Analysis of a continuous heavy rain event in Shaanxi produced by a low vortex
LIU Huimin1, MA Xiaohua2, LIANG Shengjun2, KANG Lei1, JIANG Yirong1, AI Rui3, LOU Panxing1
1. Yulin Meteorological Office of Shaanxi Province, Yulin Key Laboratory of Energy and Chemical Meteorological Services, Yulin 719000;
2. Shaanxi Meteorological Observatory, Xi'an 710015;
3. Shaanxi Institute of Meteorological Science, Xi'an 710015
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.