Abstract:Based on normal observation data, NCEP reanalysis data, and the intensive observation data of automatic weather station, a comparative analysis on two severe torrential rain processes during Meiyu period in Jiangsu province on June 17-18 and July 11-13, 2011 is performed. The results show that "617" case is caused by Yangtze-Huaihe River shear line, beside which, strong southwest low level jet as well as low level strong cold air activities supply abundant water vapor and energy, which ensure the occurrence and development of the heavy rainstorm; while "711" case is resulted from tropical depression trough, with middle-low level weak cold air overlying warm-wet air transported by southeast low leve jet, the unstable condition for heavy rain occuring and maintaining is produced. The coupling of helicity with a negative(positive) value at upper(lower) levels and ageostrophic vertical upward column provide favorable dynamic lifting mechanism. The two rainstorms occur in the south side area of energy front and negative area near zero isoline of the vertical component of moist potential vorticity. The potential unstability changes from strong to weak and unstable energy begins to release, which all indicate the beginning of heavy rain.