Abstract: A diagnostic study with a dataset of mesoscale resolution for the heavy rain process occurred over a region of Yangtze River during the third decadel days of June in the 1983 Meiyu season is presented in this paper. Comparisons are made of the mesoscale environment characteristics in a disturbed and an undisturbed period. The results showed that significant differences were found in the momentum field, Q₁ and Q₂, as well as the vorticity budget in the two periods. However not much difference was observed in the heat and moisture fields. In the disturbed period with heavy rain, the feedback effects of cumulus convection are important. Q₁ and Q₂ are mostly contributed by the vertical advection terms. The condensation heating is almost balanced by the cooling effect of ascending motion. The vertical transport of moisture is the major source of Water vapor for the cumulus convection. As to the vorticity budget, positive vorticity is produced by the convergence and twisting terms at low levels and is transported upward by the cumulus convection. In both upper and lower layers, accumulated positive vorticity is dissipated by nonlinear process of advection.