CONDITIONS FOR THE OCCURRENCE OF LARGE AREA HAILFALLS AHEAD OF COLD FRONT OVER ZHEJIANG

Seven cases have been selected to analyze the occurrence conditions of large area hailfalls ahead of cold front over Zhejiang. It is found that large potential instability energy and strong boundary layer convergence must exist prior to the occurrence of the hail events, however, strong vertical wind shear is not necessary. The results of computation show that large scale convergence and asending motion are not necessarily to occure in the middle and lower tropo-sphere during the 6-12h in advance of hailfall occurrence. The warm, moist air in boundary layer is very vigorous, but the behavior of cold, dry air is weaker before hail occurrence. The investigation suggests that the formation of instability energy is mainly due to the warm, moist air advection in boundary layer. The difference between the vertical profiles of moisture flux divergence in the hail events and those in heavy rainfalls has been found. In the hail cases, the moisture convergences are almost concentracted in the boundary layer. Probably, it is a effective information for predicting the occurrence of hailfall ahead of cold front to combine four factors: the latent instability, the convective instability, the boundary layer convergence and the location of cold front.