The results show that rain storm is most likely happen in a Q * convergent area and it locates near the ascending branch of a secondary circulation,whose intensity and direction can be indicated by vertical distribution of Q *.

The mesoscale numerical model MM4 was used to simulate the heavy rain process asso-ciated with Meiyu front occurred in the Changjiang-Huaihe Basin during the period from08: 00 July 5 to 20: 00 July 6, 1991．The hourly model products with high spatial and temperalresolution were then used for detailed diagnosis studies of the frontal secondary circulation.

This paper,using NCEP reanalysis data(horizontal resolution 1°×1°) and the simultaneous conventional observational data,analyzed its climatic background,circulation situation,helicity,divergence,temperature advection and frontogenetical second circulation.