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.

Diagnosis has been done of the July 1991 Meiyu front torrential rainfall event by using the diagnostic equation of frontogenesis circulation in the primitive equation model.

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.