Abstract: A heavy rainfall event, mainly caused by the successive mesoscale convective systems (MCSs) along the meiyu front, occurred within the mid-lower reaches of the Yangtze river on 4-5 July, 2003. A mesoscale numerical model of the Penn State/NCAR Mesoscale Model Version 5 (MM5) was employed to numerical simulate this heavy rainfall event. The stable, deep northeast-southwest oriented short-wave trough to the north of the meiyu front coupled with the low vortex systems moved from the southwest was favorable to the genesis and development of the anti-cyclonic disturbance located northward to the meiyu front. And the subsequent formation and enhancement of a lower anti-cyclonic vortex, was expected to be crucial for the initiation and maintenance of the low-level shear line and the low-level vortex along the meiyu front.The latent heating from the MCSs along the Meiyu front resulted in the formation of mesoscale convective vortex (MCV), and meanwhile the MCV enhanced the low-level convergence near the low-level shear line and thus lead to the initiation of the low-level vortex embedded in the low-level shear line. There were four different vertical circulations within the meiyu front, which were important for the interactions among the different scales weather systems in both the upper and lower-levels. These vertical circulations possessed the different structures and dynamic roles at different rainfall stages. The development of convection disturbance within the meiyu frontal system and the initiation of subsequent heavy rainfall were associated with the vertical circulation across the front and upper level ageostrophic vertical circulation, while the development of the convection would modify the vertical circulation structure. At last when the heavy rainfall weakened the vertical circulation would restore again.