Large-Scale Circulation Features Typical of Wintertime Extensive and Persistent Low Temperature Events in China

A pair of northeast-southwest-tilted mid-tropospheric ridges and troughs on the continental scale was observed to be the key circulation feature common among wintertime extensive and persistent low tempera-ture events (EPLTE) in China.During the persistence of such anomalous circulations,the split flow over the inner Asian continent and the influent flow over the southeast-ern coast of China correspond well to the expanded and amplified Siberian high with tightened sea level pressure gradients and hence,a strong,cold advection over south-eastern China.The western Pacific subtropical high tends to expand northward during the early stages of most EPLTEs.     

Influence of Major Stratospheric Sudden Warming on the Unprecedented Cold Wave in East Asia in January 2021

An unprecedented cold wave intruded into East Asia in early January 2021 and led to record-breaking or historical extreme low temperatures over vast regions.This study shows that a major stratospheric sudden warming(SSW)event at the beginning of January 2021 exerted an important influence on this cold wave.The major SSW event occurred on 2 January 2021 and subsequently led to the displacement of the stratospheric polar vortex to the East Asian side.Moreover,the SSW event induced the stratospheric warming signal to propagate downward to the mid-to-lower troposphere,which not only enhanced the blocking in the Urals-Siberia region and the negative phase of the Arctic Oscillation,but also shifted the tropospheric polar vortex off the pole.The displaced tropospheric polar vortex,Ural blocking,and another downstream blocking ridge over western North America formed a distinct inverted omega-shaped circulation pattern(IOCP)in the East Asia-North Pacific sector.This IOCP was the most direct and impactful atmospheric pattern causing the cold wave in East Asia.The IOCP triggered a meridional cell with an upward branch in East Asia and a downward branch in Siberia.The meridional cell intensified the Siberian high and low-level northerly winds,which also favored the invasion of the cold wave into East Asia.Hence,the SSW event and tropospheric circulations such as the IOCP,negative phase of Arctic Oscillation,Ural blocking,enhanced Siberian high,and eastward propagation of Rossby wave eventually induced the outbreak of an unprecedented cold wave in East Asia in early January 2021.     

2004/2005年冬季强寒潮事件与大气低频波动关系的研究

2004/2005年冬季中国出现了两次大范围寒潮过程,造成长时期的降温和严寒天气,从而改变了从1986年以来中国大部分地区连续出现18个暖冬的局面。利用2004年10月1日～2005年3月31日中国740站逐日平均温度和NCEP/NCAR的逐日高度场和风场资料,分析了2004/2005冬季的强寒潮事件,并对寒潮爆发的周期、低频波动特征及其影响寒潮爆发的可能原因进行研究。结果表明:2004/2005年冬季两次主要强寒潮事件（12月22日～1月1日和2月14～21日）是在强10～20天低频振荡背景下发生的。在一次季节内低频振荡过程中,寒潮经历了高空形势相似的3个阶段,它们分别是乌拉尔东侧阻塞脊建立、阻塞脊下游横槽发展、横槽转竖与南支槽耦合重建东亚大槽最后导致寒潮爆发。进一步分析表明,乌拉尔东侧阻塞脊和里海高度脊是导致这两次寒潮的主要系统。研究还表明在寒潮爆发过程中,10～20天低频波动在40°N南北独立传播,南北波列耦合寒潮全面爆发;寒潮爆发后期低频波动引起我国东南部低空辐散异常、高空辐合异常,从而导致我国南部地区偏北风增强,使寒潮向南爆发程度和高空东亚急流明显加强。     

Extensive Cold-Precipitation-Freezing Events in Southern China and Their Circulation Characteristics

Concurrence of low temperature,precipitation and freezing weather in an extensive area would cause devastating impacts on local economy and society.We call such a combination of concurrent disastrous weather“extensive coldprecipitation-freezing”events(ECPFEs).In this study,the ECPFEs in southern China(15°?35°N,102°?123°E)are objectively defined by using daily surface observational data for the period 1951?2013.An ECPFE in southern China is defined if the low temperature area,precipitation area and freezing area concurrently exceed their respective thresholds for at least three consecutive days.The identified ECPFEs are shown to be reasonable and reliable,compared with those in previous studies.The circulation anomalies in ECPFEs are characterized by a large-scale tilted ridge and trough pairing over mid-and high-latitude Eurasia,and the intensified subtropical westerlies along the southern foot of the Tibetan Plateau and the anomalous anticyclonic circulation over the subtropical western Pacific.Comparative analysis reveals that the stable cold air from the north and the warm and moist air from the south converge,facilitating a favorable environment for the concurrence of extensive low-temperature,precipitation and freezing weather.     

冬季大范围持续性极端低温事件与欧亚大陆大型斜脊斜槽系统研究进展

本文总结了近年来关于我国冬季大范围持续性极端低温事件（EPECE）及其对应的欧亚大陆大型斜脊斜槽系统的研究成果。EPECE和普通寒潮是冬季影响我国的两类不同时间尺度大型冷空气活动,对它们的异同点进行梳理和深入理解是非常必要的。最新研究进展可概括为如下:（1）基于极端低温站点的范围和极端低温过程的持续性特点,客观界定了我国冬季EPECE。近年来的研究表明,欧亚大陆大型斜脊斜槽系统是冬季EPECE形成和维持的主要关键环流系统。同时,鉴于大型斜脊斜槽系统的重要应用意义,建立了客观识别方法。（2）从前兆信号、环流演变、阻塞高压和反气旋式波破碎活动的角度,揭示了EPECE和普通寒潮事件之间的关键区别。全国类EPECE的发生具有一周之前的前兆信号,而普通寒潮并不存在这么早的前兆信号。EPECE以从乌拉尔山到东北亚的广阔区域的阻塞高压活动为关键特征,而普通寒潮则主要以区域性阻塞高压为其主要特征。这两类事件对应的阻塞高压活动的差异可由天气尺度波破碎活动的差异加以解释。（3）最新的研究解释了大型斜脊斜槽系统形成和维持的动力学机理。基本流场对位涡扰动的正压作用是大型斜脊斜槽系统的形成和维持最重要的动力学机制。基本流场通过变形场作功和线性平流使大型扰动维持和向下游发展。与阻塞高压不同,非线性作用并非大型斜脊斜槽系统维持的主要原因。     

乌兰察布市2010年一次大风、寒潮天气的分析

运用常规气象观测资料和欧洲数值预报图,对2010年1月19—22日从西北方向入侵的强冷空气引发大面积的大风降温天气过程进行分析。结果表明：此次寒潮主要是由贝加尔湖附近的横槽内聚积一股较强的冷空气（高空500hPa冷中心强度达-45℃、地面冷高压中心强度达1060．0hP）。西西伯利亚的阻塞高压与贝加尔湖高空冷槽共同作用,西北路冷空气不断向东南侵袭,乌兰察布市处于西风带控制,横槽沿西风带向东移动分裂为两股冷空气。阻塞高压前偏北冷空气不断东移南下,引发横槽转竖并向南加深。持续性降温导致强寒潮天气过程。     

Precursory Signals of Extensive and Persistent Extreme Cold Events in China

The distinct precursory signals of countrywide extensive and persistent extreme cold events (CECs) were investigated and contrasted with those of countrywide cold wave events (CCWs). It is shown that most CECs were accompanied by a CCW in the initial stages. From the comparison between the CECs and the CCWs that were independent of any CEC, it is found that a south- west-northeast-oriented tilted ridge at 500 hPa was present around the Europe-Barents Sea regions approximately 10 days prior to the start of the CEC. Consistent with this feature, a high sea level pressure and strong cold air accumulation occurred over a broad extent of northern Eurasia one week prior to the start of the CEC. The tilted ridge and the strong cold air accumulation were the precursory signals that were absent for the CCW, and they provide important clues for the early prediction of whether a CCW event might evolve into a CEC.     

Analysis of Isentropic Potential Vorticity for a Strong Cold Wave During 2004/2005 Winter

Using the NCAR/NCEP daily reanalysis data from 1 December 2004 to 28 February 2005, the isentropic potential vorticity （IPV） analysis of a strong cold wave from 22 December 2004 to 1 January 2005 was made. It is found that the strong cold air of the cold wave originated from the lower stratosphere and upper troposphere of the high latitude in the Eurasian continent and the Arctic area. Before the outbreak of the cold wave, the strong cold air of high PV propagated down to the south of Lake Baikal, and was cut off by a low PV air of low latitude origin, forming a dipole-type circulation pattern with the low PV center （blocking high） in the northern Eurasian continent and the high PV one （low vortex） in the southern part. Along with decaying of the low PV center, the high PV center （strong cold air） moved towards the southeast along the northern flank of the Tibetan Plateau. When it arrived in East China, the air column of high PV rapidly stretched downward, leading to increase in its cyclonic vorticity, which made the East Asian major trough to deepen rapidly, and finally induced the outbreak of the cold wave. Further analysis indicates that in the southward and downward propagation process of the high PV center, the air flow west and north of the high PV center on isentropic surface subsided along the isentropic surface, resulting in rapid development of Siberian high, finally leading to the southward outbreak of the strong cold wave.     

我国南方两次低温雨雪事件天气成因对比分析

2018年和2021年末我国南方分别发生了一次大范围的低温雨雪天气,对生活、生产造成了严重影响,因此对比分析这两次低温雨雪天气成因具有重要意义。结果表明：两次过程期间,对流层中层中高纬阻塞流场显著,阻高位于贝加尔湖西侧,脊前偏北气流在下游横槽后部堆积,使得西伯利亚高压强度增强。东传的Rossby波在阻高区域发生能量频散,利于阻高减弱、崩溃,横槽转竖引导槽后冷空气南下,导致地面强烈降温,同时在西伯利亚高压东侧和南侧,低频风温度平流是造成强降温的主要原因。低纬南支槽活跃,向北的暖湿空气与中高纬南下的冷空气汇合,造成我国南方大范围的低温雨雪、冻雨天气。与2018年过程相比,2021年过程持续时间较短,降水范围小,关键区降温幅度更大,是因为2021年过程期间Rossby波能量频散更快,阻高维持时间较短,冷空气从中高纬地区直接南下侵袭我国,而2018年冷空气在贝加尔湖附近发生堆积、西折,向南渗透时势力减弱。     

近53年中国寒潮的变化特征及其可能原因

利用1951～2004年中国740站逐日温度资料, 对中国寒潮频次的气候特征及其变化进行了分析, 并在此基础上对中国寒潮频次减少的可能原因进行了讨论: 冬季西伯利亚高压和东亚冬季风强度与中国寒潮频次呈显著的正相关.秋冬季节西伯利亚上空低层冷堆温度和中国寒潮频次呈显著的负相关.在气候变暖的大背景下, 西伯利亚高压和冬季风强度的减弱使得冬季中国地表温度持续升高, 而温度的这种变化与中国寒潮频次及其相伴随大风频次的减少均有密切的联系.西伯利亚高压和冬季风强度的减弱, 西伯利亚上空低层冷堆温度和中国地表温度的显著升高是中国寒潮及其相伴随大风频次减少的可能原因.     

强厄尔尼诺事件下2016年1月中国南方超级寒潮的动力学机制:瞬变波对大气长波异常的调制

本文采用Plumb三维波活动通量和局地Eliassen-Palm通量诊断方法,利用欧洲中期天气预报中心的逐日再分析资料ERA-Interim,分析了超强厄尔尼诺背景下2016年1月下旬中国南方超级寒潮的动力学机制：瞬变波对大气长波异常的调制。前期2015年12月的北大西洋海表热通量正异常,有助于后期大西洋阻塞形势的维持。大气长波能量沿大圆路径从大西洋阻高经乌拉尔地区向东亚中低纬度传播并在此辐合,导致了乌拉尔阻高和华北横槽的经向结构,更多强冷空气聚集在异常偏南的纬向槽线附近。寒潮爆发前夕,2支瞬变波列活跃在亚欧大陆。北支瞬变波列调制了北方的大气长波,使横槽转竖;南支瞬变波列协同调控了南方的大气长波,使南支印缅槽减弱;两者共同作用,促使冷平流大举南下,直达华南沿海,南方寒潮发生。     

中国大范围持续性低温事件与中国南方降水异常

本文利用1951~2009年的数据资料,探讨了与冬季大范围持续性低温事件对应的中国南方降水多寡的原因以及不同降水类型事件的不同前兆信号。主要结果如下:(1)欧亚大陆中高纬大型斜脊斜槽是中国大范围持续性低温事件的关键环流特征。降水偏多型和偏少型低温事件的中高纬环流系统存在显著差异:当大型斜脊的纬向尺度较大且其正高度距平中心偏强时,斜槽的负距平中心靠近东亚大槽区,西伯利亚高压大范围加强,南侵冷空气活动偏强,使中国南方地区降水偏少。相反,当大型斜脊的纬向尺度相对小时,斜槽位置偏西,西伯利亚高压加强范围偏小,南侵冷空气活动也偏弱,有利于中国南方地区降水偏多。(2)在低温事件中我国南方地区的降水多寡,也取决于西太副高和孟加拉湾南支槽。当西太副高和孟加拉湾南支槽加强时,来自西太平洋、南海和孟加拉湾的三支暖湿气流到达我国南方地区,导致南方降水偏多,反之亦然。(3)不同降水类型低温事件的前兆环流信号(之前1~3侯)显著不同。降水偏多型低温事件前期,源自北大西洋地区的波列向东亚地区传播。降水偏少型事件前期,极涡在欧亚大陆一侧的次极区显著减弱,同时极涡呈两极型,一极伸向南欧及地中海地区,另一极伸向阿留申群岛。在中期—延伸期时间尺度上,这一结果可为持续性低温事件降水形势的预测提供依据。     

The Large Scale Circulation of the Snow Disaster in Southern China in the Beginning of 2008

This study analyzed the heaviest snowfalls or icy-rainfalls occurring in southern China from January to the beginning of February 2008.The results are summarized as follows：the disaster was induced by the persistent front of warm/cold air masses in southern China,which displayed an interaction between the weather systems in higher and lower latitudes.There was an adjustment for circulation at hemisphere scale during mid January by a variation of sign of the Arctic Oscillation（AO）index from negative to positive.The long lasting precipitation well coincided with a blocking situation centered near 80°E from mid January to the beginning of February.A diagnostic analysis indicates that stationary waves with an energy dispersion accompanying the blocking high propagated from upstream region in high latitudes to the south of the Yangtze River,which formed a maintaining energy source for the cyclonic circulation in situ.This resulted in that the large mass of cold air in high latitudes could not easily invade into the south but slowly shifted southward.On the other hand,the sea surface temperature（SST）over the warm pool of the western Pacific increased with a new history record due to the effect of the strong La Nina episode,which also blocked the cold air mass from the north.The blocking high collapsed around 30 January and the energy source for the local cyclonic circulation was cut off.Thus,the precipitation in southern China ceased after 1 February.     

Maintenance and development of the Ural high and its contribution to severe cold wave activities in winter 2020/21

2020年底至2021年初,连续两次强寒潮入侵中国东部地区,导致大范围强降温.本文研究这两次寒潮的特征和形成机理.主要结果如下:从2020年12月中旬到2021年1月中旬,乌拉尔山地区维持一宽阔的高压脊.脊前北风的维持和加强导致局地斜压性增加,导致下游横槽槽底等高线梯度加大,冷平流加强,西伯利亚高压加强和向南扩张.准定...     

南海季风爆发前罕见连续3场暴雨特征及成因

2010年5月上中旬南海季风尚未爆发,广东一周内出现罕见的连续3场区域性暴雨 (下称连场暴雨)。利用常规气象观测资料和NCEP分析资料,从降水时间特征和环流形势对比了连场暴雨和持续性暴雨的异同,并应用局地经向环流数值模式诊断探讨其可能形成机制。结果表明:中高纬度地区阻塞形势建立对广东5月连场暴雨和6月持续性暴雨发生均尤为关键,连场暴雨期间阻塞高压位于乌拉尔山附近,降水与中纬度短波槽南下密切相关;而持续性暴雨期间阻塞高压偏东位于亚洲大陆中部,降水主要受热带西南季风北推影响。尽管大尺度环流背景相似,但3场暴雨过程天气系统配置差异较大。数值诊断结果进一步表明:激发连续3场暴雨的主要物理因子为潜热加热、温度平流和西风动量输送。潜热加热是此次连场暴雨的正贡献和正反馈的最直接因子,而西风动量输送和温度平流对暴雨发生有一定触发作用和指示意义 (超前0~1.5 d)。因此,分析和预报季风爆发前的连场暴雨过程,应注意中高纬度地区西风动量输送、冷暖平流活动和相应的天气形势演变。     

南京"2003.2.10"寒潮降温降雨(雪)天气过程初步分析

对2003年2月10日至11日发生在南京的一次寒潮降温降雨(雪)的主要影响系统，如阻塞高压、高空槽、横槽、中低层暖式切变线、地面冷锋以及地面暖低压倒槽等进行了描述和分析，并对某些物理量进行了分析。分析得出：这次寒潮降温降雨(雪)天气是在高空500hPa乌拉尔山阻塞高压崩溃、巴尔喀什湖至准噶尔盆地的横槽转竖，冷空气从西北路径东移南下，中低层冷槽与暖式切变线接合以及地面冷锋切入暖低压倒槽等天气系统的作用下发生的，并归纳出此类天气预报的指示系统，对于做好寒潮天气预报具有指导作用。     

中国北方冬季气温的年际变化对北太平洋东部海温异常的响应

利用地面气温观测资料及NCEP/NCAR逐月再分析资料,分析了中纬度北太平洋东部海温异常变化对中国北方地区冬季气温的可能影响。结果表明,前期夏、秋季中纬度北太平洋东部海温与北方地区冬季气温存在持续稳定的正相关关系,并且这种相关性在年代际尺度上较年际尺度更为显著。这种联系与中纬度北太平洋东部关键区海温在对流层中低层激发出的一种类似北美—大西洋—欧亚遥相关型波列有关。当前期关键区海温偏高（低）时,其激发的波列使得乌拉尔山阻塞高压偏弱（强）,西伯利亚高压偏弱（强）,导致贝加尔湖以南大部地区受正（负）高度距平控制,亚洲地区中高纬以纬（经）向环流为主,有利于北方大部地区气温偏高（低）。研究表明,中纬度北太平洋东部海温异常通过激发出一个从关键海区到我国北方地区的跨越东西半球的遥相关型波列,引发北半球中高纬度大气环流异常,进而影响北方冬季气温。     

2020年2月大气环流和天气分析

2020年2月大气环流的主要特征是北半球极涡呈偶极型分布且较常年同期明显偏强,欧亚地区中高纬环流呈三波型,环流呈纬向型,经向度较小。西太平洋副热带高压较常年偏强;下旬南支槽趋于活跃。2月,我国冷空气过程较常年偏少,出现今年首个全国型寒潮过程;全国平均气温为-0.1℃,较常年同期偏高1.6℃;全国平均降水量21.1 mm,较常年同期偏多18%。此外,前半月中东部多过程性雾 霾天气;西北地区出现今年首次沙尘天气过程。     

近11年中国寒潮频发的机理分析

利用全国840个站点观测资料以及ERA5再分析资料,分析了过去63 a(1960—2022年)冬半年(前一年10月—当年4月)全国性寒潮频次的年代际变化特征。统计结果表明:在全球变暖背景下,寒潮频次的下降趋势在2012年发生了转折,2012—2022年呈现显著上升趋势。2012—2022年,乌拉尔山阻塞频率与全国性寒潮发生频次呈显著负相关。乌拉尔山阻高会抑制高空急流发生发展,大气经向环流减弱,北极内冷气团南下受阻,从而减少寒潮发生的频次。同时乌拉尔山阻高与东亚中高纬地区地面2 m温度经向分布联系紧密,乌拉尔山阻塞的频繁发生往往会出现更多的北极增温、中纬度地区降温的天气尺度现象。此外,北极增暖效应本身也与我国寒潮频次有显著的负相关,北极增暖会减小中高纬温度差,西风带减弱,进而减弱冷空气的输送。近11 a,乌拉尔山阻塞频率呈显著减少趋势,相应地,等熵面位涡经向梯度具有线性增加趋势,不利于阻塞高压的维持,伴随北极增暖效应也呈现出减弱态势,这样的大气环流配置有利于近年来我国寒潮频次的增加。     

瞬变波活动与江淮地区夏季旱涝的关系

利用1951-2000年中国160站逐月降水量资料划分了江淮地区夏季旱涝年。通过对旱涝年500hPa高度距平场的合成分析发现：多雨年鄂霍茨克海阻塞高压稳定维持，少雨年鄂霍茨克海附近的高压脊减弱。为了了解江淮地区旱涝年500hPa中高纬环流形势异常的原因，计算了Eliassen-Palm通量，结果表明多雨年瞬变波对平均气流的强迫作用使经、纬向风异常分布有利于鄂霍茨克海阻塞形势的维持，冷空气向南输送增强，江淮地区降水偏多；少雨年瞬变波对平均气流的强迫作用使鄂霍茨克海高压脊减弱，冷空气向南输送减弱，江淮地区降水偏少。