不同天气尺度强迫下陕西暴雨的成因对比

Comparative Analysis of the Causes of Rainstorm in Shaanxi under Different Synoptic Scale Forcing

选取常规高空观测资料、地面加密观测资料和ERA5(0.25°×0.25°)再分析资料,对2020年8月4—7日陕西持续性暴雨过程进行诊断分析,并探讨了暴雨发生发展的物理机制。结果表明:暴雨过程分为两个阶段,分别是5日陕北区域暴雨和7日陕西中南部暴雨,二者的环流背景差异显著;5日暴雨的主要影响天气系统是高空槽、低空急流和低涡,中低层急流强且维持时间长,系统涡度大、辐合强,天气尺度强迫强;7日暴雨的主要影响天气系统是短波槽和低层切变,中低层大气风速小,系统涡度小,辐合相对弱,天气尺度强迫较弱。4—5日中低层大气水汽输送强,水汽输送较气候态偏大3~4σ;700 hPa西南急流和850 hPa东南急流给陕北带来充沛的水汽,中低层大气存在明显低涡,大气湿斜压性强,同时低涡南侧有明显锋生,次级环流增强上升运动,水汽在低涡中心及右侧强烈辐合抬升,强降水得以维持;800~700 hPa存在条件对称不稳定,进一步增强上升运动,异常充沛的水汽供应和偏强的低涡造成持续的上升运动是本阶段产生极端大暴雨的主要原因。6—7日陕西中南部水汽输送较弱,水汽辐合强度较小,但本地可降水量大;暴雨区为暖湿大气控制,对流不稳定较强,对流有效位能大,切变线辐合抬升触发对流,降水对流性更强;本阶段强降水较为分散,但雨强大且持续时间短,与地形关系密切。

Based on the conventional high-altitude observation data,ground densified observation data and ERA5 reanalysis data(0.25°×0.25°),the cause diagnosis and predictability of the continuous rainstorm process in Shaanxi from August 4 to 7,2020 were analysed.The results show that the rainstorm process can be divided into two stages,namely,the rainstorm in northern Shaanxi on August 5 and the rainstorm in central and southern Shaanxi on August 7,the circulation background of the two rainstorms was significantly different.The main synoptic systems affecting the rainstorm on August 5 were upper trough,low level jet and low vortex.The middle and low level jet was strong and maintained for a long time,the vorticity was large,the convergence was strong,the positive vorticity advection was significant,and the synoptic scale forcing was strong.The main synoptic systems affecting the rainstorm on August 7 were short-wave trough and low-level shear,the wind speed in the middle and lower atmosphere was weak,the vorticity of the system was small,the convergence was weak,and the synoptic scale forcing was weak.From August 4 to 5,the water vapor transport in the middle and lower atmosphere was strong,and the water vapor transport was 3~4σlarger than the climatic state.The southwest jet stream at 700 hPa and southeast jet stream at 850 hPa brought abundant water vapor supply to northern Shaanxi.There was an obvious vortex in the middle and lower atmosphere,and the vorticity center was 4~5σstronger than the climatic state in the eastern part of Northern Shaanxi,at the same time,there was an obvious frontogenesis on the south side of the vortex,and the secondary circulation strengthened the updraft,the water vapor strongly converged and lifted in the center and right side of the vortex,and the rainstorm was maintained.The conditional symmetric instability existed between 800 and 700 hPa,which further enhanced the upward motion,the main reason for the extreme rainstorm in northern Shaanxi was the abnormally abundant water vapor supply and continuous upward movement caused by the abnormally strong low vortex.From August 6 to 7,the water vapor transport in central and southern Shaanxi was weak,and the intensity of water vapor convergence was small,but the local precipitable water was large.The rainstorm area was controlled by warm and wet atmosphere,with strong convective instability and large convective available potential energy,the convergence and uplift of shear line triggered the development of convection,the convective nature of precipitation was more obvious,the heavy precipitation was more dispersed,the rain was strong and the duration was short,which was closely related to the terrain.