中文摘要：

利用NCEP 1°×1°的6 h再分析资料和常规气象观测资料,对2012年7月21日发生在北京地区的一次大暴雨天气过程进行非地转湿Q矢量（Q^＊）和湿位涡等物理量诊断分析,研究暴雨期间Q^＊散度、锋生函数和湿位涡的时空分布特征,以及它们与强降水之间的关系。结果表明,Q^＊在850 hPa高度层上对暴雨表现出良好的诊断特性,冷、暖气流的汇聚加强了锋生作用,强锋生中心出现几小时后即出现暴雨。暴雨区位于Q^＊辐合区内,Q^＊散度对6 h后暴雨的落区有很好的指示意义。暴雨落区基本位于MPV1正、负值交界处的等值线密集带上以及MPV2负值区内。暴雨区上空,从近地面到对流层低层的对流性不稳定与条件性对称不稳定同时存在,两者共同作用,这很可能是此次暴雨的中尺度对流系统发生发展的重要条件之一。

英文摘要：

Using NCEP 1°×1° reanalysis data and conventional observation data,the wet Q-vector and moist potential vorticity of a rainstorm process on July 21,2012 in Beijing are analyzed,and the temporal and spatial distribution of un-geostrophic wet Q-vector divergence,the frontogenesis function and moist potential vorticity are studied.Results show that the wet Q-vector at 850 hPa level presents good diagnostic characteristics of rainstorm.The convergence of cold and warm air strengthens frontogenesis,and the heavy rain occurs a few hours after the strong frontogenesis center forms.Analysis of moist potential vorticity shows that the heavy rainfall basically locates in the junction area of positive and negative MPV1 values and the negative MPV2 value area.Both the convective instability and conditional symmetric instability exist simultaneously from the ground to the middle-lower troposphere,which is likely to be one of the important conditions for the occurrence and development of mesoscale convective system during this rainstorm.