Causes Analysis of the Southwest Vortex Extremely Heavy Rainfall on 21 July 2012

Based on conventional observation data,ECMWF analysis field data,regional automatic station data,Doppler radar data,and the products of SWAN etc.the southwest vortex rainstorm process on 21July 2012 and extreme short-time severe rainfall observed at Panlong Station in the process are analyzed.The results indicate that;the rainstorm is triggered by surface cold air which contributes to the development of convergence ascent movement in the east of the southwest vortex and upper-level divergence with the backgroud of northern trough and southern vortex.The intrusion of cold air from the west of the southwest vortex,which is also the important factor for the formation of surface convergence line,is the direct cause of the formation of 925 hPa cold front with S pattern.Extreme short-time severe rainfall occurs in the north to the middle part of mesoscale rainbelt that is in the east of southwest vortex combining with surface convergence line,and meanwhile,TBB reaches the lowest in the center of MCC when the rainfall is the most intensive.The radar echoes show that the intersection of air in the two sides of southwest vortex advances the formation of(3 meso-scale cyclonic circulation.The low-level jet stream provides plenty of water vapor and energy conditions for the severe precipitation and reinforces vertical wind shear between middle and lower levels,contributing to the development and sustainment of heavy precipitation supercell storm.The extreme short-time precipitation of Panlong Station results from the sustainable development of meso-β scale cyclonic circulation associated with deep meso-cyclone near the center of meso-βscale cyclonic circulation in southwest vortex.