MESOSCALE CONVECTIVE SYSTEMS DURING 20-23 JUNE 2002 REVEALED BY SATELLITE OBSERVATION

The 20-23 June 2002 mesoscale convective systems,which produced heavy rainfalls over the middle and lower reaches of the Changjiang River Basin,are studied using satellite imagery, satellite products and conventional sounding data. Results demonstrate that the torrential rain was caused by three MαCSs (Meso-a scale Convective System) and some MβCS (Meso-β-scale Convective System) activities in succession. The TBB (black-body temperature) analysis depicts that several meso-β-scale cloud clusters and convective cells were embedded and alternately developed within an MαCS.As the strongest convection gradually decreased,the cold TBB area expanded quickly in hours before dissipation. However,the heavy rainfall occurred in MαCS developing and maturing stages.And the minimum TBB fluctuation matched well to the precipitation trend with the lower TBB for the heavier rainfall. A kind of favorable synoptic environment for MαCS genesis and activities could be described as follows.The West Pacific subtropical high is stable with its western ridge reaching to the west region of South China.To its south and west sides,there is the robust ITCZ (Intertropical Convergence Zone),the active Indian-Bengal monsoon cloud surges carry warm and moist air by low-level jet (LLJ) to mid-latitudes where a frontal zone existed for days,meanwhile a 500 hPa short-wave trough moved eastward.At the upper troposphere,the southeasterly divergent flow dominates the environment due to the South-Asian high enhancing and moving eastward,and a monsoon water vapor plume (WVP) is stretched from the Bay of Bengal to the Changjiang River Basin.With a certain favorable configuration including a monsoon WVP,a frontal cloud system,a shortwave trough cloud system,and monsoon cloud clusters,MαCSs could initiate and develop successively in the same region.