Abstract: By using a regional sixteen level η coordinate model which considerated the steep topography,a numerical simulation on a local heavy rainfall which occured in Yuan′an,Hubei province on August 14-15,1990 with the maximum rainfall 366.8mm in 4 hours is performed in this paper.On the basis of the well simulation,the developing characteristics of MPV of this heavy rainfall event were studied.The main results show that,in the developing stage of the heavy rainfall,the precipitation center accompanied by a negative center of moist relativity potential vorticity (MRPV).Results from the diagnosis of MPV equation indicate that,before the peak value of the precipatation,the varying rate of MPV is positive.It means that the convective instability has been released at that time.The dominant physical effects are the vertical flux convergence of mesh scale MPV and the twisting effect on subgrid scale.After the peak value,the varying rate of MPV is negative.This is resulted from the rapid strenthening of the cyclonic vorticity.The dominant physical effects are the horizontal flux divergence on mesh scale and the vertical flux divergence on subgrid scale.