Meso-scale Analysis of Heavy Rainfall Event Occurred in the Late Autumn 2007 in Beijing

To analyse the characteristics of the synoptic system structure as well as the characteristics of the occurrence and development mechanism of a strong convective rainstorm process took place in Beijing in the late autumn(October 27,2007) based on the AWS,microwave radiometer and wind profiler data.Analysis indicates that in the context of the favorable large-scale circulation background composed by upper cold trough and surface cold front,the meso-scale systems caused by the special topography and heat island effect leads to the heavy rainfall directly.Previous research achievements about the strong convection in summer can be used to explain the physical mechanism of occurrence and development of the severe convective weather.The interaction between the low-level easterly airstreams and the western mountains of Beijing increases the cyclonic vorticity.This results in the formation of the lower vertical wind shear and enhances the precipitation on the windward slop.The horizontal temperature gradient between the western suburbs and the down-town caused by the urban heat island effect leads to the vertical wind shear on the windward slope,and this is an important factor to enhance the local heavy rainfall.At the same time,the positive feedback process between the wind speed of the wind blowing to the windward slop and the precipitation intensity maybe also is a main factor to enhance the local precipitation.Comparing with the strong convection events in summer,there are some obvious differences on the occurrence and development of the weather systems,which happened in fall is mainly caused by the strong cold air and better water conditions,while the process is weak under the conditions of high temperature and humidity,as well as the invasion of weakly cold air in summer.