Abstract:By using the high resolution output data from mesoscale model WRF succ essfully simulating typhoon “Khanun”, with the diagnostic method of isentropic potential vorticity(IPV) and moist potential vorticity(MPV), the evolvement cha racteristic of mesoscale system is analyzed in the course of rainstorm and the m echanism of typhoon rainstorm development and maintenance is discussed. The resu lts show that the study of PV on isentropic surface reveals PV evolvement characteristic of typhoon low pressure and nearby, the rainstorm region lies i n the no rtheast part of high PV center on the lower isentropic surface or lies in the ri ght side of the maximum PV gradient on the upper isentropic surface. It also sho ws that high PV, including in dry and cold air, spreads downward from the upper layer to intensify the turbulence of lower layers along with the isentropic surf ace furling to the center of positive PV anomaly, which helps the release of pot ential instability energy and the amplification of rainstorm. Conditional symmet ry instability and convective instability can be considered as significant mecha nism of typhoon rainstorm development and maintenance. Meanwhile, the developmen t of mesoscale system in the rainstorm region agrees with slantwise vorticity de velopment theory.