Diagnosis of a Heavy Rain Occurred in the Hexi Corridor

The regional heavy rain occurred in the middle of Hexi Corridor on August 18 2009 was analyzed from the synoptic situation,physical quantity fields,water vapor conditions and satellite image's features.Results show that the development of 500 hPa blocking high from Balkhash Lake to Novosibirsk made the cold trough before the ridge strengthen and extend south,and it provided the cold air for this regional heavy rain.The western and northern extension of the subtropical high provided adequate water vapor for the rain.The convergence of the vortex at the level of 700 hP resulted in increase of upward air movement,which promoted convection unstable stratification emergence and development.The low and middle level instability energy triggered mesoscale convective activities in large-scale frontal rainfall cloud system,which was the main reason for this heavy rain event.The configuration of negative helicity at up level and positive helicity at low level has a good indication for forecasting heavy rainfall and rainfall area.