An Observational Study of Mesoscale Convective Systems Producing Severe Heavy Rainfall in the Huaihe River Basin During 3-4 July 2003

Three long life-cycle severe heavy rainfall induced the flooding of the Huaihe River basin from 29 June to 11 July 2003,the most severe heavy rainfall during 3-5 July,which moved from upper reaches to lower reaches of the Huaihe River basin,cause the flooding more dangerous.In this paper,firstly,synoptic condition of the heavy rainfall process is diagnosed. Secondly,the upper-air sounding,satellite and Doppler radar data are employed to analyze the mesoscale convective system producing the most severe heavy rainfall at Chuzhou and Nanjing.The synoptic weather pattern analysis shows that the eastward propagation of trough results in the rainfall area moving from upper to lower reaches of the Huaihe River basin.The invasion of dry air in the middle troposphere prior to the rainfall intensifies the convective instability in the lower and middle troposphere.The transportation of vapor increases the humidity and Convective Available Potential Energy(CAPE).The inversion near surface produced by downdraft during the heavy rainfall is not favorable for the maintenance of convective systems.The large area rainfall is produced by meso-α convective system,but the local severe rainfall at Chuzhou and Nanjing is caused by meso-β or meso-γ convective systems.The rainfall most frequently occurs while the convective system develops to the mature and dissipating periods.It is found that the rainfall at the mature stage is not always stronger than that at the dissipating stage.The analysis of infrared brightness temperature and radar reflectivity reveals that severe radar echo is not consistent with the area enclosed by-52℃.For present case,the strong radar echo is only located in the central and southern part of the area of-52℃ cloud brightness temperature,and there is the cloud anvil to the north of strong radar echo.The radar reflectivity from Hefei radar indicates that the top of radar echo is higher than 10 km during the heavy rainfall period and the reflectivity more than 45 dBZ occurs below 5 km.The two periods of Chuzhou rainfall happen during the mature and dissipating stages of the convective systems.The retrieval data of dual Doppler radar from Hefei and Maanshan reveal that the mesoscale convective line and its cells along the mesoscale convergence line produce directly the heavy rainfall at Chuzhou.The mesoscale convergence lines appear in the lower troposphere and tilt northward from low to middle level.The location of convection line is consistent with the convergence line at the height of about 3 km.