The mesoscale severe convective storm process companied with tornadoes and hailstorms occurred at 17:30 BST, July 12 2004 in Nantong ,Jiangsu Province China was numerical simulated by using the NCAR/PENS MM5 mesoscale numerical model based on the conventional observational data and NCEP 1^o×1^o reanalysis data analyzed in this paper . The results showed that the atmospheric instability energy accumulating and releasing process play a key role for the formation of the severe convective weather. The unstable stratification formed under the situation with upper level dry and cold advection and lower level warm and moist advection over Nantong region, and the dry air intrusion as well as strong convergence of the warm and moist air in boundary layer caused the instable energy releasing and the tornado forming. Based on the numerical simulation results, the parameters BCAPE, CIN, NCAPE, EHI and SWEAT Index etc were calculated. The parameters and indexes were good indicators for the severe weather including the tornado.

Key words: convective available energy SWEAT index severe convective weather tornado dry intrusion

Fig1. simulated temporal variation of energy parameters over Nantong region

a. BCAPE (dashed line , unit: J·Kg^-1 ) and BCAPE calculated by density temperature (solid line , unit: J·Kg^-1) ; b. EHI (unit: J·Kg^-1·m^{s-2 ) ; c. SWEAT index ;d. wind direction shear of SWEAT index}