华北“7·19”暴雨中低涡系统演变及多尺度相互作用机制研究

为探究华北暴雨的维持及中尺度系统演变机制,利用NCEP/NCAR的GFS资料、地面自动站观测资料等,借助数值模拟、涡度收支分析和尺度分离等方法,对2016年7月19日前后一次华北暴雨过程进行了观测分析和模拟研究。(1)本次极端降水过程与东移低槽切断形成的深厚低涡密切相关。低涡与副高脊线形成“东高西低”形势且雨区始终处于高层辐散低层辐合的动力配置下,有利于对流维持。涡旋与低空急流的配合使来自西南侧和东侧的水汽在华北辐合,并使雨区处于能量锋区,对流层中低层形成深厚逆温层,为暴雨维持提供水汽和能量保障。(2)低涡系统总体呈增强趋势,中心涡度最高达55×10-5 s-1以上。成熟阶段呈现贯穿对流层的直立正涡度柱,但涡度变化集中在500 hPa以下,中心维持在850 hPa附近。涡度增长主要受正涡度区与辐合中心重合产生的拉伸效应以及干侵入等因素的促进作用。(3)低层辐合中心由三种不同尺度系统叠加而成,其中中尺度系统对中心的强度和位置影响最大,而大中尺度风场间的辐合也使辐合区更大、强度更强。低层涡旋增长与风场辐合加强之间形成正反馈调节,有利于低涡和降水的维持。      

引发暴雨天气的中尺度低涡的数值研究

2008年7月17—19日发生在山东的大到暴雨天气是由“海鸥”台风和副热带高压共同向山东输送水汽,与弱冷空气相互作用造成的。对流层低层的中尺度低涡是暴雨天气的直接制造者。利用常规观测资料和中尺度模式WRF（Weather Research and Forecasting）的模拟资料对该中尺度低涡的结构及形成机制进行了分析研究。结果表明,数值模拟可以清楚地捕捉到中尺度低涡东移过程中有新的涡旋中心形成,并与原来的涡旋中心合并的过程,而不是简单的沿切变线东移。中尺度低涡形成在增温增湿明显、上升运动为主的对流区内;中尺度低涡形成后其中心转为下沉运动,对流区东移,降水区位于低涡的东北和东南象限。中尺度低涡上空近地面层的冷池、600～400hPa的弱冷空气堆、900～850hPa的弱风区及高低空急流耦合发展是中尺度低涡形成和发展阶段的重要特征。中尺度低涡减弱阶段,下沉运动变强,低空急流和高空出流都明显减弱。涡度方程的收支表明,对流层低层的散度项、倾侧项及对流层中层的水平平流项和铅直输送项是正涡度的主要贡献者。中低层的水平辐合、涡度由低层向高层的垂直输送都有利于中尺度低涡的形成和发展。倾侧项对中尺度低涡的形成也有重要贡献。中尺度低涡形成后期,低层辐合、高层辐散及垂直输送的减弱导致正涡度制造的减弱,从而使中尺度低涡减弱。     

一次河北大暴雨的华北低涡结构和涡度收支分析

利用Micaps系统下常规资料、云图资料和NCEP全球再分析资料,对2004年7月11-12日发生在河北省中南部的一次局地大暴雨个例进行了诊断分析。结果表明：200hPa西风槽、500hPa华北低涡和中低层偏东风,是这场暴雨的直接影响系统,该过程中的华北低涡为低层冷心、高层暖心结构,具有随高度向西北倾斜的特征,涡度场特征明显。暴雨区对应着深厚的正涡度区,散度场结构零乱,非高层辐散、低层辐合的配置,暴雨区上空上升运动较强。冷空气来源于500hPa以下的东北地区,水汽的源地来自渤海,低层偏东风可能是暴雨区所需水汽的主要携带者。水平涡度平流项和水平涡度辐散项作用相反,水平涡度辐散项对总涡度起直接作用且为正贡献,而垂直平流项对总涡度贡献比较小。     

一次双台风影响下暴雨过程的中尺度涡旋模拟分析

本文对上海地区一次双台风环境影响的暴雨过程进行数值模拟及分析,探讨了强降水过程中大气中低层的涡旋特征及发展机理。结果表明:(1)暴雨过程处于双台风、大陆高压的共同影响下,中低层伴随有较明显的中尺度低涡发展。(2)与涡旋相关的局地垂直涡度由低层开始发展,先期涡度发展集中于850 hPa以下,之后向大气中上层发展增强,涡旋尺度强度也随之发展,最终形成在对流层下半部具有闭合式气旋性环流的深厚涡旋。(3)影响局地涡度变化的水平平流项、垂直平流项、散度制造项和倾斜项对不同时间、不同高度的涡度作用各不相同,其中散度制造项是中低层涡度的主要来源,垂直平流项的输送作用对中上层的涡度发展有重要作用,倾斜项对涡旋发展移动也有部分贡献。(4)通过敏感性试验考察了对流潜热反馈的贡献,发现潜热释放过程通过加热改变大气温压场结构,从而维持并改变局地涡度倾向的中低层辐合及对流上升运动,对涡旋的发展和移动起了重要影响。     

陕南一次暴雨天气过程的诊断分析

应用常规气象观测资料、NCEP（1°×1°）再分析资料和FY-2C气象卫星资料,对2007年7月28—29日发生在陕南丹凤的暴雨天气进行综合分析。结果表明：贝加尔湖的阻塞高压使得中高纬环流形势稳定,河套低涡、副高位置稳定少动,商洛处于稳定的东高西低的环流形势是本次暴雨的大尺度环流背景;低涡切变,低空西南急流为本次暴雨过程提供了充沛的水汽和能量条件,暴雨区上空存在着明显的水汽通量辐合中心,中层干冷空气由东北向下侵入暖气团,在雨区上空形成对流不稳定区。暴雨区上空900～300hPa都为正涡度区,为一深厚的气旋性涡旋,有利于对流的发展,雨区垂直运动发展旺盛且深厚,为大降雨提供了抬升条件。散度垂直分布从对流层低层到高层呈现辐合一辐散的双重结构,有利于对流的发生发展。     

一次引发南亚大暴雨的季风低压结构、涡度与水汽收支分析

对2003年夏季风期间,7月24～28日印度季风槽内季风低压发展西移与阿拉伯海中尺度低压合并引发南亚的一次大暴雨过程进行了诊断分析,探讨了印度季风槽、季风低压的三维结构以及低压区域的涡度、水汽收支.揭示和确认了一些事实:1)印度季风槽区对流层中下层存在明显的风场切变,槽区高温高湿,为单一性质的热带气团,低层为对流不稳定,槽区对应正涡度区;2)季风低压是一较深厚系统.动力结构为低层正涡度,高层负涡度,低层辐合,高层辐散.其西移速度约500 km·d.季风低压北侧整层为深厚的东风,南侧在对流层中低层为西风,在高层为东风.热力结构在低层(700～800 hPa)间存在弱冷区,而中高层几乎为暖心结构.低压对应高湿区,低压中心西侧整层为相对湿度大值区;3)季风低压的发展过程中,低层的辐合场制造正涡度,促进低压的发展;4)低压区水汽强烈辐合,西边界输入量最大.在此研究工作的基础上,作者还比较了夏季风期间南亚印度季风槽和东亚梅雨锋系统的异同.     

引发暴雨的西南低涡特征分析

利用NCEP1°×1°再分析资料、逐时卫星云图资料和自动站资料,分析了与4次暴雨过程密切相关的中尺度系统西南低涡。结果表明,引发暴雨的西南低涡相对于环境场是湿涡,南边界是主要水汽输送方向。在西南低涡形成阶段,整层均为正涡度,一般维持深厚的上升气流,具有较为深厚的暖心结构。在其发展旺盛阶段,正涡度呈上升趋势,对流层中低层...     

低涡影响下的西北地区东部暴雨个例分析

利用常规气象观测资料、陕西区域自动站观测资料、多源融合逐小时0.05°×0.05°格点降水资料、 NCEP1°×1°再分析资料和卫星探测资料对2019年8月2—4日西北地区东部一次由低涡向东北方向移动引发的暴雨过程进行了诊断分析。结果表明：500 hPa中纬度低槽、副热带高压、低槽携带的弱冷空气及来自台风外围和副高外围的暖湿空气为此次暴雨过程提供了非常有利的条件;大气整层水汽通量及水汽通量散度在本次低涡暴雨预报中具有一定的意义,水汽通量的突增对应降水的增强,水汽通量大值中心叠加强水汽通量辐合区对应强降水落区;500 hPa正涡度平流使低涡移动发展,对暴雨的发展和移动有很好的指导意义,对流层低层温度平流对低涡移动路径方面有良好预报指示作用;对流层上层高位涡向下伸展,分裂的高值扰动可促使中低层气旋涡度发展,影响低涡加强,对流层中低层700 hPa附近上正下负的分布形态促进了不稳定能量的释放,有利于低涡暴雨的发生;暴雨期间卫星云图上表现为逗点型云带,与低涡系统对应良好。     

一次伴有高原低涡和热带气旋活动的持续性暴雨过程分析

为了进一步考察持续性暴雨发生机制,针对2010年7月下旬川、陕、甘地区的一次持续暴雨过程,应用MICAPS资料,FY-2E辐射亮温资料,TRMM卫星降水资料,NCEP每6 h 1°×1°分辨率的分析资料,主要分析了暴雨发生的环流背景,暴雨直接影响系统-高原低涡、热带气旋、中尺度对流系统、冷暖平流等对持续性暴雨的影响。结果表明:(1)本次持续性暴雨过程发生在对流层高层南亚高压由纬向型转为经向型,对流层中层副热带高压东退西进,热带气旋登陆西行,高原低涡东移受阻,中尺度对流系统不断生消的有利条件下。(2)高原低涡与热带气旋相互作用使两者移速减缓,涡区切变流场加强,正涡度平流输送使低涡加强与维持。(3)低涡为暴雨发生提供了有利的抬升条件,使降水期间涡区呈现较强的正涡度和辐合上升运动,降水最大值出现时间对应辐合上升运动最强时,降水过程中对流层中低层为垂直正螺旋度,有利于低涡系统维持和降水持续,垂直正螺旋度大值区及出现时间对强降水发生及落区有一定的指示性。(4)对流层低层暖平流输送使暴雨区能量持续积累,同时也使暴雨区中尺度对流系统生肖不断,降水得以发生和持续。     

引发四川盆地东部暴雨的西南低涡结构特征研究

利用1951-2008年四川盆地(27°-32°N,105°-110°E)54个地面气象观测站网监测的日雨量资料,分析了四川盆地东部暴雨发生的气候特征。结果表明,四川盆地东部暴雨(或伴有雷雨大风、冰雹大风等)多发生在6-9月,川东北和渝东北是单站暴雨的高发区,重庆西部是大范围暴雨的多发区;引发四川盆地东部(宜宾、南充和重庆西部)暴雨的主要天气系统是西南低涡。对2007-2010年6次西南低涡暴雨过程进行了合成分析,分析表明,西南低涡热力结构特征具有200hPa存在明显增暖现象,对流层中低层则由暖转冷;西南低涡初期大气对流性不稳定明显;西南低涡动力结构特征具有200hPa西风急流在36°N附近,500hPa低槽东移,槽前正涡度加强,从对流层底垂直伸展到300hPa以上,正涡度中心随高度向西倾斜,850～500hPa平均正涡度大值区与低涡中心对应,对流层中低层北风大值区与南风大值区在低涡中心附近形成强水平风切变,同时低涡中心附近的垂直风切变也较明显。促使西南低涡发展的水汽主要来自南海,低空急流由南向北输送水汽,将对流层低层到大气边界层内的水汽输送到低涡中心附近。西南低涡发生、发展过程中在红外卫星云图上具有MCC等中-α尺度特征,发展强盛的西南低涡在多普勒天气雷达回波上有"列车效应"和中气旋特征。     

一次引发川东暴雨的西南涡特征分析

利用NCEP再分析格点资料、常规观测资料、自动站降水资料、0.1°×0.1°的FY-2E云顶亮温资料,对2013年6月29日至7月1日发生在四川东部的大暴雨过程进行分析,结合涡度收支方程重点分析了引发这次大暴雨的西南涡结构。结果表明：在西南低涡发生发展过程中,对低涡发展起直接作用的是水平辐合辐散项和水平平流项,低涡形成前水平辐合辐散项起主要贡献,低涡形成后水平平流项贡献增大,并在对流层中低层以正贡献为主,扭转项贡献最小,而垂直输送项在低涡形成前期以正贡献为主,低涡减弱阶段以负贡献为主;在西南低涡形成前期,对流层高层有位涡大值区向下传输至中层,中高层正位涡叠加在低层负位涡之上,有利于低层低涡的发展及不稳定能量的存储与释放,是低涡维持发展的重要因素。     

Heavy rainfall caused by interactions between monsoon depression and middle-latitude systems in Australia: a case study

The heavy rainfall caused by interactions between the monsoon depression and the middle-latitude systems in Australia has been investigated in this paper. For a better understanding of the Australian monsoon depression (AMD) and its synoptic-scale interaction with the middle-latitude systems, some key meteorological parameters have been calculated, including the vorticity budget, moisture budget, temperature advection, frontogenesis function and potential vorticity. The results show that interaction between the lower and mid-latitude systems does exist leading to the merging of the extratropical low with frontal systems and the AMD, meanwhile both the low-level cold air from the mid-latitude and the warm moist air that was lifted by the front were very favorable for the formation and the intensification of heavy rainfall, which was quite different from the rainfall caused by the AMD alone. Second, the obvious temperature advection and gradient were detected, so the baroclinicity was favorable to the intensification of the front, as well as to the development of the upper-level jet. Next, isentropic analysis revealed that the south-west cold-flow sank and met the warm flow coming from the northern part of Australia, thereby forming the obvious baroclinic zone in the lower troposphere. A high-PV anomaly area located in the upper level of the troposphere, which overlaid the low-level frontogenesis zone, also existed. The upper-level PV maximum extended downwards forming a vertical PV column when the extratropical low intensified. Furthermore, the AMD is a warm-cored vortex located in middle and upper troposphere with a deep and thick moisture layer, and there were some differences in the vorticity and moisture budgets of the two different stages. Finally, based on the above-mentioned analysis, a conceptual model describing the interactions between the lower and middle-latitude systems in the southern hemisphere was proposed.     

A Study on a Heavy Rainfall Event Triggered by an Inverted Typhoon Trough in Shandong Province

A heavy rainfall event that occurred in Shandong Province in 26 28 August 2004 was caused mainly by Typhoon Acre and cold air activities related to a westerly trough. The event was triggered by an inverted typhoon trough, which was closely associated with the intensification of the low-level southeasterly flow and the northward transport of heat and momentum in the periphery of the typhoon low. A numerical simulation of this event is performed using the nonhydrostatic mesoscale model MM5 with two-way interactive and triply-nested grids, and the structure of the inverted typhoon trough is studied. Furthermore, the formation and development mechanism of the inverted typhoon trough and a mesoscale vortex are discussed through a vorticity budget analysis. The results show that the heavy rainfall was induced by the strong convergence between the strong and weak winds within the inverted typhoon trough. Dynamic effects of the low-level jet and the diabatic heating of precipitation played an important role in the development of the inverted typhoon trough and the formation of the mesoscale vortex. The vorticity budget analysis suggests that the divergence term in the low troposphere, the horizontal advection term, and the convection term in the middle troposphere were main contributors to positive vorticity. Nonetheless, at the same pressure level, the effect of the divergence term and that of the adveetion term were opposite to each other. In the middle troposphere, the vertical transport term made a positive contribution while the tilting term made a negative contribution, and the total vorticity tendency was the net result of their counteractions. It is found that the change tendency of the relative vorticity was not uniform horizontally. A strong positive vorticity tendency occurred in the southeast of the mesoscale vortex, which is why the heavy rainfall was concentrated there. The increase of positive vorticity in the low （upper） troposphere was caused by horizontal convergence （upward transport of vorticity from the lower troposphere）. Therefore, the development of the inverted typhoon trough and the formation of the mesoscale vortex were mainly attributed to the vorticity generated in the low troposphere, and also the vertical transport of vorticity from the low and middle troposphere.     

在“鞍”型大尺度环流配置下西南低涡发展的物理过程及其对川东暴雨发生的作用

利用2005年7月6～9日川东地区暴雨过程的观测资料，从大尺度环流、水汽输送和温度平流，并利用湿位涡的垂直和水平分量(Pm1和Pm2）以及相当位温，分析诊断了此次暴雨发生的大尺度环流背景特征以及西南涡发展的物理过程, 其结果表明如下:(1)在此次暴雨发生期间，四川盆地北部由于受中高纬长波东移调整的影响, 不断有低压槽分裂出来并影响此地区, 在盆地的西南方向的孟加拉湾季风槽比较活跃, 南海季风向北输送由于受到西风输送的作用在四川盆地东南部也出现弱的横槽, 并且西太平洋副高西伸到四川盆地东部以及存在于高原中部的高压共同作用, 从而形成明显“鞍”型大尺度环流配置; (2)在此“鞍”型场大尺度环流背景下, 强西南气流绕流高原东侧直接进入四川盆地, 而弱西南气流则绕流云贵高原输送进入四川盆地东部, 受地形的阻挡和西伸的西太平洋副高的作用在四川盆地东部形成向北的急流辐合带, 同时, 由于两支气流输送着大量的水汽, 暖湿空气在川东地区形成高温高湿的辐合区; (3)在此“鞍”型场作用下, 盆地上空的低层不断聚集季风气流输送的大量暖湿空气, 而在高层有冷干空气侵入, 从而导致盆地内低涡系统强烈发展; (4) 由湿位涡的垂直分量和水平分量的诊断表明了在暴雨发生期间, 在四川盆地北部上空的高层不断有干空气入侵, 引起了垂直对流不稳定, 即Pm1<0, 并向盆地东北部发展, 从而使此区域气旋性涡度不断加强, 即低涡强烈发展; 并且, 在盆地上空低层暖湿空气相当位温的水平梯度对于西南低涡的发展和暴雨的发生同样起了重要作用, 正的Pm2中心与暴雨发生区域有很好的一致性, 这表明暴雨往往发生在高温高湿的强垂直不稳定区域。     

大陆高压对强热带风暴碧利斯内陆强降水影响

强热带风暴碧利斯 (0604)(简称碧利斯) 登陆后造成极其严重的灾害。对该次强降水过程诊断分析发现,除强盛的西南季风外,深厚的大陆高压也起着重要作用。自高层向低层逐渐加强的大陆高压,其东南侧深厚的偏东北气流,与碧利斯登陆后西北象限的东北风相叠加,配合强盛的西南季风,使得碧利斯残涡以及槽区得以长时间维持。涡度收支表明:长时间维持的台风槽,其低层强烈的辐合效应,对暴雨区低涡发展作用显著,致使中尺度对流系统不断发展而造成内陆连续强降水。数值试验表明:在强盛西南季风背景下,大陆高压的增强东伸,不仅能够提供高空强辐散,而且能增强碧利斯西北象限东北风,使得登陆后减弱的碧利斯残涡和台风槽得以长时间维持,致使内陆暴雨区各层涡度增加而导致暴雨维持与增幅。     

Heavy Rainfall Associated with a Monsoon Depression in South China: Structure Analysis

A heavy rainfall associated with the deepening of a monsoon depression happened in the summer of 2005. This process was first diagnostically analyzed and the 3D structure of the monsoon depression was discussed~ then this structure was compared with those of the monsoon depression in South Asia and the low vortex in the Meiyu front. The results showed that the heavy rainfall directly resulted from a monsoon depression in South China~ and the large-scale environment provided a favorable background for the deepening of the monsoon depression. The 3D structure of the monsoon depression was as follows. In the horizontal direction, there existed a convective cloud band to the south of the monsoon depression, which lay in a convectively instable area, with a relatively strong ascending motion in the mid and low levels of the troposphere, and the ascending motion matched well with a moist tongue, a convergence area, and a band of positive vorticity in the mid and low levels of the troposphere. In the vertical direction, the depression had an obviously cyclonic circulation in the mid and low levels of the troposphere, but no circulation from above 300 hPa. The monsoon depression corresponded to convergence and positive vorticity in the low levels, but to divergence and negative vorticity in the upper levels. The upward draft of the depression could reach the upper levels of the troposphere in the west of the depression, while the descending motion lay in the east. There was a low-level jet to the south of the depression, while the upper-level jet was not obvious. The depression was vertically warm in the upper levels and cold in the low levels, and the axis of the depression tilted southeastward with height, whose characteristics were different not only from the monsoon depression in South Asia but also from the low vortex in the Meiyu front.     

Study on cut-off low-pressure systems with floods over Northeast Asia

Summary The cut-off lows (COLs) during the period from June to August 1998 leading to the record flood in Northeast Asia, especially in Northeast China, has been investigated in this paper. The results are as follows: the blocking highs benefited significantly the formation and maintenance of COLs over Northeast China; an obvious frontogenesis zone existed in Northeast China and it implies that baroclinity played an important role in the initiation of COLs, especially in middle and upper troposphere; the maxima of the potential vorticity anomaly were located in the upper troposphere, then extended downwards to the middle and the lower troposphere. The pronounced interaction between systems in upper-middle and low troposphere can be revealed; the moisture supply was from South China, and even from East China Sea and South China Sea. The strong southerly current transported very rich moisture to Northeast China. The maximum of the convergence of moisture flux was below 850 hPa. Obvious interaction between the middle and lower latitude systems was found in the study. Also, the summer monsoon showed significant impacts on the sustained heavy rainfalls related with the COLs over Northeast China; the upward motion could be caused by the lifting of the large scale dynamic forcing and there was no obvious releasing of latent heating in the upper-middle troposphere. The cold dome in the COLs was quite different both from the warm core in tropical cyclone and from the weaker warm core in Meiyu (Baiu) front low. The calculation of vorticity budget shows that both the horizontal advection term and horizontal divergence term contributed importantly to the maintenance and the strengthening of positive relative vorticity. Finally, the complex dynamical characteristics regarding the COLs are discussed and further investigation is proposed.     

东北冷涡下辽宁省短时强降水统计与合成分析

利用2000—2014年6—8月常规资料、FNL资料和辽宁省逐时降水资料,将东北冷涡分为北涡、中间涡和南涡,统计每类冷涡短时强降水特征,并进行动态合成分析。结果表明:短时强降水共755次,冷涡下227次,冷涡强降水多发生在1~3 h内。6月短时强降水主要由中间涡引起,7、8月中间涡与北涡共同影响,有一定周期变化;而南涡没有在辽宁产生强降水。北涡水汽输送充沛,中间涡水汽条件较差,切变辐合场与水汽输送的结合是有利于强降水的重要因子。降水基本处于斜压区内,冷涡中心降水处在斜压区北侧和高空急流左前方,高空槽前或槽后的降水处在斜压区南侧和急流中心右后方,降水区附近多有高空急流形成的次级环流配合。槽后降水区干侵入活动明显,冷涡中心降水主要通过高位涡诱发气旋性环流而触发上升运动。     

一次东北冷涡过程的数值模拟与降水分析

本文利用WRF模式对2007年7月8日至12日的东北冷涡过程进行模拟。通过分析天气尺度背景场可知, 在对流层中高层出现干侵入过程, 干空气主要来源于我国内蒙古西部和东北冷涡的西北部, 随着东北冷涡一起呈涡旋状运动, 在对流层中层的干侵入更加明显; 东北冷涡的东部为水汽通量辐合区, 说明此处水汽丰沛, 该处的暖湿空气与干冷空气相汇, 有利于降水的生成。通过分析中尺度对流系统的地面和对流层中下层的风场结构可知, 中尺度对流系统易发生在东北冷涡东南侧和东北侧的气旋性曲率最大处, 此处的对流层低层易形成强辐合区, 而风场的水平辐合运动激发出垂直上升运动, 在对流不稳定区配合着强上升运动, 有利于对流系统发展而产生降水, 因此在低压系统东南侧及东北侧的 “气旋曲度” 区易形成降水。从对流涡度矢量的垂直分量可知, 在东北冷涡中的东南侧和东北侧, 对流涡度矢量的垂直分量对于中尺度系统的发生位置和降水区域有很好的指示作用。