西太平洋副热带高压控制下上海地区一次局地短时强降水成因的Q矢量分析

在西太平洋副热带高压控制的天气背景下，2016年8月19日下午上海地区发生一次局地短时强降水过程，此次过程历时3 h、水平范围20~40 km，呈现出生命史短、局地性强的特点。基于上海地区地面自动气象站2分钟平均资料，采用仅需一层资料计算的非地转 Q 矢量分析方法，研究分析了此次局地短时强降水发生发展演变成因，结果如下:(1)地面温度场和风场叠加分析表明，上海“城市热岛”特征与长江沿岸及邻近水域的热力不均匀分布引发了江风，江风将江岸邻近水域的湿、冷空气向城市陆地输送，并与陆地上干、热空气交汇，激发产生局地短时强降水，而降水的发生，导致地面温度下降、“城市热岛”特征减弱，从而减小水陆温度差，进而减弱江风，这直接减弱了有助于降水发生发展的动、热力强迫条件，促使降水趋于衰亡结束。(2)地面 Q 矢量散度辐合场和温度露点差叠加分析表明:在降水发生发展阶段， Q 矢量散度辐合强迫产生垂直上升运动较强，而空气湿度条件相对较弱；在降水强盛阶段， Q 矢量散度辐合强度和空气湿度的强度不仅增至最强，且上升运动区与高湿区重合；在降水衰亡阶段，地面空气一直维持高湿条件而 Q 矢量散度辐合强度明显减弱。这从地面大气中垂直上升运动条件和水汽条件揭示出致使降水强度发展演变的内在因素，且二者重叠区对降水落区有较好指示意义。最后，对地面 Q 矢量散度辐合场在局地短时强降水短临预报工作中的潜在应用前景进行了有意义的讨论。 

ANALYZING THE CAUSE OF A LOCAL SHORT-LIVED SEVERE PRECIPITATION IN SHANGHAI UNDER THE CONTROL OF WESTERN PACIFIC SUBTROPICAL HIGH BASED ON Q VECTOR

Under the control of a Western Pacific subtropical high, a local short-lived severe precipitation occurred in Shanghai in the afternoon of August 19, 2016 that lasted 3 hours in a range of 20 to 40 km. Based on 2-minute average ground automatic meteorological observations, the ageostrophic Q vector diagnostic method, which was only based on a single layer data, is used to diagnose the cause of the precipitation. The main results are as follows: (1) Analysis of surface air temperature overlapped with winds shows that the heat contrast between the "urban heat island" and the adjacent of Yangtze River leads to alongshore-wind, which transports moist and cold air above the river banks to the urban land, which meets with the dry and warm air there, causing the local short-lived severe precipitation to appear. The occurrence of precipitation then cools the surface air temperature, weakens the "urban heat island" effect, dampens land-water temperature contrast and thus weakens the alongshore winds, which directly cripples the dynamical and thermal forcing conditions conducive to precipitation and then results in its decline. (2) Analysis of ground convergence of Q vector divergence field overlapped with the difference of dew-point shows that during the developing stage, the convergence of Q vector divergence forces relatively strong vertical ascending motion, while the moisture condition in the ascending air is weak. During the enhancing stage, the convergence of Q vector divergence and air moisture not only becomes strongest, but also the ascending region is coincided with the high-humidity region. During the decaying stage, the air moisture remains high while the convergence of Q vector divergence is weak. Analysis of Q vector divergence superposed with the difference of dew-point reveals the inherent factors which contribute to the development and evolution of precipitation, and their coincidence region is a good indicator for the precipitation area. Finally, there is a discussion on the potential application of the convergence of Q vector divergence to nowcasting of local short-lived severe precipitation in the future.