A Numerical Simulation And Diagnostic Study Of Heavy Rain Event On July 7, 2007 In Nanjing

Based on NCEP reanalysis and routine observation data, a heavy rain process occurred in Nanjing during the period from 14:00 BST July 7 to 00:00 BST July 8, 2007 is analyzed and diagnosed. It was found that the heavy rain occurred in a well weather situation with the low-level trough and the shear line at low level.In using of the new generation mesoscale model of the Weather Research and Forecast model, the heavy rain was simulatedt. Simulated results show that the development of the mesoscale rainstorm system is well described. With the simulated results of the high resolution of space-time,the direct effect of mesoscale system was analyzed, including mesoscale low level jet,mesoscale vortex etc.The model outputs were used to diagnostically analize and find that there were low-level vortex building and moving eastwardly in low-level convergence field north of the rain-band.The low-level vortex could be maintained and developed due to the strong upward current which caused by interaction of the low-level jet inflow in its southwest and high-level jet outflow in its northeast. Above the rain area there were the instability area of MPV<0,and the negative focus of MPV located at the focus of heavy rain when MPVl﹥0,MPV2﹤0 and‖MPV1‖≧‖MPV2‖. The positive focus of MPV slidding toward low level along the equivalent potential temperature ribbon leaded to the enhancement the negative focus of MPV and convective instability in the low level.