The development of a powerful Mongolian cyclone on 14–15 March 2021: Eddy energy analysis

Intense and extensive dust, caused by a strong Mongolian cyclone, hit Mongolia and northern China on 14–15 March 2021. In this study, the development process of this cyclone is analysed from the perspective of high-frequency eddy energetics. During the low-frequency circulation field of early March of 2021, an amplified polar vortex intruding towards central Asia and a ridge straddling eastern and northeastern Asia worked in concert to comprise a strong baroclinic zone from central Asia to Lake Baikal. Under these favourable conditions, on 13 March, a migratory trough triggered the Mongolian cyclone by crossing over the Sayan Mountains. The downwards transfer of kinetic energy from the eddy at 850 hPa played a key role in the intensification and mature stage of the cyclone. This mechanism was primarily completed by the cold air sinking behind the cold front. The frontal cyclone wave mechanism became crucial once the cyclone started to rapidly develop. The authors emphasize that the anomalously large growth of high-frequency available potential energy, which characterized this super strong cyclone, was obtained by extracting energy first from the time-mean available potential energy and then from the low-frequency available potential energy. The interannual temperature anomaly pattern of “north cold south warm” facilitated the additional time-mean available potential energy, and the temperature anomaly pattern of “northwest cold southeast warm” conditioned the extra low-frequency available potential energy. The analysis results suggest that the interaction between high- and low-frequency waves was also important in the development of the intense cyclone.摘要2021年3月14-15日, 强蒙古气旋引起的大范围强沙尘天气袭击了蒙古国和中国北方地区. 本文从高频涡动能量学的角度分析了这一超强气旋的发展过程. 2021年3月初, 加强的极涡向中亚伸入, 并与横跨东亚和东北亚的一个大型脊协同作用, 由此形成了从中亚到贝加尔湖地区的强大斜压带. 在这一有利的低频环流条件下, 3月13日一个移动性小槽越过萨彦岭后触发了蒙古气旋. 850 hPa涡旋动能的下传在气旋的加强和成熟阶段起到了关键作用. 而这一机制主要由冷锋后侧的冷空气下沉过程完成. 一旦气旋开始快速发展, 锋面气旋波机制就变得至关重要.我们强调, 高频涡动有效位能是首先从时间平均有效势能中提取能量, 然后从低频有效位能中汲取能量而剧烈增长的, 这正是该超强气旋的鲜明特征. “北冷南暖”的近地面温度气候异常型为时间平均有效位能的增多和向高频涡动有效位能的转换提供了条件, 而“西北冷东南暖”的温度异常型则有利于低频有效位能的增加和向高频涡动有效位能的转换. 分析结果表明, 高低频波之间的相互作用对蒙古气旋的增强也很重要.     

Strengthened influence of the East Asian trough on spring extreme precipitation variability over eastern Southwest China after the late 1980s

The relationship between variations in the East Asian trough (EAT) intensity and spring extreme precipitation over Southwest China (SWC) during 1961–2020 is investigated. The results indicate that there is an interdecadal increase in the relationship between the EAT and spring extreme precipitation over eastern SWC around the late 1980s. During the latter period, the weak (strong) EAT corresponds to a strong and large-scale anomalous anticyclone (cyclone) over the East Asia–Northwest Pacific region. The EAT-related anomalous southerlies (northerlies) dominate eastern SWC, leading to significant upward (downward) motion and moisture convergence (divergence) over the region, providing favorable (unfavorable) dynamic and moisture conditions for extreme precipitation over eastern SWC. In contrast, during the former period, the EAT-related circulation anomalies are weak and cover a relatively smaller region, which cannot significantly affect the moisture and dynamic conditions over eastern SWC; therefore, the response in extreme precipitation over eastern SWC to EAT is weak over the period. The interdecadal change in the relationship between eastern SWC spring extreme precipitation and the EAT could be related to the interdecadal change in the EAT variability. The large (small) variability of the EAT is associated with significant (insignificant) changes in spring extreme precipitation over eastern SWC during the latter (former) period.摘要本文研究表明东亚大槽强度与中国西南地区东部春季极端降水的关系在20世纪80年代末后显著增强, 这可能与东亚大槽自身变率的年代际变化有关. 在80年代末之后, 东亚大槽的变率显著增强, 其对应的大气环流异常也偏强, 范围偏大, 可以显著影响西南地区东部的水汽和动力条件, 从而引起该地区春季极端降水的显著变化. 而在80年代末之前, 东亚大槽的变率偏弱, 其对应的大气环流异常也偏弱, 范围偏小, 因此不能对西南地区东部春季极端降水的变化产生显著影响.     

Response of a westerly-trough rainfall episode to multi-scale topographic control in southwestern China

This study aims to quantify the response of a westerly-trough rainfall episode that occurred in summer 2020 to multi-scale topographic control in southwestern China, based on observations and numerical simulations. The multi-scale topography is composed of the Tibetan Plateau, Hengduan Cordillera (HC), and Sichuan Basin (SB). The westerly trough was characterized by southeastward deepening together with an in-phase propagating rainfall episode. By utilizing the results of numerical experiments, how the multi-scale topography impacted this westerly trough rainfall episode is explored. It is found that HC was the pivotal topographic factor affecting the southeastward extension of the trough and related rainfall, while SB accerelated the eastward movement of the westerly trough and changed the tilting direction of the trough line, thus further changing the location and orientation of precipitation. For extreme rainfall with intensity exceeding 10 mm h^?1, a roughly threefold rise in the cover ratio (from 1.8% to 7.2%) and fourfold increase in the areal rainfall amount per hour occurred by removing the HC barrier, due to the strongest vorticity and long-distance transport capacity to potential vorticy mass accompanying the southeast-stretching trough. Our results quantitatively reveal a strong response of westerly trough rainfall to multi-scale topographic control in southwestern China, therefore serving as an important reference for future decision making and effective model improvement.摘要中国西南部地形复杂, 降水频发, 地形对降水的影响至关重要. 本文基于观测和数值模拟, 定量揭示了青藏高原, 横断山脉和四川盆地多尺度地形对该地区西风槽降水的影响. 发现横断山脉是影响槽东南伸展, 降水传播的关键地形要素, 而四川盆地可加速西风槽东移, 改变槽线倾斜方向, 进而改变降水的位置和方向. 对于极端降水事件, 移除横断山脉屏障后, 降水覆盖率约增加3倍 (从1.8%增至7.2%), 小时面雨量增强4倍. 这些研究, 可为地形复杂地区降水的未来预报决策和有效模式改进提供参考.     

On the Two Successive Supercold Waves Straddling the End of 2020 and the Beginning of 2021

Two supercold waves straddling 2020 and 2021 successively hit China and caused record-breaking extremely low temperatures.In this study,the distinct features of these two supercold waves are analyzed on the medium-range time scale.The blocking pattern from the Kara Sea to Lake Baikal characterized the first cold wave,while the large-scale tilted ridge and trough over the Asian continent featured the second cold wave.Prior to the cold waves,both the northwest and hyperpolar paths of cold air contributed to a zonally extensive cold air accumulation in the key region of Siberia.This might be the primary reason why strong and extensive supercold waves occur even under the Arctic amplification background.The two cold waves straddling 2020 and 2021 exhibited distinct features:(1)the blocking circulation occurred to the north or the east of the Ural Mountains and was not confined only to the Ural Mountains as it was for the earlier cold waves;(2)the collocation of the Asian blocking pattern and the polar vortex deflection towards East Asia preferred the hyperpolar path of cold air accumulation and the subsequent southward outburst;and(3)both high-and low-frequency processes worked in concert,leading to the very intense cold waves.The cold air advance along the northwest path,which coincides with the southeastward intrusion of the Siberian High(SH)front edge,is associated with the high-frequency process,while the cold air movement along the hyperpolar path,which is close to the eastern edge of the SH,is controlled by the low-frequency process.     

A spatiotemporal 3D convolutional neural network model for ENSO predictions: A test case for the 2020/21 La Niña conditions

Many coupled models are unable to accurately depict the multi-year La Niña conditions in the tropical Pacific during 2020–22, which poses a new challenge for real-time El Niño–Southern Oscillation (ENSO) predictions. Yet, the corresponding processes responsible for the multi-year coolings are still not understood well. In this paper, reanalysis products are analyzed to examine the ocean–atmosphere interactions in the tropical Pacific that have led to the evolution of sea surface temperature (SST) in the central-eastern equatorial Pacific, including the strong anomalous southeasterly winds over the southeastern tropical Pacific and the related subsurface thermal anomalies. Meanwhile, a divided temporal and spatial (TS) 3D convolution neural network (CNN) model, named TS-3DCNN, was developed to make predictions of the 2020/21 La Niña conditions; results from this novel data-driven model are compared with those from a physics-based intermediate coupled model (ICM). The prediction results made using the TS-3DCNN model for the 2020–22 La Niña indicate that this deep learning–based model can capture the two-year La Niña event to some extent, and is comparable to the IOCAS ICM; the latter dynamical model yields a successful real-time prediction of the Niño3.4 SST anomaly in late 2021 when it is initiated from early 2021. For physical interpretability, sensitivity experiments were designed and carried out to confirm the dominant roles played by the anomalous southeasterly wind and subsurface temperature fields in sustaining the second-year cooling in late 2021. As a potential approach to improving predictions for diversities of ENSO events, additional studies on effectively combining neural networks with dynamical processes and mechanisms are expected to significantly enhance the ENSO prediction capability.摘要2020–22年间热带太平洋经历了持续性多年的拉尼娜事件, 多数耦合模式都难以准确预测其演变过程, 这为厄尔尼诺-南方涛动(ENSO)的实时预测带来了很大的挑战. 同时, 目前学术界对此次持续性双拉尼娜事件的发展仍缺乏合理的物理解释, 其所涉及的物理过程和机制有待于进一步分析. 本研究利用再分析数据产品分析了热带东南太平洋东南风异常及其引起的次表层海温异常在此次热带太平洋海表温度(SST)异常演变中的作用, 并构建了一个时空分离(Time-Space)的三维(3D)卷积神经网络模型(TS-3DCNN)对此次双拉尼娜事件进行实时预测和过程分析. 通过将TS-3DCNN与中国科学院海洋研究所(IOCAS)中等复杂程度海气耦合模式(IOCAS ICM)的预测结果对比, 表明TS-3DCNN模型对2020–22年双重拉尼娜现象的预测能力与IOCAS ICM相当, 二者均能够从2021年初的初始场开始较好地预测2021年末 El Niño3.4区SST的演变. 此外, 基于TS-3DCNN和IOCAS ICM的敏感性试验也验证了赤道外风场异常和次表层海温异常在2021年末赤道中东太平洋海表二次变冷过程中的关键作用. 未来将神经网络与动力 模式模式间的有效结合, 进一步发展神经网络与物理过程相结合的混合建模是进一步提高ENSO事件预测能力的有效途径.     

Link between Arctic ozone and the stratospheric polar vortex

The stratospheric ozone layer protects life on earth by preventing solar ultraviolet radiation from reaching the surface. Owing to the large population in the Northern Hemisphere and extreme ozone loss in the Arctic, changes in Arctic stratospheric ozone (ASO) and their causes have attracted broad attention recently. Using monthly mean data during the period 1980–2020 from MERRA-2, the relationship between the stratospheric polar vortex (SPV) and ASO, along with the relative contributions of chemical and dynamic processes associated with the SPV to changes in ASO, were examined in this study. Results showed that the ASO in March has a strong out-of-phase link with the strength of the SPV in March, with no obvious lead–lag correlations, i.e., an increase (decrease) in ASO corresponds to a weakened (strengthened) SPV. Further analysis suggested that the strong out-of-phase link between the SPV and ASO is related to changes in Brewer–Dobson circulation (BDC). Strong SPV events, accompanied by a low temperature condition and weakened upward propagation of planetary waves over the Arctic in the stratosphere, result in weakened BDC. The weakened downwelling at high latitudes tends to transport less ozone-rich air in the upper stratosphere at lower latitudes into the lower stratosphere at high latitudes, facilitating a decrease in ASO. The BDC's vertical velocity plays the dominant role in modulating ASO.摘要利用1980–2020年MERRA-2资料, 分析了平流层极涡 (Stratospheric polar vortex, SPV) 和北极臭氧 (Arctic stratospheric ozone, ASO) 的关系, 评估了与SPV相关的化学, 动力过程在其中的相对作用. 结果表明, 3月份ASO与同期SPV强度反相关最大. SPV-ASO二者反相关与平流层剩余环流 (Brewer-Dobson circulation, BDC) 变化密切相关. 强SPV伴随的北极平流层低温条件和行星波向上传播减弱, 导致BDC减弱, 减弱的BDC下沉支将低纬度平流层上层臭氧含量较低的空气输送到北极平流层低层, 从而导致ASO减少. BDC垂直速度在其中起主导作用.     

The exceptionally strong and persistent Arctic stratospheric polar vortex in the winter of 2019–2020: 2019-2020冬季北半球超强极涡事件

The Arctic stratospheric polar vortex was exceptional strong, cold and persistent in the winter and spring of 2019–2020. Based on reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research and ozone observations from the Ozone Monitoring Instrument, the authors investigated the dynamical variation of the stratospheric polar vortex during winter 2019–2020 and its influence on surface weather and ozone depletion. This strong stratospheric polar vortex was affected by the less active upward propagation of planetary waves. The seasonal transition of the stratosphere during the stratospheric final warming event in spring 2020 occurred late due to the persistence of the polar vortex. A positive Northern Annular Mode index propagated from the stratosphere to the surface, where it was consistent with the Arctic Oscillation and North Atlantic Oscillation indices. As a result, the surface temperature in Eurasia and North America was generally warmer than the climatology. In some places of Eurasia, the surface temperature was about 10 K warmer during the period from January to February 2020. The most serious Arctic ozone depletion since 2004 has been observed since February 2020. The mean total column ozone within 60°–90°N from March to 15 April was about 80 DU less than the climatology.摘要2019-2020冬季北极平流层极涡异常并且持续的偏强,偏冷.利用NCEP再数据和OMI臭氧数据, 本文分析了此次强极涡事件中平流层极涡的动力场演变及其对地面暖冬天气和臭氧低值的影响.此次强极涡的形成是由于上传行星波不活跃.持续的强极涡使得2020年春季的最后增温出现时间偏晚.平流层正NAM指数向下传播到地面, 与地面AO指数和NAO指数相一致, 欧亚大陆和北美地面气温均比气候态偏暖, 在欧亚大陆的一些地区, 2020年1月和2月的气温甚至偏高了10K.2020年2月以来北极臭氧出现了2004年以来的最低值, 2020年3-4月60°–90°N的平均臭氧柱总量比气候态偏低了80DU.     

Impacts of strong El Niño on summertime near-surface ozone over China

The influences of strong El Niño events (1997/98 and 2015/16) on summertime near-surface ozone (O₃) concentrations over China are investigated using the GEOS-Chem model. The results show that near-surface O₃ concentrations increased by a maximum of 6 ppb (parts per billion) during the summer of the developing phase of the 1997/98 El Niño in northeastern China, mainly due to the increased chemical production related to the hot and dry conditions. Besides, the O₃ concentration increased by 3 ppb during the developing summer of both the 1997/98 and 2015/16 El Niño in southern China. It was linked to the weakened prevailing monsoon winds, which led to the accumulation of O₃ in southern China. In contrast, in the summer of the decaying phase of the two El Niño events, O₃ concentrations decreased over many regions of China when the El Niño reversed to the cooling phase. This highlights that El Niño plays an important role in modulating near-surface O₃ concentrations over China.摘要利用全球大气化学三维模式 (GEOS-Chem) 模拟研究两次强厄尔尼诺事件 (1997/98和2015/16) 对中国夏季近地面臭氧 (O₃₎ 浓度的影响. 结果表明1997/98年厄尔尼诺事件发展期夏季中国东北区域O₃浓度升高, 最大值超过6ppb, 这主要归因于高温晴朗低湿等气象因素导致O₃化学生成升高. 此外, 两次厄尔尼诺事件发展期夏季O₃浓度在中国南部均增加了3ppb, 这与盛行季风减弱导致中国南方O₃局地积累有关. 相反, 在两次强厄尔尼诺衰减期夏季, 中国大部分地区O₃浓度下降伴随着海温模态转变为拉尼娜事件. 这表明厄尔尼诺在调节中国近地面O₃浓度中发挥着重要作用.     

Subseasonal reversal of haze pollution over the North China Plain

China has been frequently suffering from haze pollution in the past several decades. As one of the most emission-intensive regions, the North China Plain (NCP) features severe haze pollution with multiscale variations. Using more than 30 years of visibility measurements and PM_2.5 observations, a subseasonal seesaw phenomenon of haze in autumn and early winter over the NCP is revealed in this study. It is found that when September and October are less (more) polluted than the climatology, haze tends to be enhanced (reduced) in November and December. The abrupt turn of anomalous haze is found to be associated with the circulation reversal of regional and large-scale atmospheric circulations. Months with poor air quality exhibit higher relative humidity, lower boundary layer height, lower near-surface wind speed, and southerly anomalies of low-level winds, which are all unfavorable for the vertical and horizontal dispersion and transport of air pollutants, thus leading to enhanced haze pollution over the NCP region on the subseasonal scale. Further exploration indicates that the reversal of circulation patterns is closely connected to the propagation of midlatitude wave trains active on the subseasonal time scale, which is plausibly associated with the East Atlantic/West Russia teleconnection synchronizing with the transition of the North Atlantic SST. The seesaw relation discussed in this paper provides greater insight into the prediction of the multiscale variability of haze, as well as the possibility of efficient short-term mitigation of haze to meet annual air quality targets in North China.摘要中国近几十年来频受雾霾污染问题困扰, 其中华北平原作为排放最密集的区域之一, 常遭遇不同尺度的严重雾霾污染. 本文利用30余年的能见度和颗粒物 (PM_2.5) 观测数据, 发现了华北平原地区在秋季和早冬时雾霾污染在次季节尺度上“跷跷板式”反向变化的关系. 研究发现, 当9–10月污染较轻 (重) 时, 11–12月的污染倾向于加重 (减轻) . 这种突然的变化与局地和大尺度环流的反向变化有关. 污染较重的月份常伴随有更高的相对湿度, 更低的边界层高度和近地面风速以及低层的南风异常, 均不利于污染的垂直和水平扩散和传输, 从而导致了次季节尺度上霾污染的加重. 进一步的研究发现环流场的突然转向与在次季节尺度上活跃的中纬度波列的传播密切相关, 而此波列可能主要与大西洋海温转变及引起的EA/WR遥相关型有关. 这一次季节反向变化为霾污染多尺度变率预测提供了新的理解, 同时为华北地区年度空气质量达标的短期目标提供了具有可行性的参考方法.     

The anthropogenic acceleration and intensification of flash drought over the southeastern coastal region of China will continue into the future

The frequent and rapid onset of flash drought poses a serious threat to agriculture and ecosystems. Detecting human influences on flash droughts and estimating their future risks under climate change have attracted great attention. Focusing on a record-breaking flash drought event in the southeastern coastal region of China in summer 2020, the authors found that the suppression of convective precipitation and high temperature caused by the persistent high geopotential height anomalies and land–atmosphere dry coupling were important reasons for the rapid onset and strong intensity of the flash drought. Event attribution analysis with the latest CMIP6 data showed that anthropogenic climate change has not only increased the likelihood of an onset speed and intensity like those of the 2020 flash drought event, by about 93% ± 20% and 18% ± 15%, respectively, but also increased the chance of their simultaneous occurrence, by about 86% ± 38%, according to their joint probability distribution. Under a business-as-usual future scenario (SSP2-4.5), the likelihood of such an onset speed, intensity, and their simultaneous occurrence will further increase, by 85% ± 33%, 49% ± 8%, and 81% ± 48%, respectively, as compared with current climate conditions. This study highlights the importance of anthropogenic climate change for accelerating and intensifying flash drought in the southeastern coastal region of China.摘要快速爆发的骤旱对农业生产, 生态环境等造成严重威胁, 亟须量化当前及未来气候变化对骤旱爆发过程的影响. 2020年夏季, 在持续高压异常和陆气干耦合的控制下, 我国东南沿海地区出现高温少雨天气并引发极端骤旱事件. 基于第六次国际耦合模式比较计划 (CMIP6) 数据开展归因分析, 本文发现相比天然情景, 温室气体排放等人为因子导致的气候变化不仅使类似2020年骤旱爆发速度和强度的发生概率分别增加93%和18%, 还使其联合概率增加86%. 在目前排放水平下, 此类骤旱爆发速度和强度发生概率及其联合概率在21世纪末将提高85%, 49%, 和81%, 极大增加了干旱适应的挑战.     

Distribution of lightning spatial modes and climatic causes in China

This paper investigates the distribution of spatial modes of cloud-to-ground (CG) lightning activity across China's land areas during the period 2010–20 and their possible causes based on the CG lightning dataset of the China National Lightning Detection Network. It is found that the first empirical orthogonal function mode (EOF1) occupies 32.86% of the total variance of the summer CG lightning anomaly variation. Also, it exhibits a negative–positive–negative meridional seesaw pattern from north to south. When the SST of the East Pacific and Indian Ocean warms abnormally and the SST of the Northwest Pacific becomes abnormally cold, a cyclonic circulation is stimulated in the Yellow Sea, East China Sea, and tropical West Pacific region of China. As the water vapor continues to move southwards, it converges with the water vapor deriving from the Bay of Bengal in South China, and ascending motion strengthens here, thus enhancing the CG lightning activity of this area. Affected by the abnormal high pressure, the corresponding CG lightning activities in North China and Northeast China are relatively weak. The ENSO phenomenon is the climate driver for the CG lightning activity occurring in land areas of China.摘要本文利用中国气象局国家雷电监测网 (CNLDN) 的地闪观测数据集, 分析了2010–2020年中国陆地区域地闪空间模态分布特征及其可能的气候成因. 研究发现, 夏季地闪第一模态的方差贡献率为32.86%, 其分布从北到南呈现出“−+−”的经向跷跷板模式. 当东太平洋和印度洋的海温异常增暖, 西北太平洋的海温异常变冷时, 在中国黄海, 东海及热带西太平洋地区激发出气旋性环流. 随着水汽南下至华南地区, 与来自孟加拉湾的水汽汇合, 上升运动在此加强, 从而使得该地区的雷电活动增强. 表明厄尔尼诺-南方涛动 (ENSO) 现象, 是发生在中国陆地区域的地闪活动的气候驱动因子.     

Analysis of climatic features and major meteorological disasters over the Three Gorges Region of the Yangtze River Basin in 2021

Based on daily observation data in the Three Gorges Region (TGR) of the Yangtze River Basin and global reanalysis data, the authors analyzed the climate characteristics and associated temporal variations in the main meteorological factors in 2021, as well as the year's climatic events and meteorological disasters. The 2021 average temperature was 0.2°C above the 1991–2020 average and the 13th-warmest year since 1961. Seasonally, winter and autumn were both warmer than usual. The annual mean precipitation was 12.8% above normal, and most regions experienced abundant rainfall throughout the year. The seasonal variation in precipitation was significant and the TGR had a wetter-than-normal spring and summer. The number of rainstorm days was higher than normal; the wind speed was above normal; and the relative humidity was higher than normal. In terms of rain acidity, 2021 was tied with 2020 as the lowest since 1999. From mid-September to early October 2021, the TGR experienced exceptional high-temperature weather, which was driven by abnormal activity of mid- and high-latitude atmospheric circulation over the Eurasian continent and the western Pacific subtropical high (WPSH). In addition, a strong blocking high over the Ural Mountains accompanied by intense mid-latitude westerly winds prevented cyclonic disturbances from extending to the subtropical region. As a result, under the combined effect of the weaker-than-normal cold-air activities and the anomalous WPSH, the TGR experienced extreme high-temperature weather during early autumn 2021.摘要本文分析了2021年长江三峡地区气候特征及主要天气气候事件. 2021年, 三峡地区年平均气温较常年偏高0.2°C, 降水量偏多12.8%, 年暴雨日数偏多, 年平均风速偏大, 年平均相对湿度偏高. 2021年与2020年同为有记录以来酸雨强度最弱年. 汛期区域强降水频发; 春季多低温阴雨; 秋冬寒潮过程降温幅度大影响广. 9月中旬至10月初, 三峡地区遭遇罕见“秋老虎”, 分析表明该时段内异常高温与中高纬度大气环流及西太平洋副热带高压活动异常等因子密切相关.     

Distinct influential mechanisms of the warm pool Madden–Julian Oscillation on persistent extreme cold events in Northeast China

This study investigates whether and how the Madden–Julian Oscillation (MJO) influences persistent extreme cold events (PECEs), a major type of natural disaster in boreal winter, over Northeast China. Significantly increased occurrence probabilities of PECEs over Northeast China are observed in phases 3 and 5 of the MJO, when MJO-related convection is located over the eastern Indian Ocean and the western Pacific, respectively. Using the temperature tendency equation, it is found that the physical processes resulting in the cooling effects required for the occurrence of PECEs are distinct in the two phases of the MJO when MJO-related convection is consistently located over the warm pool area. The PECEs in phase 3 of the MJO mainly occur as a result of adiabatic cooling associated with ascending motion of the low-pressure anomaly over Northeast Asia. The cooling effect associated with phase 5 is stronger and longer than that in phase 3. The PECEs associated with phase 5 of the MJO are linked with the northwesterly cold advection of a cyclonic anomaly, which is part of the subtropical Rossby wave train induced by MJO-related convection in the tropical western Pacific.摘要 本文利用高分辨率气温数据和热带季节内振荡 (MJO) 实时指数, 研究了1979–2015年冬季MJO活动对中国东北持续性极端低温事件 (PECE) 的影响特征和机理.结果表明:当MJO对流分别位于暖池地区的东印度洋 (位相3) 和西太平洋 (位相5) 时, 中国东北PECE的发生频率显著增加.利用温度方程诊断分析发现MJO两个位相所导致的冷却过程不同: 当 MJO处于位相3时, 中国东北地区为低压异常, 上升运动引起绝热冷却作用; 而位相5所形成的气旋性环流为中国东北地区带来西北风冷平流, 降温过程更强且持续更长时间.     

Factors responsible for the increasing trend of mei-yu season rainfall during 1979–2020 over the western and eastern mei-yu domain

The mei-yu season (June–July) rainfall over the mei-yu monitoring domain (MMD) in the Yangtze–Huaihe Basin has shown an increasing trend in recent decades. This study examines the dominant factors responsible for this increasing trend for the period 1979–2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation. Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD, the magnitude of the trends is slightly larger over the eastern MMD (EMMD) than over the western MMD (WMMD). Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD. The increasing rainfall trend over the WMMD (EMMD) is attributable to increased evaporation (enhanced vertical moisture advection), which is dependent on an anomalous cyclonic circulation in the middle-lower troposphere over the MMD. Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 600 hPa over the EMMD, leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa. By contrast, the anomalous cyclone favors increasing local evaporation over the WMMD.摘要近几十年来, 江淮流域梅雨监测区 (MMD) 的梅雨期 (6–7月) 降水呈增加趋势. 本文基于1979–2020年台站观测降水资料和ERA5再分析数据, 从大气环流变异的角度揭示了这种长期增加趋势的主要影响因素. 发现在MMD范围内, 梅雨期降水趋势的增幅东部大于西部. 水汽收支定量诊断表明, 异常的蒸发和水汽平流对MMD西部和东部降水增加趋势的相对贡献是不同的. MMD西部 (东部) 的降水趋势主要归咎于增强的局地蒸发 (增强的垂直水汽平流) , 后者又取决于MMD对流层中, 低层的异常气旋环流. 这种位于气候平均的西太平洋副热带高压西北侧的异常气旋有助于MMD东部600 hPa以上的水汽辐散增加, 伴随加强的850 hPa水汽辐合, 从而导致垂直水汽平流的增强. 相反, 该异常气旋则有利于增强MMD西部的局地蒸发.     

Evaluation of the Madden–Julian oscillation in HiRAM

The aim of this study was to understand the cause of Madden–Julian oscillation (MJO) bias in the High Resolution Atmospheric Model (HiRAM) driven by observed SST through process-oriented diagnosis. Wavenumber-frequency power spectrum and composite analyses indicate that HiRAM underestimates the spectral amplitude over the MJO band and mainly produces non-propagating rather than eastward-propagating intraseasonal rainfall anomalies, as observed. Column-integrated moist static energy (MSE) budget analysis is conducted to understand the MJO propagation bias in the simulation. It is found that the bias is due to the lack of a zonally asymmetric distribution of the MSE tendency anomaly in respect to the MJO convective center, which is mainly attributable to the bias in vertical MSE advection and surface turbulent flux. Further analysis suggests that it is the unrealistic simulation of MJO vertical circulation anomalies in the upper troposphere as well as overestimation of the Rossby wave response that results in the bias.摘要本研究评估了高分辨率大气环流模式HiRAM模拟的MJO. 结果表明, HiRAM模拟的MJO东传很弱. 我们通过计算整层积分的湿静力能 (MSE) 收支来诊断MJO东传模拟偏差的原因. 结果发现, MSE倾向相对于MJO对流中心的纬向非对称分布很弱是导致东传模拟偏弱的原因, 这主要是由MSE垂直平流和地表湍流通量的模拟偏差造成的. 进一步研究表明, 对流层上层MJO垂直环流结构的模拟偏差和MJO对流西侧的Rossby波环流偏强共同导致了模式的偏差. 本研究中指出的MJO传播模拟偏差的原因与之前基于多模式结果的结论不同, 这意味着要想了解特定模式的模拟偏差, 有必要对该模式进行具体分析.     

Budget analysis of mesoscale available potential energy in a heavy rainfall event over the eastern Tibetan Plateau: 青藏高原东部暴雨的中尺度有效位能收支分析

Using model simulated data, the distribution characteristics, genesis, and impacts on precipitation of available potential energy (APE) are analyzed for a heavy rainfall event that took place over the eastern Tibetan Plateau during 10–11 July 2018. Results show that APE was mainly distributed below 4 km and within 8–14 km. The APE distribution in the upper level had a better correspondence with precipitation. Northwestern cold advection and evaporation of falling raindrops were primary factors leading to positive anomalies of APE in the lower level, while positive anomalies of APE in the upper level were caused by a combination of thermal disturbances driven by latent heat and potential temperature perturbations resulting from the orography of the Tibetan Plateau. Budget analysis of APE indicated that APE fluxes and conversion between APE and kinetic energy (KE) were the main source and sink terms. Meridional fluxes of APE and conversion of KE to APE fed the dissipation of APE in the lower level. Vertical motion enhanced by conversion of APE to KE in the upper level was the major factor that promoted precipitation evolution. A positive feedback between APE and vertical motion in the upper level generated a powerful correlation between them. Conversion of KE to APE lasted longer in the lower level, which weakened vertical motion; whereas, northwestern cold advection brought an enhanced trend to the APE, resulting in a weak correlation between APE and vertical motion.摘要针对2018年7月10-11日青藏高原东部一次暴雨过程, 利用模式模拟资料分析了有效位能分布特征,成因及其对降水发展演变的影响.结果表明, 有效位能主要分布在对流层低层4km以下和高层8-14km, 高层有效位能和降水有更好的对应性西北冷平流和降水粒子下落的蒸发作用是低层有效位能高值中心的主要成因, 而降水过程释放潜热带来的热力扰动叠加高原大地形造成的位温扰动是导致高层有效位能高值的主要原因.有效位能收支分析表明, 有效位能的通量输送项以及与动能间的转换项是主要源汇项.低层有效位能的经向通量输送和动能向有效位能的转化补给了有效位能的耗散;高层有效位能向垂直动能转化增强垂直运动是促进降水发展演变的主要因素.高层有效位能与垂直运动之间的正反馈过程使得两者相关性较强;低层较长时间内均存在垂直动能向有效位能的转化, 削弱了垂直运动, 而西北冷平流使得低层有效位能有增强的趋势, 因此二者相关性较弱.     

FY-2/D卫星反演的云特征参数与地面降水的相关分析

利用中国气象局人工影响天气中心研发的云参数卫星反演系统反演得到的产品,结合地面自动站观测资料,对2009年9月19—20日降水过程的云参数及地面雨量进行对比分析。结果发现:云顶高度、云顶温度、过冷层厚度和云光学厚度对本次降水过程指示性不强,而云粒子有效半径及云液水路径对降水有较好的指示作用,且云液水路径指示作用更强,二者的变化超前于地面降水30min到1h;云液水路径及云粒子有效半径大值区与地面雨量的大小呈正相关,云液水路径值大于400g.m-2及云粒子有效半径大于27μm区域与地面雨强中心位置基本一致。掌握云参数的演变规律,有助于监测、识别大范围人工影响天气作业条件和分析可播区。     

Observed decadal shifts and trends in global tropical cyclone activities from 1980 to 2021

This study investigates the variability of annual tropical cyclone (TC) frequency and intensity over six major ocean basins from 1980 to 2021. Statistical change-point and trend analyses were performed on the TC time series to detect significant decadal variation in TC activities. In the middle of the last decade of the 20th century, the frequency of TC genesis in the North Atlantic basin (NA) and North Indian Ocean (NIO) increased dramatically. In contrast, the frequency in the western North Pacific (WNP) decreased significantly at the end of the century. The other three basins—the East Pacific, southern Indian, and South Pacific—all experienced a declining trend in annual TC frequency. Over recent decades, the average TC intensity has decreased in the East Pacific and the NA, whereas it has risen in the other ocean basins. Specifically, from 2013 to 2021, the average peak TC intensity in the NIO has enhanced significantly. The magnitude of the Genesis Potential Index exhibits fluctuation that is consistent with large-scale parameters in the NIO, NA, and WNP, emphasizing the enhancing and declining trends in TCs. In addition, a trend and correlation analysis of the averaged large-scale characteristics with TCs revealed significant associations between the vertical wind shear and TC frequency over the NIO, NA, and WNP. Therefore, global TC trends and decadal variations associated with environmental parameters deserve further investigation in the future, mainly linked to the significant climate modes.摘要研究发现在1980–2021期间全球6个海域每年热带气旋的发生频次和强度具有显著年代际变化规律, 最近几十年, 北大西洋和北印度洋的热带气旋发生频次明显增加, 但西北太平洋的热带气旋却显著下降. 另外三个海域, 东太平洋, 南印度洋和南太平洋发现所生成的热带气旋有减少趋势. 但在过去十几年, 平均热带气旋的强度除了在东太平洋和北大西洋有所减弱但在其他几个海域有所加强, 特别是在 2013–2021期间, 北印度洋的平均热带气旋的强度增强明显. 热带气旋的潜在生成指数 (GPI) 增加或减少趋势变化与北印度洋, 北大西洋和西太平洋热带气旋变化相关的大尺度环流一致. 另外, 北印度洋, 北大西洋和西太平洋上空的垂直风切变是影响其区域热带气旋发生频次变化的主要因子, 不同的气候模态也可能对全球热带气旋的趋势变化和年代际变化有影响, 值得进一步研究.     

Dominant patterns of winter surface air temperature over Central Asia and their connection with atmospheric circulation

The dominant patterns of the winter (December–February) surface air temperature anomalies (SATAs) over Central Asia (CA) are investigated in this study. The first two leading modes revealed by empirical orthogonal function (EOF) analysis represent the patterns by explaining 74% of the total variance. The positive phase of EOF1 is characterized by a monopole pattern, corresponding to cold SATAs over CA, while the positive phase of EOF2 shows a meridional dipole pattern with warm and cold SATAs over northern and southern CA. EOF1 is mainly modulated by the negative phase of the Arctic Oscillation (AO) in the troposphere, and the negative AO phase may be caused by the downward propagation of the precursory anomalies of the stratospheric polar vortex. EOF2 is mainly influenced by the Ural blocking pattern and the winter North Atlantic Oscillation (NAO). The SATAs associated with EOF2 can be attributed to a dipole-like pattern of geopotential height anomalies over CA. The dipole-like pattern is mainly caused by the Ural blocking pattern, and the NAO can also contribute to the northern part of the dipole.摘要本文利用经验正交函数分解方法 (Empirical orthogonal function, EOF) , 针对1979–2019年冬季 (12月–2月) 中亚地区地面气温异常进行了研究. 结果表明, 中亚地区冬季地面气温异常的前两个EOF模态解释方差总占比可达74%. 其中, 第一模态 (EOF1) 正位相为一致型变化, 对应中亚地区气温冷异常; 第二模态 (EOF2) 正位相则为南北偶极型变化, 对应于中亚地区南冷北暖型气温异常. EOF1可能受到冬季北极涛动 (Arctic Oscillation, AO) 负位相的调制, 而AO的负位相则可能来自于前期平流层极涡正位势高度异常下传. EOF2则可能受到乌拉尔山阻塞及冬季北大西洋涛动 (North Atlantic Oscillation, NAO) 的共同调制. 乌拉尔山阻塞可引起中亚区域南北偶极型气温异常, 而冬季NAO可对该偶极型气温异常的北侧产生贡献.     

Analysis of aerosol distribution variations over China for the period 2045–2050 under different Representative Concentration Pathway scenarios: 不同代表性浓度路径情景下 2045-2050 年中国气溶胶分布变化的分析

The regional air quality modeling system RAMS-CMAQ was applied to simulate the aerosol concentration for the period 2045–2050 over China based on the downscaled meteorological field of three RCP scenarios from CESM (NCAR's Community Earth System Model) in CMIP5. The downscaling simulation of the meteorological field of the three RCP scenarios showed that, compared with that under RCP2.6, the difference in near-surface temperature between North and South China is weakened and the wind speed increases over North and South China and decreases over central China under RCP4.5 and RCP8.5. Under RCP2.6, from 2045 to 2050, the modeled average PM_2.5 concentration is highest, with a value of 40–50 µg m⁻³, over the North China Plain, part of the Yangtze River Delta, and the Sichuan Basin. Meanwhile, it is 30–40 µg m⁻³ over central China and part of the Pearl River Delta. Compared with RCP2.6, PM_2.5 increases by 4–12 µg m⁻³ under both RCP4.5 and RCP8.5, of which the SO₄²⁻ and NH₄⁺ concentration increases under both RCP4.5 and RCP8.5; the NO₃⁻ concentration decreases under RCP4.5 and increases under RCP8.5; and the black carbon concentration changes very slightly, and organic carbon concentration decreases, under RCP4.5 and RCP8.5, with some increase over part of Southwest and Southeast China under RCP8.5. The difference between RCP4.5 and RCP2.6 and the difference between RCP8.5 and RCP2.6 have similar annual variation for different aerosol species, indicating that the impact of climate change on different species tends to be consistent.摘要基于来自于 CMIP5 中 CESM 模式的三种 RCP 情景下的气象场的降尺度模拟, 应用区域空气质量模式系统 RAMS-CMAQ 模拟 2045-2050 年中国地区气溶胶浓度.三种 RCP 情景下气象场的降尺度模拟表明, 与 RCP2.6 相比, 在 RCP4.5 和 RCP8.5 下, 华北和华南的近地表温度差减小, 风速在华北和华南地区增加, 在中部地区下降. RCP2.6 情景下, 模拟的 2045 年到 2050 年平均的 PM _2.5浓度在华北平原, 长三角的部分地区和四川盆地最高, 约为 40-50 µg m^–3, 在中国中部和珠三角的部分地区约为 30-40 µg m^–3. 与 RCP2.6 相比, 在 RCP4.5 和 RCP8.5 下, PM_2.5增加了 4-12 µg m^–3, 其中在 RCP4.5 和 RCP8.5 下, SO₄^2–和 NH₄⁺的浓度增加, 在 RCP4.5 下, NO^3–浓度降低, 在 RCP8.5 下, NO^3–浓度升高, 在 RCP4.5 和 RCP8.5 下, BC 浓度变化很小, 而 OC 浓度下降, 其中在 RCP8.5 下, 西南和东南部分地区的 OC 有所增加.不同的气溶胶物种浓度在 RCP4.5 和 RCP2.6 之间的差异以及 RCP8.5 和 RCP2.6 之间的差异具有相似的年度变化, 这表明气候变化对不同物种的影响趋于一致.