By using the three-dimensional numerical model of convective rainstorm,IAP-CSM2,the severe rainstorm occurring in Wuhan on the morning of 21 July 1998 is simulated to analyze the structure and development of convective cloud and precipitation,and to study the microphysical mechanism of heavy rainfall deeply.As a comparison,another case occurring in Shaanxi Province is chosen.The modeling results show that the convective cloud of Wuhan develops more stably.The main developing process is warm-cloud process.The diameters of raindrops fallen to the ground are bigger than those of Shaanxi case.Though its maximum updraft velocity,water contents of precipitation particles and instantaneous precipitation intensity are all smaller,the cumulative precipitation of Wuhan is more than that of Shaanxi case.Analysis illustrates that high humidity of lower atmosphere is an important reason to lead to this phenomenon.