Mesoscale Numerical Simulation and Diagnostic Analysis of Meiyu Front Heavy Rain in Nanjing in July 2003

Mesoscale model MM5 is used in this paper to investigate the course of Meiyu front rainstorm in Nanjing on 4~5 July 2003(!03.7), and it is shown thatits simulation ability is good about this process. The results show that the time of frontogenesis is earlier 12 h than that of heavy rain occurred. The results also suggest that the developmentof jet stream is about earlier 2 h than that of heavy rain. Increasing of the lower level jet stream and enhancing of absolute value of negative divergence that produce convergence on the low-level of troposphere to cause violently the occurrence of heavy rain. But it is the heavy rain that causes the increaseof the relative vorticity. The moist potential vorticity analysis during the heavy rain indicates that the lower level of troposphere is negative averagely in large area. This means the heavy rain have a feature of moist symmetric instability. But the result from the center of the heavy rain (rainfall 20 mm·h+{-1}) isaccompanied by a small positive moist potential vorticity on 850 hPa. This shows that the center in heavy rain is almost moist symmetricstability after the release of convective energy.