Diagnostic Analysis on Maintenance Mechanism of Meiyu Front Rainstorm during 4~5 July 2003

The heavy flood over Huaihe River valley in the summer of 2003 is another serious flood event after 1991.Three long life-cycle severe heavy rainfall induced the flood from 29 June to 11 July 2003.The most severe heavy rain happened during 4～5 July,in which the intensive rain period was from 12:00 on 4 to 06:00 on 5 July,with Chuzhou in Anhui Province isolated by 24 h precipitation of 273.7 mm.By use of the upper-air sounding,satellite and Doppler radar data,and NCEP/NCAR reanalysis data, environmental condition,mesoscale feature of rainstorm and the effect of reaction between low jet and energy front on heavy rain is analyzed and the maintenance mechanism of heavy rain is diagnosed.The results are as follows:(1) Moisture transporting to the heavy rain area from the South China Sea and the Bay of Bengal is main moisture source supplying severe precipitation because the air above heavy rain area is more dry before precipitation.(2) The starting and terminating time for severe precipitation are consistent with the increase and weak of convergence and vertical motion.(3) There are the effects of reaction between low jet and energy front on the occurrence and maintenance of severe precipitation.The severe precipitation will occur and maintain when couple condition between low jet and energy front,and otherwise, the severe precipitation will terminate.(4) The barotropic non-equilibrium force is contribution to the occurrence of heavy rain,but the baroclinic coupling forces between dynamic and thermodynamic fields play main role in the maintenance of heavy rain.