夏季青藏高原及周边大气热源与四川盆地暴雨的关系

利用1960-2016年川渝逐日降水资料和NCEP/NCAR再分析资料,分析了夏季青藏高原及周边大气热源与四川盆地暴雨的关系。结果表明,青藏高原及其周边的大气热源对四川盆地夏季暴雨频数具有显著的影响。影响关键区分别位于高原中南部及其南侧和高原中东部及其东侧,由此定义了一个影响四川盆地夏季暴雨频数的高原热力差指数Itc,该指数能够较好地反映出盆地夏季暴雨频数的东、西部差异变化特征。当Itc偏高时,副高位置偏西偏北,阿拉伯海、孟加拉湾水汽输送增强,同时贝加尔湖西侧槽或低压位置偏西,盆地西部水汽辐合上升异常增强,暴雨明显偏多;而盆地东部暴雨偏少。当Itc偏低时,副高位置偏东,来自于东南沿海的水汽输送在盆地东部增强,同时贝加尔湖南侧多阻塞形势,使得水汽在盆地东部辐合上升增强,产生暴雨偏多;此时盆地西部暴雨偏少。     

高原低值系统影响下一次极端强降水天气诊断分析

采用常规观测资料、地面加密观测资料、逐时云顶亮温TBB资料和1°×1°NCEP/NCAR再分析资料,对2013年7月8~11日四川盆地持续性暴雨天气过程的中尺度对流系统活动及其发生发展的物理机制进行了分析。结果表明:(1)暴雨过程发生在对流层中层中高纬度两槽一脊稳定维持的环流背景下,由活跃的高原低值系统以及异常稳定的副高西侧偏南气流配合低层冷空气作用造成。(2)极端降水过程分为暖区强对流性降水和相对稳定的锋面降水两个阶段;暖区对流性降水阶段,偏南暖湿气流源源不断向盆地输送水汽和能量,为暴雨发生提供了高能高湿条件,大气层结极不稳定,中尺度对流云团发展旺盛;锋面降水阶段层结趋于稳定,对流云团有所减弱,但仍有充足的水汽输送且降水云系稳定少动,致使盆地西部产生持续性降水。(3)500h Pa高原低槽前的正涡度平流诱发盆地西部低层气旋性涡度增加、低涡生成和发展,致使暖湿气流持续在盆地西部形成辐合上升,为暴雨的维持提供了很好的动力条件,两个降水阶段均为明显的低层辐合高层辐散的特征,暖区对流性降水阶段正涡度发展较锋面降水阶段更强。(4)青藏高原东侧的地形作用强迫气流在盆地西部强烈辐合上升,使得暖湿水汽更加有效率地形成降水,是此次极端强降水天气出现的一个重要动力因素。      

龙门山沿线暴雨过程的诊断分析及数值试验

利用常规气象观测资料和西南区域模式WRF_RUC产品,对2012年8月17 18日四川盆地西部龙门山脉沿线出现的一次暴雨过程进行了诊断分析和数值试验。结果表明,对流层中低层明显高能、高湿,大气层结极不稳定,为中尺度对流系统的生成和发展提供了有利的热力不稳定条件;对流发展与低层偏东气流密切相关,暴雨开始前,四川盆地内低层偏东气流增强,使盆地西部沿山地区辐合和地形抬升作用增强,是造成垂直上升运动强烈发展的主要动力机制;降低高原地形高度,暴雨区明显西移;降低盆地内初始场温湿条件,降水强度明显减弱;不考虑地面热通量影响,降水强度也将有一定减弱。     

高原东侧陡峭地形对一次盆地中尺度涡旋及暴雨的数值试验

利用MM5中尺度模式,对2003年8月8~10日发生在四川盆地的一次强降水过程进行了数值模拟。分析表明:MM5模式较成功地模拟出2003年8月8~10日发生在高原东侧陡坡地形区域的四川盆地西北部的大暴雨过程。在本次暴雨过程中,中尺度涡旋有利于盆地上空正涡度输送和辐合上升运动的维持,有利于降水的维持和强降水的发生。地形对中尺度涡旋活动的影响主要表现在:复杂陡峭的地形扰动有利于中尺度涡旋的形成。保留真实地形,在青藏高原东部到盆地西部,涡旋活动频繁;降低地形高度和坡度,不仅减少了涡的个数,还减弱了涡的强度,并改变了涡的生成源地以及涡的移动路径;不考虑地形使得青藏高原到四川盆地西部涡旋活动明显减少,且涡移动速度加快;仅降低地形高度而不改变地形坡度,对该地区的涡旋活动无明显的影响;仅保留青藏高原地形,高原地区涡旋活动频繁,无地形地区涡明显减少。数值试验还表明,地形与涡旋的活动及降水的分布密切相关。     

青藏高原切变线对四川盆地西部突发性暴雨影响的数值试验

使用η模式对１９９５年８月２４日四川盆地西部一次突发性暴雨进行了数值模拟和无高原切变线、无西昌小高压的数值试验。由试验结果分析得出：（１）高原切变线活动可使四川盆地西部暴雨增强，而西昌小高压的存在则便四川盆地西部暴雨减弱；（２）高原切变线活动使暴雨增强的主要机制是暴雨区上空对流层低层流场辐合、上升运动、正涡度、水汽通量辐合和对流层中层流场辐合、水汽通量辐合等的加强；（３）对流层低层的动力、水汽条件     

青藏高原大地形对华南持续性暴雨影响的数值试验

利用新一代中尺度数值预报模式WRF3.2及NCEP/NCAR 逐日4次1°×1°的FNL再分析资料,通过有、无青藏高原以及将高原高度降低到临界高度的数值试验,研究了青藏高原大地形对我国华南地区2010 年5 月一次持续性暴雨过程的影响。试验结果表明:高原大地形对降水的影响显著,随着高原高度的升高,降水增多,高原以东地区的雨带也由北向南移动;高原地形的机械阻挡作用使迎风坡一侧的近地面层附近为强上升运动,背风坡为下沉运动,并分别对应降水的峰值和谷值区;高原对西风气流的爬流、绕流作用明显,高原升高后爬坡作用减弱,以绕流作用为主;高原的加热作用使气流过高原时南支减弱,北支加强,并加强了高原及其东部地区低层的正涡度和高层的负涡度,使高原上空为强烈的上升运动;高原的热力作用使西太平洋副热带高压位置偏南、偏西并稳定维持;高原大地形对形成稳定的高原季风环流圈有重要作用;高原地形高度的作用有利于定常波的形成,波动中心对应强上升运动,形成降水的大值区,稳定维持的定常波使得降水持续集中在同一地区,造成持续性暴雨。     

台风阻塞形势下两次四川盆地持续性暴雨过程对比研究

利用 CMORPH融合资料、地面降水资料和卫星云图资料，针对2015年6月22~24日（过程一）和2018年7月10~12日（过程二）四川省持续性暴雨过程，从动力、热力、水汽、云图等多方面进行对比分析。结果表明:两次过程的落区、强度均不相同；过程一主要降水产生于四川盆地东北部到中部，48h内共有338个区域自动站出现了暴雨以上的降水，最大雨量达到313.5mm；过程二强降水出现在四川盆地西北部，48h内共有942个区域自动站出现了暴雨以上的降水，最大雨量达到481.7mm。两次过程都有高空低槽、台风、切变线等多个不同尺度影响系统的相互作用；台风登陆点对四川盆地的暴雨落区有明显影响，台风从广西登陆，盆地降水偏北，而从东部沿海登陆，盆地降水偏西。两次过程低槽均为后倾槽，斜压特征明显；过程一降水激发了盆地涡的生成，盆地涡又使得降水持续，整个降水过程基本处于不稳定层结下，有MCC生成，多个时次的1h降水超过30mm；过程二随着副高东退，低槽东移，降水由前期的对流性转为稳定性降水。从位涡发展和移动来看，两次过程高位涡与强降水的发生时段吻合，位涡指示四川盆地暴雨落区具有重要的参考价值。      

青藏高原东侧陡峭地形对一次强降水天气过程的影响

利用高分辨率中尺度模式分析资料,研究了青藏高原东侧陡峭地形对一次暴雨天气发生发展的影响。结果显示,青藏高原地形对大气环流的动力阻挡作用形成了本次暴雨过程的水汽输送通道,青藏高原东侧陡峭地形结构造成了四川西北部和黄河上游的强水汽辐合中心,并使低层高能舌和能量锋区位于海拔较低的四川盆地,在四川盆地对流层低层建立起位势不稳定层结。青藏高原东侧陡峭地形结构引起了低层偏东气流强烈的垂直上升运动,最强的垂直上升运动出现在东西风垂直切变与陡峭地形交汇处,激发不稳定能量释放,促使强对流猛烈发展,暴雨过程中高原东侧还有一个中尺度涡旋的发生发展相伴。青藏高原东侧暴雨区最显著的热力特征是高温高湿区域仅出现在对流层低层,最显著的动力特征是强涡度柱也仅出现在对流层低层。     

中国西南部一次东移型暴雨中涡旋发展的多尺度地形影响研究

由观测和数值模拟结果分析发现,2019年8月5～6日中国西南部的东移型致灾暴雨事件中存在三涡（南北双高原涡、西南涡）相继发展并导致暴雨加强和移动的现象。借助数值试验,研究了多尺度地形因子（青藏高原、横断山脉和四川盆地三大地形）各自对涡旋演变的作用。结果表明,横断山脉对西南涡的形成起关键作用,四川盆地影响着西南涡的位置和强度。对于高原涡（南侧高原涡）的移动,四川盆地地形只影响涡旋强度演变,但不会改变高原涡的移动路径。一旦横断山脉被移除,高原涡的东移现象随之消失。进一步分析青藏高原和四川盆地交界处的陡峭地形坡度改变对涡旋发展的影响发现,发现坡度越陡,高原涡移动速度越快,且盆地内二涡合并后的西南涡强度越强。最后借助于倾斜涡度发展理论,解释了不同坡度对涡旋强度演变的影响：随着坡度变陡,倾斜涡度发展系数沿涡旋下滑路径快速减小,对垂直涡度局地倾向的强迫作用,加剧了涡旋的快速加强。     

江淮流域持续性强降水过程的多尺度物理模型

本文对江淮流域持续性暴雨事件（PHREs）的多尺度物理模型和能量转换特征以及青藏高原东部对流系统东移影响下游地区降水的研究成果进行了总结。从欧亚大陆Rossby波列能量频散的角度揭示了江淮流域PHREs中纬度系统槽脊稳定的机制,定量分析了冷暖空气的源地和输送路径,提出了江南型和江北型PHREs的多尺度物理模型。从天气尺度和次天气尺度之间的能量转换角度呈现了不同尺度系统相互作用的物理图像,指出背景场的能量供给是直接触发暴雨的次天气尺度系统维持的最重要因子,尤其是在对流层的低层,动能的降尺度级串（即能量由背景场传递给次天气尺度系统）最强。研究表明青藏高原东部对流系统东移影响江淮流域的降水是一系列天气系统配合和活跃的结果,主要由青藏高原和四川盆地、二级地形和东部平原之间的热力环流、西南涡、二级地形以东中尺度涡旋和对流系统的共同影响。除了本文总结的内容,还有一些影响PHREs的因子值得深入研究,多尺度相互作用中的Rossby波源及其波列如何影响天气系统,中尺度系统对其背景场的能量反馈等。     

夏季云贵高原地区降水特征及云水资源的匹配

基于云贵高原地区1961—2010年高分辨率(0.5°×0.5°)逐日降水格点资料,分析了云贵高原及东、西两个区域的夏季降水变化特征。并结合欧洲中期天气预报中心(ECMWF)提供的1979—2010年ERA-Interim再分析资料,计算了其夏季水汽输送通量和净水汽收支。结果表明:(1)云贵高原夏季平均降水分布不均匀,存在区域差异:云贵高原西部的中部为降水量低值区,其向南、向西逐渐增加;东部由其东南部向西北部递减的分布形式。(2)将云贵高原分成两个区域,东、西部区域的降水都呈增加的趋势,降水量较高的区域降水增长速度较快。(3)大气中的水汽从云贵高原南边界和西边边界进入,从北边界和东边界流出,全区以净水汽输出为主,输出值与降水的变化都呈增长趋势。其中东部水汽为净输入;西部为净输出,向各区域的水汽输送量逐渐增加与各区降水量呈增长趋势变化同样相一致。(4)影响西部夏季降水的水汽主要源于孟加拉湾北部、南海北部和横断山到四川盆地地区,而东部水汽主要来自南海北部和四川盆地西部。     

1961-2000年西南地区小时降水变化特征

根据西南地区112个站点1961-2000年逐时降水资料,分析了不同季节降水时数、小时雨强、极端强降水时数和极端强降水强度的变化趋势.从降水时数变化来看,夏季西南大部分地区如四川盆地西部、云南、贵州南部等地总降水时数有减少趋势,四川盆地东部和川西高原总降水时数增加;整个区域平均趋势为-0.9%/10a.相应地,极端强降...     

The Persistent Heavy Rainfall over Southern China in June 2010：Evolution of Synoptic Systems and the Effects of the Tibetan Plateau Heating

This study investigates influencing weather systems for and the effect of Tibetan Plateau （TP）’s surface heating on the heavy rainfall over southern China in June 2010, focusing on the four persistent heavy rainfall events during 14-24 June 2010. The ma jor weather systems include the South Asian high, midlatitude trough and ridge, western Pacific subtropical high in the middle troposphere, and shear lines and eastward-moving vortices in the lower troposphere. An ensemble of convection-permitting simulations （CTL） is carried out with the WRF model for these rainfall events, which successfully reproduce the observed evolution of precipitation and weather systems. Another ensemble of simulations （SEN） with the surface albedo over the TP and its southern slope changed artificially to one, i.e., the surface does not absorb any solar heating, otherwise it is identical to CTL, is also performed. Comparison between CTL and SEN suggests that the surface sensible heating of TP in CTL significantly affects the temperature distributions over the plateau and its surroundings, and the thermal wind adjustment consequently changes atmospheric circulations and properties of the synoptic systems, leading to intensified precipitation over southern China. Specifically, at 200 hPa, anticyclonic and cyclonic anomalies form over the western and eastern plateau, respectively, which enhances the southward cold air intrusion along the eastern TP and the divergence over southern China;at 500 hPa, the ridge over the northern plateau and the trough over eastern China are strengthened, the southwesterly flows along the northwestern side of the subtropical high are intensified, and the positive vorticity propagation from the plateau to its downstream is also enhanced significantly;at 850 hPa, the low-pressure vortices strongly develop and move eastward while the southwesterly low-level jet over southern China strengthens in CTL, leading to increased water vapor convergence and upward motion over the precipitation region.     

Impacts of Diurnal Variation of Mountain-plain Solenoid Circulations on Precipitation and Vortices East of the Tibetan Plateau during the Mei-yu Season简

Diurnal variations of two mountain-plain solenoid （MPS） circulations associated with ＂first-step＂ terrain [Tibetan Plateau （TP）] and ＂second-step＂ terrain （high mountains between the TP and ＂east plains＂） in China and their influence on the south west vortex （SWV） and the mei-yu front vortex （MYFV） were investigated via a semi-idealized mesoscale numerical model [Weather Research and Forecasting （WRF）] simulation integrated with ten-day average fields （mei-yu period of 1-10 July 2007）. The simulations successfully reproduced two MPS circulations related to first and second-step terrain, diurnal vari- ations from the eastern edge of the TP to the Yangtze River-Huaihe River valleys （YHRV）, and two precipitation maximum centers related to the SWV, MYFV. Analyses of the averaged final seven-day simulation showed the different diurnal peaks of precipitation at different regions： from the aftemoon to early evening at the eastern edge of the TP; in the early evening to the next early morning in the Sichuan Basin （SCB）; and in the late evening to the next early morning over the mei-yu front （MYF）. Analyses of individual two-day cases confirmed that the upward branches of the nightlime MPS circulations enhanced the precipitation over the SWV and the MYFV and revealed that the eastward extension of the SWV and its con vection were conducive to triggering the MYFVs. The eastward propagation of a rainfall streak from the eastern edge of the TP to the eastern coastal region was primarily due to a series of convective activities of several systems from west to east, including the MPS between the TP and SCB, the SWV, the MPS between second-step terrain and tile east plains, and the MYFV.     

PV Perspective of Impacts on Downstream Extreme Rainfall Event of a Tibetan Plateau Vortex Collaborating with a Southwest China Vortex

An extreme rainfall event occurred over the middle and lower reaches of the Yangtze Basin(MLY) during the end of June 2016, which was attributable to a Tibetan Plateau(TP) Vortex(TPV) in conjunction with a Southwest China Vortex(SWCV). The physical mechanism for this event was investigated from Potential Vorticity(PV) and omega perspectives based on MERRA-2 reanalysis data. The cyclogenesis of the TPV over the northwestern TP along with the lowertropospheric SWCV was found to involve a midtropospheric large-scale flow reconfiguration across western and eastern China with the formation of a high-amplitude Rossby wave. Subsequently, the eastward-moving TPV coalesced vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection,leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV-SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascending motion including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY. This led to more intense rainfall. Quantitative PV diagnoses demonstrate that both the coalescence and subsequent re-separation processes of the TPV with the SWCV were largely dominated by horizontal PV advection and PV generation due to vertically nonuniform diabatic heating, as well as the feedback of condensation latent heating on the isentropicdisplacement vertical velocity.     

CMA-SH9在川渝地区的降水日变化预报效果评估

利用CMA-SH9模式逐小时降水预报数据和地面自动站-CMORPH卫星融合降水数据,开展该模式对2020年暖季（5~9月）川渝地区降水日变化的预报效果评估。结果表明:CMA-SH9模式可以再现小时平均降水量在四川盆地偏小、盆地周边陡峭地形处偏大的空间分布特征;显著的预报正偏差分布于青藏高原东坡至四川盆地西南部一带和四川盆地以东地区,偏差来自降水频率和降水强度的共同贡献;预报负偏差分布于四川盆地,主要来自模式对降水强度的低估;降水日变化峰值时间自西向东呈午夜到上午的滞后,模式预报的降水日变化峰值时间超前于观测;模式能够较好地把握青藏高原东坡至四川盆地西南部一带和四川盆地的单峰型日变化位相,以及盆地以东地区的双峰型日变化位相,但预报的降水量值和观测存在一定偏差。      

Temporal and Spatial Characteristics of Extreme Hourly Precipitation over Eastern China in the Warm Season

Based on hourly precipitation data in eastern China in the warm season during 1961-2000,spatial distributions of frequency for 20 mm h 1 and 50 mm h 1 precipitation were analyzed,and the criteria of short-duration rainfall events and severe rainfall events are discussed.Furthermore,the percentile method was used to define local hourly extreme precipitation;based on this,diurnal variations and trends in extreme precipitation were further studied.The results of this study show that,over Yunnan,South China,North China,and Northeast China,the most frequent extreme precipitation events occur most frequently in late afternoon and/or early evening.In the Guizhou Plateau and the Sichuan Basin,the maximum frequency of extreme precipitation events occurs in the late night and/or early morning.And in the western Sichuan Plateau,the maximum frequency occurs in the middle of the night.The frequency of extreme precipitation (based on hourly rainfall measurements) has increased in most parts of eastern China,especially in Northeast China and the middle and lower reaches of the Yangtze River,but precipitation has decreased significantly in North China in the past 50 years.In addition,stations in the Guizhou Plateau and the middle and lower reaches of the Yangtze River exhibit significant increasing trends in hourly precipitation extremes during the nighttime more than during the daytime.     

2012年西南低涡影响下四川地区一次强降水过程研究

本文利用中国气象科学数据共享服务网提供的中国地面降水日值0.1°×0.1°格点数据集(V2.0)、FY-2E逐小时红外黑体辐射亮温(Temperature of Black Body,以下简称TBB)资料和欧洲中心ERA-Interim0.5°×0.5°,6小时一次温度、风场、位势高度、相对湿度等资料对2012年7月21~22日一次四川暴雨进行了初步分析。结果表明,西南低涡活动为此次四川盆地强降水过程提供了有利的发生条件。在此基础之上,副热带高压引导的暖湿空气与西伯利亚南下的冷空气相汇于四川盆地,风切变的生成,大量不稳定能量在四川盆地东部累积,和盆地上空形成的上升气流,为此次降水过程提供了有利的天气条件。随着西南低涡的东移减弱,四川盆地此次降水过程也随之结束。     

Contrasts between the Interannual Variations of Extreme Rainfall over Western and Eastern Sichuan in Mid-summer

Rainfall amount in mid-summer(July and August)is much greater over eastern than western Sichuan,which are characterized by basin and plateau,respectively.It is shown that the interannual variations of extreme rainfall over these two regions are roughly independent,and they correspond to distinct anomalies of both large-scale circulation and sea surface temperature(SST).The enhanced extreme rainfall over western Sichuan is associated with a southward shift of the Asian westerly jet,while the enhanced extreme rainfall over eastern Sichuan is associated with an anticyclonic anomaly in the upper troposphere over China.At low levels,on the other hand,the enhanced extreme rainfall over western Sichuan is related to two components of wind anomalies,namely southwesterly over southwestern Sichuan and northeasterly over northeastern Sichuan,which favor more rainfall under the effects of the topography.Relatively speaking,the enhanced extreme rainfall over eastern Sichuan corresponds to the low-level southerly anomalies to the east of Sichuan,which curve into northeasterly anomalies over the basin when they encounter the mountains to the north of the basin.Therefore,it can be concluded that the topography in and around Sichuan plays a crucial role in inducing extreme rainfall both over western and eastern Sichuan.Finally,the enhanced extreme rainfall in western and eastern Sichuan is related to warmer SSTs in the Maritime Continent and cooler SSTs in the equatorial central Pacific,respectively.