The differences between wet Q vector divergence field(WQVDF) and ω field from the modified MM4 model are examined in details by the use of data concerning a typical Meiyu front heavy rainfall in Jiang-Huai Basins during 5—6 July 1991. The results show that WQVDF convergence is more closely associated with the precipitation than ω field throughout the rainstorm; the 700hPa convergence field has indication to the surface rainfall on a synchronous basis, especially in the developing and developed stages, which serves as a better indicator to the fallout area and intensity of precipitation;the calculating scheme of vertical velocity involved in the MMM4 model needs to be further improved.