2011年9月中国华西致灾暴雨环流特征及地形强迫效应分析

采用陕西及周边地区7213个雨量站资料，分析了2011年9月1-18日期间华西地区降水量分布特征，结果表明：该时段累计降水量是历史同期水平的2-3倍，持续时间长，致灾严重。降水主要分为4-6日、11日和16-18日3次大的过程。利用NCEP 1°×1°的资料分析3次过程的水汽和环流特征，表明暴雨区低层的主要水汽输送，除了偏南气流从川东一带向北输送外，还有偏东气流从华北平原南部一带向西输送。与3次降水过程对应，华西地区500 hPa有3次西风槽过境，700 hPa偏南风和850 hPa偏北风分别出现3个明显的控制时段;700 hPa偏南气流先期输送大量暖湿空气到华西地区，形成不稳定层结，当遇到偏北风输送的干冷空气楔入其下将其抬升时，叠加槽前的动力抬升作用和地形造成的风场辐合抬升，使得对流活动增强，不稳定能量释放，暴雨发生。稳定的环流形势（乌拉尔山阻高）导致特定地形下形成持续的不稳定层结以及动力抬升条件被不断重复满足，从而不断引发上升运动释放不稳定能量，这是2011年华西秋雨稳定持续的根本原因。

Circulation characteristics of continuous disastrous rainstorms and terrain forcing effect on September of 2011 in West China

Based on precipitation data from 7213 weather stations in Shaanxi province and its surrounding areas,the spatial distributions of precipitation from September 1 to 18,201 1 in the West China were analyzed.The results in-dicate that accumulated precipitation during the period is twice to threefold as much as that during the same period of history.Precipitation is characterized with long duration and serious disaster.This precipitation event can be di-vided into three processes,i.e.the first process is from September 4 to 6 ,the second on September 1 1 and the third from September16 to 18.According to 1°×1°NCEP reanalysis data,atmospheric circulation characteristics of these three processes suggest that major water vapor transport of lower level in rainstorm region has two branches. One is the southern air flow from eastern Sichuan province to north,and the other is the eastern air flow from southern North China plain to west.Corresponding to these three precipitation processes,three westerly troughs in-fluence the circulation in 500 hPa,and 700 hPa southerly and 850 hPa northerly winds appear in three obvious con-trol periods over the West China.Unstable stratification is formed because southerly airflow in 700 hPa transports a large number of warm-moist airflow to West China in advance.When dry-cold air transported by northern wind en-ters into the bottom of warm-moist air mass and lifts it,dynamic lift action in the front of the superadded trough and wind field convergence uplift caused by the terrain occur,which makes the convective activity strengthened and potential unstable energy released,so appears heavy rain.Continued unstable stratification and dynamic lifting caused by stable circulation situation （such as blocking high）and specific terrain condition,which benefits to re-lease unstable energy,are the essential reason forming steady and continuous rainstorm in the West China.