雅安“8·26”特大暴雨的天气分析

Synoptic Study on "8·26" Ya'an Torrential Rain in August 2003

2003年8月25日晚8时30分至26日凌晨1时许，四川省雅安市雨城区发生228．1mm的特大暴雨，给当地造成了历史罕见的灾情。本文主要从天气学角度，对该降水过程的大尺度环流及天气尺度影响系统、水汽输送及源地、中尺度系统特征和地形作用等进行了分析。结果表明：雅安“8·26”特大暴雨过程是一次典型的登陆热带气旋与“雅安天漏”中β尺度地形涡旋相互作用的暴雨过程。此过程产生的大尺度环流背景及天气尺度影响系统是：对流层上层南亚高压一直稳定在青藏高原中部，雅安市上空100hPa为南亚高压的辐散场；对流层中层西太平洋副热带高压主体持续加强西伸，位于南海的“科罗旺”台风稳定西进，雅安市处于副高边缘湿润的东南气流和高原切变线前部的辐合上升气流中；对流层低层受对流层中层高原切变线和登陆台风东北侧强东风急流的激发，地形涡旋强烈发展。“科罗旺”台风东北侧和副高西南侧，850～700hPa天气尺度的低空偏东急流及其风速脉动，从热力（水汽、感热、潜热输送）和动力两个方面激发暴雨区的中β尺度系统迅速、强烈的发展。暴雨过程的水汽输送主要是靠“科罗旺”强台风的作用，水汽源地主要来自南海。暴雨中的地形作用非常显著，青衣江河谷喇叭口地形和迎风坡对暖湿空气的强迫辐合抬升，对降水有很大的增幅作用，暴雨中心地形雨的比率高达60％以上。

An extremely heavy rain in which precipitation reached 228.1 mm occurred in Ya＇ an, Sichuan province from 8：30 on 25th to 1：00 on 26th, in August 2003 （ Beijing time）. The large-scale circulation features, synoptic weather systems, water vapor source and topo- graphic effects on the severe flood were analyzed in this study. The results show that ＂8·26＂ heavy rain is resulted from a typical interaction process between landfalling typhoon and meso- β Scale topographic vortex. The mainly rain producing systems are that： the divergence controls the Ya＇ an region in upper troposphere because of the maintaining of The South Asia high over the middle parts of the Tibetan Plateau at 100hPa; the unstable wet upward flows dominate the Ya＇ an region in the middle troposphere,since Ya＇ an is at the fringe of the sub- tropical high and the front of the plateau wind shear line at 500hPa. The strong topographic vortex develops in the Ya＇ an region in lower troposphere owing to the easterlies triggered by a landfalling typhoon named ＂Ke - Luo - Wang＂. In fact, the sub - synoptic scale easterlies and its disturbance in the northeast of the typhoon and southwest of the subtropical high trigger and strengthen the meso - β Scale systems, providing dynamic and thermodynamic conditions, and the water vapor transported by typhoon from The South China Sea provides abundant preeipitable water for this heavy rain. In addition, the forced convergence lifting generated by the erase topography and the windward slope reinforces the rainfall, and the statistical ratio of the topographic rainfall can be up to more that 60% in the center regions of the heavy rain.