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

An extremely heavy rain in which precipitation reached 228.1mm 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 topographic 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 subtropical 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 precipitable water for this heavy rain.In addition,the forced convergence lifting generated by the evase 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.