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。

The Physical Process Analyses of the Southwest Vortex Development and Its Effect on Heavy Rainfall in Eastern Sichuan Under the Saddle Pattern Background of Large-Scale Circulations

The characteristics of large-scale circulation background and the physical process of the southwest vortex during the occurring period of heavy rainfall in the eastern Sichuan basin from 6 July to 9 July 2005 are analyzed from large-scale circulations, water vapor transport and temperature advection by using the observed data and are diagnosed from the vertical and horizontal components of moist potential vorticity, i.e., Pm1 and Pm2, respectively. The analyzed results are shown as follows: (1) During the period of heavy rainfall, the northern part of Sichuan basin was continuously influenced by some low-pressure troughs separated from the eastward propagation and adjustment of atmospheric long-waves at middle and high latitudes, and the Bay of Bangal monsoon trough located to the southwest of the basin was more active. At the same time, Influenced by the westerly wind zone in the process of the northward transport of the South China Sea monsoon, a cyclonic distribution was also formed in the southeast part of the Sichuan basin. Moreover, the western Pacific subtropical high shifted westward to the west of the Sichuan basin, and the Tibetan high was formed over the middle part of the Tibetan Plateau. Thus, a saddle pattern disposition of large-scale circulations was formed. (2) Under the background of the saddle pattern disposition of large-scale circulations, strong southwesterly flow could directly enter the basin around the east of the Tibetan Plateau, and weak southwesterly flow also could enter the eastern Sichuan basin over the Yunnan-Guizhou Plateau, which formed a convergent zone of northward jet stream in the east of the Sichuan basin due to the stopping effect of topography and the effect of the westward shift of the western Pacific subtropical high. Moreover, since these two flows also transported a great amount of water vapor, a convergent area of warm and wet air with high temperature and humidity was caused in the northeast of the Sichuan basin. (3) It can be shown from the diagnosis of the vertical and horizontal components of moist potential vorticity that during the occurring period of heavy rainfall, since dry air continuously entered the upper level over the Sichuan basin, the strong instability of vertical convection, i.e., Pm1<0, was caused over the basin and developed toward the northeast of the basin, which made the continuous intensification of cyclonic vorticity, i.e., a low vortex strongly developed over the area. Moreover, the horizontal gradient of equivalent potential temperature of warm and wet air in the low level over the Sichuan basin, which can caused Pm2>0, also played an important role in the development of the southwest vortex and the occurrence of heavy rainfall, and the center of positive Pm2 was also in good agreement with the area of heavy rainfall. This shows that heavy rainfall used to occur in the instable region of vertical convection of air with high temperature and humidity.