一次双高空急流背景下南疆强降水事件的动力过程和水汽源分析

本文利用欧洲中心ERA-Interim和NOAA的再分析资料并应用拉格朗日后向轨迹追踪的方法对2015年5月24日发生在南疆的一次强降水过程进行了动力诊断和水汽特征分析。结果表明此次强降水过程的直接影响系统是中亚低涡前西南气流中发展的小槽,南北两支高空急流辐散场叠加引发的对流层高层加剧的抽吸作用和高低空急流的耦合作用共同导致了深厚强烈的上升运动,是这次强降水主要的动力抬升机制。TBB（black body temperature）的演变与降水的发生、发展有很好的对应关系,TBB中心降至-50°C以下时降水开始且随其中心强度的扩大降水也持续加强。进一步诊断发现,低层850 hPa对流涡度矢量（CVV）垂直分量的正值中心在降水前6～12小时已可以大致体现未来强降水的落区。此次南疆盆地强降水的水汽主要源于黑海和里海,低空急流引导了一部分水汽进入南疆,HYSPLIT模式后向追踪的结果表明,此次强降水过程主要有两条水汽通道,均源于新疆以西的欧亚大陆但输送路径有所差异,偏西路径和转向路径分别主要输送800 hPa以上和以下的水汽,降水发生前两条路径在垂直方向上均有明显抬升,水汽辐合有利于暴雨的形成。

Analysis of Dynamic Process and Moisture Source on a Heavy Precipitation Event in Southern Xinjiang Associated with the Double Upper-level Jet

Based on the ERA-Interim reanalysis data and the HYSPLIT (hybrid single particle Lagrangian integratedtrajectory) model driven by the NOAA reanalysis data, this study examined the dynamical process and moisture source ofa heavy precipitation event that occurred in southern Xinjiang on 24 May 2015. The results indicated that the directinfluencing system of this heavy precipitation was the small perturbation along the southwesterly flow east of the centralAsian low vortex. The enhanced upper divergence pumping action caused by the two upper-level jets and the coupling ofthe upper- and low-level jet streams induced deep and intense ascending motions, which was the main dynamic upliftmechanism of the heavy precipitation. The evolution of TBB (black body temperature) corresponded well with theoccurrence and development of precipitation; the rainfall occurred when the low value center of TBB dropped below-50℃ and strengthened continually with the expansion of the low value center. A further study suggested that thepositive center of the vertical component of CVV (convective vorticity vector) at 850 hPa could roughly reflect thelocation of the heavy precipitation 6 to 12 hours ahead. Analysis of the moisture flux and trajectory tracking revealed thatthe most significant moisture sources of the heavy precipitation over the basin in southern Xinjiang were the Black Seaand the Caspian Sea, and the low-level jet entrained a part of moisture into the south of Xinjiang. The result of HYSPLITmodel tracking illustrated that two moisture paths both originated from Eurasia but differed in trajectories during theheavy precipitation; the moisture above and below 800 hPa was mainly transported by the western track and diversiontrack separately. Significant uplift occurred vertically in both trajectories before heavy precipitation, and the convergenceof moisture contributed to the event．