陕北北部一次罕见区域性暴雨天气过程分析

利用MICAPS常规气象资料、ERA-Interim 0.25°×0.25°再分析数据、地面区域气象站逐小时观测数据、FY-2G卫星云图和榆林CR/CB雷达产品，对2017年7月25日20时—26日08时陕西北部持续强降水过程进行综合分析。结果表明：（1）这次降水过程呈东西向带状分布，雨强大、范围小、移动慢、持续时间长，降水主要集中在夜间，大暴雨区具有典型的β中尺度特征；（2）西风槽的快速东移南压以及副高的稳定维持有利于槽前正涡度平流的加强及低层低值系统的发展，850 hPa新生的河套低涡和东南低空急流成为这次强降水过程的直接影响系统；（3）河套低涡是一个浅薄的热低压系统，它的发生发展可分为三个阶段，初始阶段低涡形成于弱的锋区中并具有不对称的暖心结构，成熟阶段和旺盛阶段低涡转变为对称的暖心结构，强降水产生在低涡发展成熟阶段，在低涡旺盛阶段降水达到最强；（4）河套低涡直接影响并控制着地面β中尺度低压的发生发展，β中尺度低压稳定在榆林西部，中尺度低压的西部和东部分别形成冷性辐合和暖性辐合，不断触发γ对流单体生成，不同中尺度对流云团的合并导致了降水的强烈发展。

Analysis of a continuous heavy rain event in Shaanxi produced by a low vortex

Using MICAPS data, ERA-Interim 0.25°×0.25° global high resolution reanalysis data, hourly surface observation data, FY-2G satellite cloud picture and CR/CB radar data from Yulin, a comprehensive analysis was conducted for an extreme heavy storm rain event occurred in northern Shaanxi province from 20:00 BT 25 July to 08:00 BT 26 July 2017. The results are as follows. (1) The heavy precipitation event exhibited a clear east-west zonal distribution, with slow moving high rainfall during a long time period in a small area, and mainly concentrating at night. Typical beta mesoscale characteristics showed in areas of heavy rainfall. (2) The westerly trough moved rapidly eastward and pressed down to the south. The stable maintenance of western Pacific subtropical high pressure favored the strengthening of the positive vorticity advection in front of the trough and the development of a low-value system in the surface layer. The nascent Hetao vortex at 850 hPa and southeast low-level jet are the direct impact systems to this heavy precipitation event. (3) Hetao vortex is a shallow thermal low pressure system with three development stages. In the initial stage, a low vortex formed in weak zone with warm center of asymmetry structure. In the mature and vigorous phases, the vortex changed into symmetric warm center structure, and heavy rainfall starts in the mature stage and reaches peak in the vigorous stage. (4) Hetao vortex directly affects and controls the occurrence and development of mesoscale low pressure at the ground. Mesoscale low pressure is stable in the west of Yulin. Cold and warm convergence are formed in the west and east of the mesoscale low, respectively, which continuously trigger the generation of convective monomers.