一次梅雨锋暴雨的数值模拟及位涡诊断

Numerical Simulation and Potential Vorticity Diagnosis of a Meiyu Front Heavy Rain Process

在对2005年7月11—12日发生在长江中下游地区的一次暴雨过程进行天气分析的基础上,利用MM5数值模式对这次暴雨过程进行模拟,模拟结果及客观分析对比表明,MM5基本上成功地再现了这次暴雨过程。对模式输出的高分辨率结果进行诊断分析,发现梅雨锋暴雨期间,暴雨区上空对流层中低层有平行于雨带的位涡(PζV)、湿位涡垂直分量(MζPV1)的正值区以及湿位涡水平分量(ζMPV2)的负值区,而分布在高层相当位温(eθ)线密集带上PζV、ζMPV1正值中心以及ζMPV2负值中心沿该密集带传向低空。干空气侵入在这次暴雨过程中有重要作用,对流层高层西风分量下传将高纬高层高位涡干空气带到低纬暴雨区低层,促使对流层低层对流发展,从而产生强降水。

By using the mesoscale numerical model MM5（V.3）,the simulation was conducted on the basis of synoptic analysis of a heavy rain process in the mid-and low-reaches of Changjiang River during the period from 0000 UTC 11 to 0000 UTC 12 July 2005.The comparison between the numerical simulation results and objective analysis shows that the MM5 is able to reproduce the rainstorm process.The high spatial and temporal resolution model outputs were used in diagnostic analysis and it was found that there were a positive potential vorticity（ζPV） and moist potential vorticity vertical component（ζMPV1） bands and a negative moist potential vorticity horizontal component（ζMPV2） band which ran parallel to the surface rain-band in the middle-low troposphere above the rainstorm area.The positive centers of ζPV and ζMPV1 and the negative center of ζMPV2 distributing on the dense isoline zone of equivalent potential temperature at the upper level slantly spread from the upper down to low troposphere along the dense isoline zone.Dry intrusion played an important role in the development of the heavy rain process.Dry air with high potential vorticity in the upper troposphere from high latitudes intruded into the lower level above the rainfall area due to the discending of west wind and promoted the development of convection,leading to the heavy rainfall.