一次西南低涡东移引发长江中下游暴雨的诊断研究

Diagnostic Analysis of One Heavy Rainfall Process Induced by Southwest Vortex in the Middle-Lower Reaches of the Yangtze River

利用常规观测资料和NECP再分析资料,对2013年6月6—7日西南低涡东移加强发展造成长江中下游大暴雨过程进行了诊断分析,重点探讨了西南低涡东移和发展维持的物理机制以及最强降水的变化特征。结果表明,沿着700 hPa高空切变线东移的西南低涡是造成此次长江中下游地区暴雨的直接影响系统,西南低涡沿着700 hPa切变线东移发展,深厚阶段正涡度柱伸展到400 hPa高度,自下而上呈近垂直结构。西南低涡附近低层辐合与高层辐散的大尺度环境条件、西南低涡与西南低空急流耦合发展动力结构、低空暖平流和高空槽前正涡度平流输送等条件是导致西南低涡东移到长江中下游后加强发展的主要因子。与西南低涡相伴随的强降雨区主要位于低涡南部3个纬距以内,该处的西南季风和副高西南侧东南气流两支水汽输送的汇合为暴雨发生提供了充沛的水汽和对流不稳定能量,而对流层中低层携带的冷空气侵入低层低涡的后部,不仅加强了低涡的斜压性,也促进了上冷下暖不稳定层结的产生和发展,为强降水的发生提供了不稳定对流触发条件。

A heavy rainfall event induced by the moving and developing southwest vortex occurred in the middle-lower reaches of the Yangtze River from 6 to 7 June 2013.Using conventional observation and NCEP reanalysis data,diagnostic analysis was done on this event,especially on the physical mechanism of the vortex moving and developing and on the variability of the rainfall climax.The results show that the southwest vortex moving easterly along the 700 hPa shear line is the direct impact system to the heavy rainfall.Southwest vortex moves eastward and develops,and when reaching its deep phase,positive vorticity column stretches up to 400 hPa height with a nearly vertical structure in the relative vorticity chart.Large-scale condition of lower-level convergence and upper-level divergence in the vicinity of southwest vortex,dynamic structure of the coupling of southwest vortex and southwest low-level jet,positive vorticity advection in front of the high-level trough and other favorable weather conditions are the main causes for the vortex reinforcement when it gets to the middle-lower reaches of the Yangtze River.Heavy rainfall area that accompanies the southwest vortex is mainly located within 3 latitudes to the south and east side of the vortex.Convergence of two water vapor advection provides abundant humidity and latent unstable energy for the rainfall.One is the southwest monsoon flow,and the other is the southeast flow from the southwest side of the tropical high.Besides,cold air from the middle-lower level of troposphere coming down to the rear side of the vortex reinforces the development of vortex,promoting an unstable stratification,which provides convective triggering mechanism to the heavy precipitation.