Mechanisms governing the evolution of a long-lived northwest vortex that caused a series of disasters in Northwest China

The northwest vortex (NWV) is a type of mesoscale vortex that appears with a relatively high frequency in Northwest China. To further the understanding of the NWV's evolution, in this study, the moisture and circulation budgets of a long-lived NWV (～132 h) that appeared in early August 2019 were calculated. This vortex induced a series of torrential rainfall events in Northwest China and Mongolia, which caused severe transmission line faults and urban waterlogging. Synoptic analyses indicate that the NWV was generated in a favorable background environment characterized by notable upper-level divergence and strong mid-level warm advection. The moisture budget shows that the East China Sea and Bohai Sea acted as the main moisture sources for the NWV-associated precipitation, and the water vapor was transported into the rainfall regions mainly by easterly and southeasterly winds. The circulation budget indicates that, during the developing stage, convergence-related vertical stretching was a dominant factor for the NWV's development; whereas, the vortex's displacement from regions with stronger cyclonic vorticity to those with weaker cyclonic vorticity mainly decelerated its development. In the decaying stage, divergence-related vertical shrinking and the net export of cyclonic vorticity due to the eddy flow's transport resulted in the NWV's dissipation.摘要西北涡是我国西北地区一类发生频率较高的中尺度涡旋, 其所引发灾害的强度与西南低涡, 高原涡相当, 但相关研究却远远少于前两种涡旋.为了深化对西北涡的认识, 本文利用水汽与环流收支对一次长生命史西北涡 (其在西北地区引发了一系列暴雨过程, 导致了严重的输电线路故障与城市内涝) 进行了研究, 发现, 东海与渤海是此西北涡引发暴雨的主要水汽来源, 东风与东南风则是主要的输送气流. 辐合所引起的垂直伸展是西北涡发展的主导因子, 而涡旋由较强背景涡度区向较弱背景涡度区的移动则一定程度上延缓了涡旋的发展.