| Based on conventional data, NCEP1°×1°reanalysis data, cloud image, and Doppler weather radar data,two typical severe squall line cases are analyzed.It is found that the first case which happened on August 24, 2002 in eastern China occurred under moderate vertical wind shear and extreme unstable stratification of environmental conditions. The invasion of mid-level troposphere dry and cold air behind the 500hPa trough, low level convergence, water vapor transposition makes the squall line arise and enhanced. Doppler weather radar data shows that the long-lived gust front caused widespread high wind and made the new cells to be formed and developed. It is easily to observe the midlatitude radial convergence (MARC) signature. Several persistent mesocyclones are found.The second case happened on March 22, 2005 in southern China. The synoptic background is featured by the acclivitous trough, strong vertical wind shear and the intrusion of the dry and cold air down from the upper troposphere. The low level jet (LLJ) provides abundant water vapor for the squall line. The Doppler radar data analysis results indicate that the rear inflow jets are observed, and the MARC signature is obvious in the cross section for the radial velocity at most of the time in the front of the bow echo. At the late period of the squall line, the mesocyclone appeared.Although both the severe squall lines sustained for long time and brought about high wind in large area, there are many differences between them. The first case happened in summer with weaker troughs in 500hPa synoptic charts. The bow echo divided into three parts with the gust front moving. It is featured by MARC and mesocyclone. In the second case, the acclivitous trough and the jet stream make it keep strong and move fast. The cold pool of the squall line existed from the ground to the middle of the troposphere. The mesocyclone was also observed at the late period of the life of the squall line, but persisted for a short time comparing to the first case.
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