over the southwestern China in early July 2018 caused severe floods and landslides,leading to substantial damage to normal production and life.In this work,the roles of TPVs in the rainfall are investigated based on NCEP FNL (final) operational global analysis and forecast data from the global data assimilation system (GDAS).The results show that TPVs over the eastern Tibetan Plateau favor ascending motion to the east of the plateau,which is conducive to precipitation and therefore the genesis of southwest vortices (SWVs).Generally,the ascending motion in the southwestern China is stronger when TPVs exist,compared with the other scenarios in which the TPVs are absent.Ascending motion is always observed on the east side of TPVs,which is greatly attributed to the positive vertical vorticity and warm centers in the TPVs.In this fashion,the influence of TPVs is not merely limited around their centers,but further eastward.Therefore,the effects of TPVs may have been underestimated in previous investigations; the TPVs located over the eastern Tibetan Plateau that do not move off the plateau should also be seriously considered in future precipitation predictions.(Li Lun,Zhang Renhe,Wu Peili,Wen Min,Duan Jianping)

1.12 Representation of the boreal summer tropical Atlantic-western North Pacific teleconnection in AGCMs:Comparison of CMIP5 and CMIP6

Previous studies have revealed that warm (cold) sea surface temperature (SST) anomalies in the northern tropical Atlantic (NTA) can enhance (weaken) the anomalous low-level anticyclone over the western North Pacific (WNP) during boreal summer.This study assesses the ability of current atmospheric general circulation models (AGCMs) to simulate such an NTA-WNP connection by using Atmospheric Model Intercomparison Project experiments from 23 Coupled Model Intercomparison Project Phase 5 (CMIP5) and 35 CMIP6 climate models.It is shown that both the CMIP5 and CMIP6 multimodel ensemble (MME) averages and the majority of the individual AGCMs can reasonably reproduce the observed pattern of the NTA-related anomalous anticyclone over the WNP during boreal summer.Overall,the performance of the CMIP6 AGCMs in representing the NTA-WNP connection is similar to that of the CMIP5 AGCMs,except that the former tends to have a smaller spread than the latter among models.Additionally,both the CMIP5 and CMIP6 MME averages as well as the individual models can reasonably represent the mechanism responsible for the boreal summer NTA-WNP connection,which involves a zonally westward-extending overturning circulation over the Pacific–Atlantic oceans.Furthermore,the intensity of the NTA-related WNP anomalous anticyclone is positively correlated with that of the WNP local climatological convection activity for both the CMIP5 and CMIP6 AGCMs,implying that better representation of the WNP climatological convection activity may be crucial for improving the skill of AGCMs to simulate the boreal summer NTA-WNP connection.However,model bias in the simulation of climatological convection activity over the WNP remains large for the current CMIP6 AGCMs,although the bias is reduced over most of the tropical and subtropical Pacific-Atlantic regions compared to that for the CMIP5 AGCMs during boreal summer.(Zuo Jinqing,Sun Chenghu,Li Weijing,Wu Jie,Ren Hongchang)

1.13 Evaluation of NCEP-FNL and ERA-Interim data sets in detecting Tibetan Plateau vortices in May-August of 2000-2015

Tibetan Plateau vortices (TPVs) are important rainfall triggers in the southwestern and eastern China after they move off the plateau.Characteristics of the moving-off TPVs derived from two gridded data sets,the final operational global analysis data from the global forecasting system of National Centers for Environment Prediction (NCEP-FNL) and the ERA-interim data from the European Centre for Medium-Range Weather Forecasts,are evaluated against the Yearbooks of Tibetan Plateau Vortex and Shear Line (YB).Generally,NCEP-FNL captures more moving-off TPVs in YB than ERA-interim does.Both gridded data sets perform the best in capturing the TPVs in YB in May and the worst in July.Zonal propagation distances of TPVs are apparently better revealed in NCEP-FNL than in ERA-interim,and the performances of NCEP-FNL and ERA-interim in showing the meridional propagation distances and lifespans are similar.Positional deviations of TPVs in NCEP-FNL and ERA-interim reveal that both data sets show good skills in presenting the TPVs locations,and NCEP-FNL generally performs better than ERA-interim.In both gridded data sets,positional deviations of TPVs are larger before the TPVs move off the plateau than after moving off,and zonal deviations are always larger than meridional deviations.Before the TPVs move off,NCEP-FNL shows no specific preference for the directions of positional deviations,whereas ERA-interim tends to present further west TPVs locations relative to those in YB.After the TPVs move off,TPVs in both gridded data sets are always observed to the west and north of the TPVs in YB,which is more significant in ERA-interim.(Li Lun,Zhang Renhe,Wu Peili)

1.14 Characteristics of convections associated with the Tibetan Plateau vortices based on geostationary satellite data

Tibetan Plateau vortices (TPVs) are major rain producers over the Tibetan Plateau,some of which can move eastward off the plateau and trigger heavy rainfall over the southwestern and eastern China.The infrared black body temperature (TBB) data,derived from geostationary meteorological satellite FY-2E in the period of May–August of 2010–2014,is utilized to investigate the characteristics of convections associated with the moving-off TPVs.During May–August,negative TBB anomalies appear over the tracks of the TPVs,but are located further north of the tracks in May and June than in July and August.Relative to the centre of TPVs,low TBB appears in the southeast and extends from southwest to northeast,whose values are smaller and ranges are larger before the TPVs move off than after moving off.Distributions of TBB at different evolution times show a close relationship with the characteristics of TPVs.The intensity,scales and shape of the convections linked to the TPVs are explored in the coordinates whose origin is the composite centre of the convections.Generally,convections associated with the TPVs are elliptic and stretch from southwest to northeast,with average spatial scales of approximately 3° in the zonal and 2° in the meridional direction.Convections in July and August are more intensive and lie southwest-northeast,and those in May and June are weaker with a quasizonal orientation.The horizontal scales of convections are much larger in May and July than those in June and August.Convections are usually larger and more intense before the TPVs move off than after moving off,corresponding to the changes of TPVs.(Li Lun,Zhang Renhe,Wu Peili,Wen Min,Li Bo)

1.15 Structure characteristics of the vortices moving off the Tibetan Plateau

Tibetan Plateau vortices (TPVs) are major rainfall producers generated over the Tibetan Plateau,and the ones moving off the Tibetan Plateau can trigger heavy rainfall over the eastern China.The structure characteristics of the moving-off TPVs are investigated based on the final operational global analysis data (FNL)from the National Centers for Environmental Prediction (NCEP).Generally,the TPVs show different dynamic and thermodynamic structures before and after they move off the Tibetan Plateau.Specifically,the structures of TPVs at the genesis and peak times before they move off the Tibetan Plateau are distinguishing,and different structure characteristics are also found at the times when the TPVs just move off the Tibetan Plateau and when the intensity reaches the peak after moving off.In addition,the moving-off TPVs are divided into two groups according to their lifespans after moving off the plateau,and the structure characteristics of these two groups of TPVs are further compared.Furthermore,on the basis of the evolution features of the moving-off TPVs,the relationship between the structures of TPVs and their intensity is discussed.It is inferred that the evolution of the TPVs is determined by the structures of TPVs themselves to some extent.(Li Lun,Zhang Renhe,Wen Min)

1.16 A statistical comparison of the westerly wind bursts between the positive and negative phases of the PDO

To understand the diversity of the El Niño-Southern Oscillation (ENSO) under the background of Pacific decadal oscillation (PDO) during recent decades,characteristics of westerly wind bursts (WWBs)during positive and negative phases of the PDO were analyzed.It is shown that,during the ENSO developing period,the El Niño evolution may be affected by stronger or more frequent WWBs in the positive PDO phase than in the negative PDO phase.The sustained effects of atmospheric dynamics on the equatorial ocean can be indicated by the accumulated WWB strength,which contains most WWB characteristics,including the accumulated days,occurrence frequency,strength,and spatial range of WWBs.The synoptic/climate systems that are directly related to WWBs show a wider spatial distribution in the positive PDO phase than in the negative PDO phase.(Shi Yunhao,Su Jingzhi)

1.17 A new equatorial oscillation index for better describing ENSO and westerly wind bursts

An equatorial oscillation index (EOI) is defined,based on the zonal gradient of sea surface pressure between the western Pacific and eastern Pacific along the equator,to describe the distribution of wind and pressure within the equatorial Pacific.The EOI has a stronger correlation with the Niño3.4 sea surface temperature anomaly (SSTA),as well as with westerly/easterly wind bursts (WWBs/EWBs),showing a superiority over the southern oscillation index (SOI).In general,the EOI is consistent with the SOI,both of which reflect large-scale sea level pressure oscillations.However,when there are inconsistent SSTAs between the equator and subtropical regions,the SOI may contrast with the EOI due to the reverse changes in sea level pressure in the subtropical regions.As a result,the SOI fails to match the pattern of El Niño,while the EOI can still match it well.Hence,the EOI can better describe the variability of the Niño3.4 SSTA and WWBs/EWBs.The correlation between the SOI and Niño3.4 SSTA falls to its minimum in May,due to the large one-month changes of sea level pressure from April to May in the subtropical southern Pacific,which may be related to the spring predictability barrier (SPB).The newly defined EOI may be helpful for monitoring El Niño-Southern Oscillation (ENSO) and predicting ENSO.(Shi Yunhao,Su Jingzhi)

1.18 中国夏季降水预测因子潜在技巧分布图及应用

影响我国夏季汛期降水异常的因子繁多，不同因子之间复杂的相互作用制约我国夏季降水季节预测水平。目前动力模式对降水预测技巧水平较低，如何开发客观统计预报方法，提高我国夏季降水预报技巧依然存在挑战。该文基于最小二乘法拟合和交叉检验方法，提出一种搜索预测因子潜在预测技巧的方法( 潜在技巧分布图)，并基于该方法开发预测因子自动选择器，建立中国夏季降水异常自动统计预测模型。与传统线性相关分析相比，潜在技巧分布图不受极端气候事件影响，可直观展现具有显著预测技巧的前兆信号，而预测因子自动选择器则能从潜在技巧分布图中自动筛选最优预测因子，获得逐年不同的预测因子，更符合中国夏季降水异常影响因子多样性的客观事实。在完全剔除预测当年信息的回报试验中，该预测模型对1999—2019年中国夏季汛期降水异常的历史回报技巧明显高于动力模式。通过方差订正，历史回报降水的PS评分从71.00分提高到82.10分，显示了该模型的潜在预报潜力。（刘伯奇，祝从文）

1.19 多要素表征的东亚季风区准双周振荡特征

为评估不同要素对东亚季风区大气准双周振荡的表征能力，对大气向外长波辐射（OLR）、500 hPa位势涡度、850 hPa相对涡度、850 hPa风场和750 hPa比湿等要素的准双周振荡特征进行对比。结果表明，无论是准双周振荡还是季节内振荡，南亚季风区和东亚季风区存在各自独立的活动中心，准双周振荡普遍强于季节内振荡，且准双周振荡在南海最强，各要素均表现出这种分布特征。OLR及500 hPa位势涡度、850 hPa相对涡度、850 hPa纬向风描述的准双周振荡能很好地表现出东亚季风区大气准双周振荡向西北方向传播的特征，但500 hPa位势涡度、850 hPa相对涡度、850 hPa纬向风北传更强，北传速度更快。850 hPa经向风的准双周振荡呈明显西移特征，但北移特征不明显，北传速度最慢。而750 hPa比湿准双周振荡呈东南向传播。不同要素准双周振荡的强度略有差异，其中750 hPa比湿与其他要素的差异大。总体而言，750 hPa比湿不能较好地表现出东亚季风区大气准双周振荡活动的主要西传模态特征，可能与受到降水相态转化、热量释放等复杂的物理过程影响有关；其余要素能很好地表征东亚季风区大气准双周振荡的西传模态，其中500 hPa位势涡度和850 hPa相对涡度准双周振荡特征一致性高。（李靖怡，王遵娅，温敏）

1.20 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）研究团队先后参加了16次全国或区域气候会商会以及冬奥会专题会商，为国家气候中心、国家海洋环境预报中心、水利部黄河水利委员会、东北区域气象中心、冬奥会气象中心等部门提供了及时准确的气温降水预测结果。准确预测出2020年汛期我国北方地区降水异常偏多、西北地区降水异常偏少的情况，3月预测结果对我国北方和西部地区夏季平均降水距平百分比的PS评分分别为80.8和77.4分，为我国北方和西部地区的汛期气象防灾减灾提供了重要参考；同时，于10月不仅准确预测出2020/21年冬季我国东北地区整体偏冷、其余地区整体偏暖的气候格局，还预测出12月全国性冷空气活动频繁的次季节特征，为国家防灾减灾提供了重要参考。

在决策气象服务方面，S2S团队针对新冠肺炎传播的有利气温条件，及时开展专题研究，于今年6月初报送了气象决策服务材料。入梅以来，我国南方地区发生了严重汛情。S2S团队立即组织相关科研人员开展分析研判，指出了6月长江以南地区发生持续性强降水的主要原因是中纬度频繁的冷空气活动，并预测了盛夏主雨带异常的移动和气温变化趋势，于7月上旬撰写上报气象决策服务材料。入冬以来，S2S团队根据气温预测结果，针对冬季新冠疫情传播特征和春播期南方干旱撰写了2份决策服务材料，获得良好的决策支撑效果，相关工作被评为中国气象局2020年全国气象服务优秀案例。（刘伯奇，祝从文，马双梅，鄢钰函，蒋宁，苏京志）

2 气候特征与气候变化

2 Climate characteristics and climate change

2.1 Anthropogenically-driven increases in the risks of summertime compound hot extremes

Compared to individual hot days/nights,compound hot extremes that combine daytime and nighttime heat are more impactful.However,past and future changes in compound hot extremes as well as their underlying drivers and societal impacts remain poorly understood.Here we show that during 1960–2012,significant increases in Northern Hemisphere average frequency (about 1.03 days per decade) and intensity (about 0.28 °C per decade) of summertime compound hot extremes arise primarily from summer-mean warming.The forcing of rising greenhouse gases (GHGs) is robustly detected and largely accounts for observed trends.Observationally-constrained projections suggest an approximate eightfold increase in the hemispheric-average frequency and a threefold growth in intensity of summertime compound hot extremes by 2100 (relative to 2012),given uncurbed GHG emissions.Accordingly,endof-century population exposure to compound hot extremes is projected to be four to eight times the 2010s level,dependent on demographic and climate scenarios..(Wang Jun,Chen Yang,Simon F,B,Tett,Yan Zhongwei,Zhai Panmao)

2.2 Increasingly uneven intra-seasonal distribution of daily and hourly precipitation over East China

It has been long appreciated that precipitation falls unevenly in time,but the degree of unevenness and its changes with warming have been seldomly quantified.These quantifications,however,matter to various sectors (e.g.crop and livestock yields) for addressing evolutionary hydro-meteorological hazards.Using gauge observations at hourly- and daily-resolution,precipitation unevenness is measured by the number of wettest days/hours for half of seasonal precipitation totals over East China,a major breadbasket vulnerable to precipitation volatility intra-seasonally.Across the region,half of seasonal totals needs only 11 days or even more unexpectedly just 44 hours to precipitate.During 1970–2017,though seasonal precipitation amount changed little,the intra-seasonal distribution of precipitation,in both frequency and amount,has been getting significantly more uneven,with more widespread and faster changes manifesting in hourly records.The regional-scale unevenness increase is unlikely modulated by internal variability alone,suggesting detectable contributions from anthropogenic climate change.The increased unevenness has led to significant lengthening of the longest dry spells,exposing the region to a more volatile precipitation mode—burstier-but-wetter storms with prolonged droughts in-between.(Chen Yang)

2.3 Tropical Pacific cold tongue mode triggered by enhanced warm pool convection due to global warming

A cold tongue mode (CTM) formed in the 1980s as a La Niña-like stepwise responses to recent global warming; however,a consensus has not been reached on the mechanism underlying the CTM formation.Here,we attribute the CTM to the enhanced deep convection of the warm pool regions over the western Pacific and south of North America.Increases in the sea surface temperatures in the two Pacific warm pool regions that occurred due to global warming exceeded the threshold of deep convection after the 1980s,which resulted in two opposite anomalous vertical circular circulation patterns and induced the CTM via the intensification,contraction,and westward shift of the Walker circulation and the uplift of the thermocline.Our results provide a novel explanation of the La Niña-like response under recent global warming.(Jiang Ning,Zhu Congwen)

2.4 Characteristics of summer regional rainfall events over Ili River Valley in Northwest China

The Ili River Valley is a trumpet-like terrain facing westward.From west to east,the elevation of the valley floor increases and the width of the valley decreases.Using hourly rainfall data of 2009-2016 obtained from 100 gauge stations over the Ili River Valley,the climatic features of summer regional rainfall events(RREs) are investigated.The mean amount and frequency of rainfall during summer show considerable spatial variabilities in terms of along-valley and across-valley directions.Along the valley,the rainfall gradually increases with increasing elevations.Across the valley,the rainfall also increases from the valley floor to the northern or southern slopes.Three distinct types of RREs are identified:western-local,eastern-local,and eastward-movement RREs.The eastern-local and western-local RREs are located over the east and west of the valley,respectively.The eastern-local RREs have more numbers and longer duration than the western-local ones,resulting in more rainfall in the east than in the west of the valley.In contrast,the eastward-movement RREs with the longest duration initiate from the west of the valley and gradually move to the east of the valley.For these RREs,the spatial distribution of the composite rainfall exhibits a similar pattern to that of the summer mean rainfall.These contribute to more than half of the summer total rainfall in most stations.It is found that the circulation structure accompanying the eastward-movement RREs exhibits a consistent feature with that of the Central Asia vortex/trough systems,and the eastward movement of the entire circulation system is in correspondence with that of the rainfalll center.(Li Liangliang,Li Jian,Yu Rucong)

2.5 Mesoscale convective system precipitation characteristics over East Asia.Part I:Regional differences and seasonal variations

Mesoscale convective systems (MCSs) play an important role in modulating the global water cycle and energy balance and frequently generate high-impact weather events.The majority of existing literature studying MCS activity over East Asia is based on specific case studies and more climatological investigations revealing the precipitation characteristics of MCSs over the eastern China are keenly needed.In this study,we use an iterative rain cell tracking method to identify and track MCS precipitation during 2008–2016 to investigate regional differences and seasonal variations of MCS precipitation characteristics.Our results show that the middle-to-lower reaches of the Yangtze River basin (YRB-ML) receive the largest amount and exhibit the most pronounced seasonal cycle of MCS precipitation in the eastern China.MCS precipitation over YRBML can exceed 2.6 mm day−1in June,contributing over 30.0% of April–July total rainfall.Particularly longlived MCSs occur over the eastern periphery of the Tibetan Plateau (ETP),with 25% of MCSs over the ETP persisting for more than 18 hours in spring.In addition,spring MCSs feature larger rainfall areas,longer durations,and faster propagation speeds.Summer MCSs have a higher precipitation intensity and a more pronounced diurnal cycle except for the southeastern China,where MCSs have similar precipitation intensity in spring and summer.There is less MCS precipitation in autumn,but an MCSprecipitation center over the ETP still persists.MCSs reach peak hourly rainfall intensities during the time of maximum growth (a few hours after genesis),reach their maximum size around 5 hours after genesis,and start decaying thereafter.(Li Puxi,Christopher Moseley,Andreas F.Prein,Chen Haoming,Li Jian)

2.6 Changes in compound drought and hot extreme events in summer over populated eastern China

Compound extreme events always cause severe impacts on human society and the natural system,especially in the populated areas.However,studies on their changes depend on definitions of the related extreme events.In this study,a newly defined compound drought and hot extreme events (CDHEEs) index based on the day-night concurrent hot extreme index and daily drought monitoring index is defined to explore possible changing features in summer-time CDHEEs from 1961 to 2018 in the densely populated areas of the eastern China.Results reveal that the occurrence of CDHEEs shows a significant increase in the densely populated areas with the strengthening interaction of drought and hot extreme events,especially since the late 1990s.Further analyses indicate that urbanization and regional climate also moderate changes in CDHEEs,as we found that CDHEEs increased more prominently in the cities,especially in the drier regions.As a result,the population exposed to CDHEEs has doubled since the late 1990s in the densely populated areas of the eastern China.(Yu Rong,Zhai Panmao)

2.7 More frequent and widespread persistent compound drought and heat event observed in China

Compound drought and heat event (CDHE) causes severe impacts on agriculture,ecosystem,and human health.Based on daily maximum surface air temperature and meteorological drought composite index data in China,changing features of CDHEs in warm season from 1961 to 2018 are explored at a daily time scale based on a strict and objective definition in this study.Results reveal that CDHEs have occurred more frequently and widely in China,especially since the late 1990s.Notably,such changes are more obvious in Southwest China,the eastern Northwest China,the northern North China,and the coastal area of the southeastern China.A prominent feature is that persistent CDHEs on a daily scale have increased significantly.To better understand climate change of compound extreme events,further studies on the physical mechanism,especially attribution analyses at a regional scale,are urgently needed.(Yu Rong,Zhai Panmao)

2.8 New perspectives on “warming-wetting” trend in Xinjiang,China

Recently,a hot topic about warmer and wetter climate change in the arid region of Northwest China,especially in Xinjiang,has attracted much attention by general public and scientific community.Our study revisits this topic especially for Xinjiang in the Eurasian continental context from multiple perspectives based on most updated CRU high-resolution grid data and China’s homogenized station data for 1961−2019.We conclude that such “warming-wetting” trend is not a regional phenomenon for Xinjiang but has much larger spatial scale.Regions having experienced both temperature and precipitation increases reflecting “warmingwetting” trend account for more than half of the Eurasian continent since 1961.Nevertheless,the “warmingwetting” trend in Xinjiang suggests some unique regional features in response to the global warming.Although drought seems to have relieved to some extent,especially in the mountainous regions in the western Xinjiang,the nature of arid and semi-arid climate regime has not changed.Noticeably,the interannual variability of precipitation has enlarged and the increase in extreme precipitation events has a major contribution.These findings suggest that “warming-wetting” trend in Xinjiang is asymmetric regarding warming and wetting in seasons and intensifying interannual variability and increasing contribution of extreme precipitation to the total.Thus,the current “warming-wetting” trend in Xinjiang possibly brings us some beneficial impacts for the ecosystem but also increases challenges for water resources utilization and risk management.(Wang Qian,Zhai Panmao,Qin Dahe)

2.9 Differing mechanisms for the 2008 and 2016 wintertime cold events in southern China

The southern China was hit by a severe cold event during January 21−25,2016,characteristic of a drastic and sudden temperature drop,with new records of low temperatures set in widespread areas.This cold event was dynamically linked to the activity of the Arctic oscillation (AO),whose evolution led the event by about 2 weeks.Different from this short lasting but intense cold event in 2016,another cold event in the 2008 winter lasted much longer and occurred in tandem with freezing precipitation.Their differences in the duration and precipitation are essentially determined by differing behaviours of blocking highs at mid–high latitudes,the India-Burma Trough and the western Pacific subtropical high.The rapid southward invasion of cold air masses during the 2016 event resulted from the rapid decay of the blocking high,which emanated wave energies downstream and likewise lost wave energies to the local mean flow.By contrast,during the 2008 cold event,continuous upstream wave energy dispersion recurrently stimulated disturbances in the blocking domain,which then extracted energies from the mean flow efficiently to survive markedly longer.Moreover,in the 2008 case,the deepening of the India-Burma Trough and the westward extension of the western Pacific subtropical high jointly enhanced the moisture transport,leading to the freezing rain.This favourable configuration for precipitation was absent in the 2016 case,dictating its dry nature accordingly.(Liao Zhen,Zhai Panmao,Chen Yang,Lu Hong)

2.10 An overview of the integrated meteorological observations in complex terrain region at Dali National Climate Observatory,China

Systematically observing components of the climate system as well as their processes and interactions are crucial to understand the weather,climate,climate change,etc.In order to launch long-term,continuous,stereoscopic,and integrated meteorological observations for key regions of the climate system in the southwestern China where it is sensitive to interactions among multiple layers and exchanges of mass and energy,the Dali National Climate Observatory (DNCO) was established in May 2006.To date,the DNCO has gradually performed an integrated meteorological observation network in a complex terrain region over the southeastern Tibetan Plateau including the conventional observations of weather and climate,and the special observations of radiation,lightning,soil moisture,wind profile,water vapor,water quality,water level,water temperature profile,turbulent fluxes of momentum,sensible heat,latent heat,carbon dioxide,and methane,etc.Furthermore,the DNCO mainly focuses on the field observation experiments and scientific research activities for mountain meteorology.This paper presents an overview of the DNCO including its location,climatology,scientific objectives,research tasks,and existing observation projects.The progresses in observation and associated research including data quality controls and assessments,recent observation results,and regional numerical model tests are summarized.Future works are also discussed.(Xu Anlun,Li Jian)

2.11 Contribution of changes in synoptic-scale circulation patterns to the past summer precipitation regime shift in eastern China

During the past few decades,the eastern China has experienced a summer precipitation regime shift characterized by a “southern flood-northern drought” pattern.Among numerous studies explaining this phenomenon,few have quantified dynamic-thermodynamic contributions at daily-synoptic scales.Using a self-organizing map approach,summertime daily atmospheric flows during 1961−2015 are clustered into 20 circulation patterns (CPs),each of which is assigned to an attribute among wet,dry,and neutral according to their synchronous precipitation anomalies.We find that decreases in wet CPs for the North and increases(decreases) in wet (dry) CPs for the South are robustly significant and can well explain the contrasting precipitation trends.Dynamic and thermodynamic processes jointly produce more precipitation in the South but less precipitation in the northern half of the north region,with thermodynamic contributions being 30%−40% larger.Dynamic influence and its interaction with thermodynamic factors dictate the latitudinal boundary between the drying and wetting regions.Summer precipitation in the eastern China has exhibited a“southern flood-northern drought (SFND)” pattern during the past few decades.Despite of substantial literature attempting to explain the phenomenon,efforts devoted to the quantification of contributions from dynamic processes (changes in daily atmospheric circulations) and thermodynamic processes (other physical processes such as the warming-induced increasing water vapor in the air) appear to be deficient.Using a cluster method called self-organizing map (SOM),the daily circulations in the summers during 1961−2015 are classified into 20 circulation patterns (CPs).The 20 CPs are further assigned to a wet,dry,or neutral attribute according to their synchronous precipitation anomalies.We demonstrate that the significant declining wet CPs for the North and significant increasing wet but decreasing dry CPs for the South can well explain the negative and positive precipitation trends in the North and South,respectively.Dynamic and thermodynamic processes both produce more precipitation in the South and less precipitation in the northern half of the north region,with thermodynamic contributions being 30%−40% larger.Dynamic processes and its acting on thermodynamic processes play a particularly vital role in determining the latitudinal boundary between the positive and negative precipitation trends in the eastern China.(Zhou Baiquan,Zhai Panmao,Chen Yang)

2.12 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 the 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 occur 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 to 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)

2.13 The influence of soil moisture and solar altitude on surface spectral albedo in arid area

Using data collected from a specially designed experiment at the Dunhuang Station (40°10' N,94°31' E,1150 m) from September 2017 to September 2018,we have characterized the influences of soil moisture and solar altitude on surface spectral albedo in an arid area.The specific settings of our experiment allowed us to minimize the influences of underlying surface,cloud cover,aerosol and weather conditions,and thus highlight the influence of soil moisture and solar altitude.During the timespan of the experiment,we observed the annual mean surface albedo of global radiation (GR),ultraviolet radiation (UV),visible radiation (VIS) and near-infrared radiation (NIR) to be 0.24,0.11,0.24 and 0.25.A significantly negative linear correlation between surface albedo and soil moisture was identified,with the correlation coefficients between GR,UV,VIS,NIR and soil moisture being −0.68,−0.75,−0.70 and −0.61.In addition,we identified an exponential relationship between surface albedo and solar altitude.The exponential regression coefficients are −0.21,−0.077,−0.53 and−0.21,respectively.From these analyses,we derived a new two-factor parametric formula for depicting the influence of soil moisture and solar altitude on surface spectral albedo.Using observation data,we demonstrate that the formula recapitulates the real-world relationship between soil moisture,solar altitude and surface spectral albedo with little deviation.These findings may help us gain a deeper understanding of improving land surface parameterizations and have potential implications for solar energy research and applications.(Yang Jiaxi,Li Zhenchao,Zhai Panmao)

2.14 极端天气气候事件变化对荒漠化、土地退化和粮食安全的影响

土地是人类赖以生存的重要资源，在受气候变化影响的同时其状况变化也在气候系统中起着关键作用。IPCC最新发布的气候变化与土地特别报告（SRCCL）系统反映了关于荒漠化、土地退化、可持续土地管理、粮食安全和陆地生态系统碳通量方面的最新科学认知，并探讨了如何进行更加可持续性的土地利用和管理以应对与土地相关的气候变化问题。文中从极端事件变化及其影响的角度，结合SRCCL 与其他相关文献，予以分析和总结。结果表明，在全球变暖的背景下，极端天气气候事件的变化已经并将继续影响荒漠化和土地退化进程并对粮食安全造成冲击；而土地对气候系统的反馈作用，又会加剧气候变化并提高极端事件发生的概率和严重程度。面对气候变化尤其是极端事件给土地带来的巨大压力，必须坚持可持续的土地管理，通过减少包括土地和粮食系统在内的所有行业的排放，才有可能实现到21 世纪末将全球平均升温控制在相对工业化前水平2 ℃以内的目标，以减轻气候变化对土地和粮食系统的负面影响。（黄萌田，周佰铨，翟盘茂）

2.15 海洋和冰冻圈变化有关的极端事件、突变及其影响与风险

《气候变化中的海洋和冰冻圈特别报告》（SROCC）于2019年9月在IPCC第一工作组和第二工作组第二次联合大会上得到审议通过，并得到了IPCC第51届全会接受和批准。文中主要对该报告中海洋和冰冻圈变化有关的极端事件、突变及其影响与风险的有关评估内容进行了综合分析。SROCC评估得到的最新结果显示：气候变化背景下冰冻圈变化引起的山体滑坡、雪崩和冰川洪水事件频发。海洋有关的海洋热浪频发，极端El Niño事件加强，大西洋经向翻转环流减弱。同时，沿海地区极端海平面上升，极端海浪增高，极端热带气旋影响增加。这些变化，比如海洋热浪等，是可以归因于人为增暖的。预估结果表明，海洋和冰冻圈变化引起的极端事件未来会进一步加剧。而这些变化已经影响了高山、极地以及沿海地区人群的生产和生活，以及海洋和冰冻圈的生态系统服务功能。应对这一系列变化，需要更加精准的预测和预警，包括对极端事件和突变的季节预测和年际、年代际预测，以便做好充足的准备来降低极端事件风险。同时，加强应对极端事件的科普教育和提供因地制宜的灾害重建措施等也是风险管理的重要环节。（余荣，翟盘茂）

2.16 IPCC特别报告SRCCL关于气候变化与粮食安全的新认知与启示

气候变化对粮食安全的影响是广泛的，不但影响粮食产量和品质，还会影响到农户的生计以及农业相关的产业发展等；而粮食系统在保障粮食安全的同时会产生一系列的环境问题，其中农业源温室气体（GHG）的排放加剧全球变暖。IPCC在2019年8月份发布的《气候变化与土地特别报告》（SRCCL），从粮食生产、加工、储存、运输及消费的各个环节评估气候变化对粮食安全的影响及粮食系统的温室气体排放对气候系统的影响；系统梳理粮食系统供给侧和需求侧的适应与减缓措施、适应与减缓的协同和权衡问题，以及气候变化条件下保障粮食安全的政策环境等。SRCCL评估结论认为，由于大量施用氮肥和消耗水资源，目前粮食系统GHG排放占全球总排放的21%～37%；农业和粮食系统是全球应对气候变化的重要方面，供给侧和需求侧的综合措施可以减少食物浪费、减少GHG排放、增加粮食系统的恢复力。未来工作的重点应丰富和扩展气候变化影响评估内容，量化适应效果，加深对适应、减缓及其协同和权衡的科学认知，大力加强应对气候变化能力建设。（许吟隆，赵运成，翟盘茂）

2.17 对基于土地应对气候变化的新认知

2019年8月7日IPCC《气候变化与土地特别报告》决策者摘要获得通过，报告涉及对基于土地应对气候变化不同措施的评估，取得一些新的认知，包括定量评估基于土地的40种综合措施对提高气候变化减缓和适应能力、防治荒漠化和土地退化、增强粮食安全、保护生物多样性和水资源及促进可持续发展的效益，提出在防治荒漠化和土地退化及保障粮食安全过程中提高气候变化适应和减缓能力的措施，评估了实施不同措施的风险、限制、障碍和保障政策等。在措施的分类、气候变化适应的主体对象，以及风险和政策分析等方面还存在一些不足。评估报告结论将对我国建立基于土地的应对气候变化的对策，促进把应对气候变化与防治荒漠化和土地退化、保护生态环境与增强粮食安全工作结合，制定相关政策等方面都具有重要的启示意义。（吴建国，翟盘茂，武亚堂）

2.18 气候变化科技支撑工作

圆满完成了本年度IPCC第一工作组联合主席技术支撑工作任务，完成了IPCC AR6第一工作组报告第二稿的内部审稿。气候变化团队成员作为副主编、主要作者、技术支持等角色参与到《中国气候与生态环境演变：2021》《第四次气候变化国家评估报告》《新疆气候变化科学评估报告》等三部国内气候变化评估报告的编写工作中。（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 A multiscale dynamical model in a dry-mass coordinate for weather and climate modeling:Moist dynamics and its coupling to physics

A multiscale dynamical model for weather forecasting and climate modeling is developed and evaluated in this study.It extends a previously established layer-averaged,unstructured-mesh nonhydrostatic dynamical core (dycore) to moist dynamics and parameterized physics in a dry-mass vertical coordinate.The dycore and tracer transport components are coupled in a mass-consistent manner,with the dycore providing time averaged horizontal mass fluxes to passive transport,and tracer transport feeding back to the dycore with updated moisture constraints.The vertical mass flux in the tracer transport is obtained by reevaluating the mass continuity equation to ensure compatibility.A general physics-dynamics coupling workflow is established,and a dycore-tracer-physics splitting strategy is designed to couple these components in a flexible and efficient manner.In this context,two major physics-dynamics coupling strategies are examined.Simple physics packages from the 2016 Dynamical Core Model Intercomparison Project (DCMIP2016) experimental protocols are used to facilitate the investigation of the model behaviors in idealized moist-physics configurations,including cloud-scale modeling,weather forecasting,and climate modeling,and in a real-world testcase setup.Performance evaluation demonstrates that the model is able to produce reasonable sensitivity and variability at various spatiotemporal scales.The consideration and implications of different physics-dynamics coupling options are discussed within this context.The appendix provides discussion on the energetics in the continuousand discrete-form equations of motion.(Zhang Yi,Li Jian,Yu Rucong,Liu Zhuang,Zhou Yihui,Li Xiaohan)

3.2 Configuration and evaluation of a global unstructured mesh atmospheric model (GRIST-A20.9)based on the variable-resolution approach

Targeting a long-term effort towards a variable resolution (VR) global weather and climate model,this study systematically configures and evaluates an unstructured mesh atmospheric model based on the multiresolution approach.The model performance is examined from dry dynamics to simple physics and full physics scenarios.In the dry baroclinic wave test,the VR model reproduces comparable fine-scale structures in the refined regions as a fine-resolution quasi-uniform (QU) mesh model.The mesh transition zone does not adversely affect the wave pattern.Regional kinetic energy spectra show that the fine-scale resolving ability improves as the fine resolution increases.Compared to a QU counterpart that has equivalent degrees of freedom,the VR model tends to increase the global errors,but the errors can be reduced when the resolution of the coarse region is increased.The performance over the coarse region is generally close to that of a low-resolution QU counterpart.Two multi-region refinement approaches,the hierarchical and polycentric refinement modes,further validate the model performance under the multiresolution refinement.Activating hyper diffusion for horizontal velocity is helpful with respect to VR modeling.An idealized tropical cyclone test is further used to examine its ability to resolve fine-scale structures.In the simple physics environment,the VR model can have the tropical cyclone stably pass the transition zone in various configurations.A series of sensitivity tests examines the model performance in a hierarchical refinement mode.The simulations exhibit consistency even when the VR mesh is slightly perturbed by one of the three parameters that control the density function.The tropical cyclone,starting from the second refinement region and passing through the inner transition zone,gets intensified and covers a smaller area in the refined regions.Such variations are consistent with the behavior that one may observe when uniformly refining the QU mesh.In the full physics environment with a highly variable mesh that reaches sub-10 km resolution,the VR model also produces a reasonable evolution for the tropical cyclone.The explicit diffusion shows its usefulness in terms of suppressing some unrealistic isolated-scale structures that are far away from the initial vortex and does not adversely affect the physically important object.The fine-scale structure is determined mainly by the fine-resolution area,although the systems may have larger differences before they move into the fine-resolution area.Altogether,this work demonstrates that the multi-resolution configuration is a reliable and economic alternative to high-resolution global modeling.The adverse impact due to mesh transition and the coarse region can be controlled well.(Zhou Yihui,Zhang Yi,Li Jian,Yu Rucong)

3.3 Investigation of the effect of the time step on the physics-dynamics interaction in CAM5 using an idealized tropical cyclone experiment

To understand the effect of the time step on the physics-dynamics interaction in a model,we used an idealized tropical cyclone test to evaluate the sensitivities to the physics time step in the Community Atmosphere Model Version 5 (CAM5).The investigated time steps were 450,900 and 1800 s at a resolution of 1°,and 225,450,900 and 1800 s at a resolution of 0.25° in the corresponding ensemble simulations.We found that the intensity and precipitation of the simulated tropical cyclone and the physics parameterizations are fairly sensitive to the time step.These sensitivities are affected by the dynamical core and the physicsdynamics coupling strategy and vary with the horizontal resolution.In low-resolution runs,the intensity of the simulated tropical cyclone varies little with physics time step in the finite volume (FV) dynamical core,but it tends to weaken with decreasing time steps in the spectral element (SE) dynamical core.The horizontal circulation of the tropical cyclone in both the FV and SE simulations increases as the length of the time step decreases in high-resolution runs,where large-scale condensation dominates.The sensitivities in the physical parameterizations to time step play an important role in regulating the impact of time step on the physicsdynamics interaction,especially in high-resolution simulations.Compared with the sequential coupling approach (ftype1) with a sudden adjustment at each physics time step in the SE core,the dribbling coupling strategy (ftype0) that adjusts the state more gradually weakens the effect of the physical parameterizations.(Li Xiaohan,Peng Xindong,Zhang Yi)

3.4 Southeastern Pacific error leads to failed El Niño forecasts

In El Niño-Southern Oscillation (ENSO) dynamical predictions,the ensemble members may show a large spread,leading to low prediction accuracy.The reasons for unreasonable forecast spreads in dynamical predictions are investigated based on hindcast/forecast results from the North American multimodel ensemble system.A category of failed-forecasting members is defined if a negative Niño34 index in winter is forecasted by one member for the observed El Niño events.Compared with reasonable-forecasting members,the failedforecasting members show significant cold sea surface temperature anomalies (SSTAs) in the southeastern Pacific (SEP).Such cold SSTAs can be traced back to the initial cold error in the SEP region.The initial cold error can be enhanced by positive feedback near the SEP region and further hinder warm SSTAs in equatorial regions,leading to a failed prediction.This result highlights the essential role of the SEP region,providing possible contributions to enhance the ENSO forecast skill.(Hua Lijuan,Su Jingzhi)

3.5 Obtaining more information about precipitation biases over East Asia from hourly-scale evaluation of model simulation

The hourly summer precipitation simulations over East Asia by the Chinese Academy of Meteorological Sciences Climate System Model (CAMS-CSM) high-resolution Atmospheric Model Intercomparison Project(AMIP) runs (T255,about 50 km) were evaluated based on the merged hourly precipitation product released by the China Meteorological Administration (CMA).The results show that the simulation biases are closely related to the topography,with the precipitation amount and frequency overestimated (underestimated),and duration of precipitation events being longer (shorter),over the western high-altitude (eastern plain) regions of China.Six regions with large discrepancies were further analyzed.In terms of the frequency-intensity structure,the overestimation of precipitation frequency is mainly due to the excessive simulated weak precipitation over the four regions with positive biases:the southern edge of the Tibetan Plateau (STP),the northeastern edge of the Tibetan Plateau (NETP),the eastern periphery of the Tibetan Plateau (EPTP),and the mountainous area of North China (NCM); while the underestimation of frequency is mainly due to the insufficient precipitation with moderate intensity over the two regions with negative biases:lower reaches of the Yangtze River (LYR)and the South China coast (SCC).Based on the duration-diurnal structure analysis,two kinds of precipitation events with different natures can be distinguished.The long-duration night to early morning precipitation events have a significant contribution to the precipitation amount biases for all the six key regions,and this kind of precipitation mainly affects the precipitation diurnal variation over the mountainous areas or steep terrain.Although the short-duration afternoon precipitation events only have a greater contribution to the precipitation amount biases over the SCC region,this kind of precipitation affects the diurnal variation over the NCM region and the two key regions with negative biases.Such a detailed hourly-scale evaluation is helpful for enriching the understanding of simulation biases and to further improve model performance.(Li Nina,Li Jian,Rong Xinyao,Chen Haoming,Xin Yufei,Su Jingzhi)

3.6 Comparison of Indian Ocean warming simulated by CMIP5 and CMIP6 models

Under the ongoing global warming,the sea surface temperature (SST) over the entire Indian Ocean (IO)has been warming saliently at a rate of 0.014 °C per year since the 1950s,which is larger than that in other regions of the globe.The salient IO warming reflects the synergistic effect of global warming and the internal variability of the climate system,and the warming could lead to climate anomalies in peripheral regions.The simulation performance of the sustained IO warming was evaluated by comparing 37 CMIP5 and 37 CMIP6 models with observed data.The results show that the warming in the IO can be captured by nearly all the CMIP models,but most tend to underestimate the magnitude of IO warming trends.There is no qualitative improvement in the simulation of the salient IO warming from CMIP5 to CMIP6.In addition,six metrics were used to investigate the performance of all models.Concerning the spatial pattern of warming trends,the CMIP5 models reveal a better simulation performance than those in CMIP6 models.Only nine best models(seven CMIP5 models and two CMIP6 models) can simulate a high warming trend in the IO region of 0.014 ±0.001 °C per year during 1950–2005,but these nine models still have some disadvantages among other metrics.The overall evaluation here provides necessary information for future investigation about the mechanism of the sustained IO warming based on the climate models with better performances.(Li Jingyi,Su Jingzhi)

3.7 Improved leading modes of interannual variability of the Asian-Australian monsoon in an AGCM via incorporating a stochastic multicloud model

In the present study,we investigated improvements in simulating the two major modes of the Asian-Australian monsoon (AAM) interannual variability by incorporating the stochastic multicloud model (SMCM)into the state-of-the-art ECHAM6.3 atmospheric model.Model results show that the modified ECHAM6.3,i.e.,with the SMCM,improves the simulation of seasonal evolution of anomalous rainfall and low-level circulation.Analysis reveales that the improvement in the simulation of precipitation anomalies of the first mode is associated with the improvement in both enhanced and suppressed convection.In addition,the enhanced easterly vertical shear over the Maritime Continent in the modified ECHAM6.3 contributes to the improvement in an equatorial asymmetry of precipitation anomalies of the first mode.Moreover,the moisture budget analysis demonstrates that the modified ECHAM6.3 improves the seasonal anomalous rainfall of the first mode by ameliorating the vertical mass integral of the moist flux divergence.In addition,the second mode of the AAM interannual variability produced by the modified ECHAM6.3 potentially exerts stronger influence on the ENSO variability compared to the default ECHAM6.3.(Ma Libin,Jiang Zijun)

3.8 Effects of a stochastic multicloud parameterization on the simulated Asian-Australian monsoon rainfall in an AGCM

The Asian-Australian monsoon (AAM) plays a vital role in modulating the global and local climates.Current numerical models,including climate system models (CSMs) and atmospheric general circulation models (AGCMs),have difficulties in capturing the fundamental and intrinsic features of the AAM.By applying the stochastic multicloud model (SMCM) to the state-of-the-art ECHAM6.3 atm model,we investigated the impacts of SMCM on the simulation of AAM precipitation.Though the modified model with the SMCM improves simulations of the AAM precipitation in June–July–August–September season,the solstice mode,the AAM domain,and the AAM precipitation intensity,deficiencies also exist in other aspects,that is,simulations of rainfall in December–January–February–March and April–May seasons,and the equinox mode.Analyses also unravelled the modified model improves in simulating the spatial patterns of seasonal variation of precipitation in the Indian monsoon region,western North Pacific monsoon region,and Australian monsoon region in terms of pattern correlation coefficient and normalized root-mean square error scores.The enhanced simulation of solar radiation in the modified model favours improving the performance of the simulated solstice mode.In addition,moisture budget analysis was applied to investigate physical processes that modulate the variations and distributions of AAM precipitation.Analyses revealed that variations and distributions of the vertical integral of moisture flux convergence is closely associated with changes in precipitation,while the stratification of vertical moist transport is associated with changes in precipitation over land in the AAM region.In addition,the modified model influences the AAM precipitation mainly through the variations of moisture flux convergence.The present work potentially provides a method to improve the simulated capability of AGCMs.(Ma Libin,Jiang Zijun,Cao Jian)

3.9 Impacts of the stochastic multicloud parameterization on the simulation of western North Pacific summer rainfall

This study investigated the sensitivity of the western North Pacific (WNP) summer precipitation to the convection schemes and discussed the associated dynamical processes.Two convection schemes were compared:one is the default mass-flux convection scheme used in the state-of-the-art ECHAM6.3 atmosphere model and the other incorporates the stochastic multicloud model (SMCM) into ECHAM6.3.Incorporation of the SMCM reduces the bias of cloud cover and shortwave and longwave radiation by regulating the shortwave and longwave cloud radiative forcing over the WNP.Compared to the default model,the modified model with the SMCM alleviates the dry bias in the WNP,which is associated with enhanced ascending motion.The moist static energy balance revealed that improved simulation of precipitation in the modified model is contributed by enhanced horizontal advection of moist enthalpy and increased net energy in the atmosphere,which is attributed to increased total cloud cover,over the WNP.Additionally,intensified latent energy advection over the WNP dominates enhanced horizontal advection of moist enthalpy in the modified model.On the other hand,the moisture budget analysis of the WNP demonstrated that strengthened convergence of moisture flux in the modified model plays the most influential role in reducing precipitation bias.Further analysis unraveled that enhanced zonal-mean moisture transported by the stationary eddy zonal flow convergence in the WNP dominates intensified zonal moisture convergence,thus increased horizontal convergence of moist flux in the modified model.(Ma Libin,Yang Shuangyan)

3.10 Impact of seawater equation of state on the simulation of Atlantic meridional overturning circulation

The Atlantic meridional overturning circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat.However,realistic simulation of the AMOC,i.e.,its strength and spatial structure,remains a challenge for ocean general circulation models(OGCMs) and coupled climate models.Here,we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM.Two EOSs used in this study:the UNESCO EOS80,and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80.Compared to the model using the UNESCO EOS80,the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea,the Irminger-Iceland-Scotland Basin,and the Greenland-Iceland-Norwegian (GIN) seas,which leads to an improvement in the simulation of the AMOC:Along 26.5°N,the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km,both matching the observations better.Taken the Labrador Sea as an example,the processes,including both direct and indirect causes,that in part responsible for the improved AMOC are as follows.The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water.Moreover,the enhanced cabbeling and thermobaric effect strengthen the vertical advection,intensifying the deep convection and increasing formation of deep water,which eventually improves the simulation of the AMOC.The intensified AMOC,in turn,speeds up the surface return flow,transporting more warm and saline water to the high latitudes in the North Atlantic,which contributes to the densification of surface water.Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas.Thus,the enhanced deep convection and formation of deep water in the Labrador Sea,as well as in the Iceland-Scotland Basin and GIN seas,improve the simulated AMOC.(Ma Libin,Wang Bin,Zhang Xiao)

3.11 Impacts of atmosphere-sea ice-ocean interaction on Southern Ocean deep convection in a climate system model

Deep convection in polar oceans plays a critical role in the variability of global climate.In this study,we investigate potential impacts of atmosphere–sea ice–ocean interaction on deep convection in the Southern Ocean (SO) of a climate system model (CSM) by changing sea ice-ocean stress.Sea ice-ocean stress plays a vital role in the horizontal momentum exchange between sea ice and the ocean,and can be parameterized as a function of the turning angle between sea ice and ocean velocity.Observations have shown that the turning angle is closely linked to the sea-ice intrinsic properties,including speed and roughness,and it varies spatially.However,a fixed turning angle,i.e.,zero turning angle,is prescribed in most of the stateof-the-art CSMs.Thus,sensitivities of SO deep convection to zero and non-zero turning angles are discussed in this study.We show that the use of a non-zero turning angle weakens open-ocean deep convection and intensifies continental shelf slope convection.Our analyses reveal that a non-zero turning angle first induces offshore movement of sea ice transporting to the open SO,which leads to sea ice decrease in the SO coastal region and increase in the open SO.In the SO coastal region,the enhanced sea-ice divergence intensifies the formation of denser surface water descending along continental shelf by enhanced salt flux and reduced freshwater flux,combined with enhanced Ekman pumping and weakened stratification,contributing to the occurrence and intensification of continental shelf slope convection.On the other hand,the increased sea ice in the open SO weakens the westerlies,enhances sea-level pressure,and increases freshwater flux,whilst oceanic cyclonic circulation slows down,sea surface temperature and sea surface salinity decrease in the open SO response to the atmospheric changes.Thus,weakened cyclonic circulation,along with enhanced freshwater flux,reduced deep-ocean heat content,and increased stability of sea water,dampens the open–ocean deep convection in the SO,which in turn cools the sea surface temperature,increases sea-level pressure,and finally increases sea-ice concentration,providing a positive feedback.In the CSM,the use of a non-zero turning angle has the capability to reduce the SO warm bias.These results highlight the importance of an accurate representation of sea ice–ocean coupling processes in a CSM.(Ma Libin,Wang Bin,Cao Jian)

3.12 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 show that the two MSM have 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)

3.13 建立基于CAMS-CSM动力预测系统，完成1980—2019历史逐月回报试验

基于CAMS-CSM模式，发展了模式的同化模块，建立了气候动力预测系统，并完成了1980—2019年历史回报试验。回报试验中，每月20、21日00:00、06:00、12:00、18:00起报6个月，3月1、6日00:00、06:00、12:00、18:00起报12个月，每个起报月份共有8个集合成员。回报结果表明，与中国多模式集合（CMMEv1.0）的其他模式成员相比，CAMS-CSM模式对于全球几个主要海温模态指数的整体预测性能较好，对ENSO和印度洋偶极子（IOD）预测技巧在CMMEv1.0模式中处于中上水平，加入CAMS-CSM后对CMMEv1.0集合平均预报技巧有正贡献。在此基础上，利用CAMS-CSM从2020年3月起逐月开展滚动预测试验，首次将CAMS-CSM模式应用于汛期气候预测，初步建立了基于CAMS-CSM的短期气候动力预测系统，为一体化模式未来开展天气—气候预测提供了重要的技术积累。（苏京志、容新尧、李建、刘波、马利斌、唐彦丽、华莉娟、陈昊明、刘伯奇）