贺兰山两次特大致洪暴雨的数值模拟与地形影响对比

Numerical Simulation and Comparative Analysis of Topographic Effects on Two Extraordinary Severe Flood Rainstorms in Helan Mountain

基于天气研究和预报模式WRF,利用常规观测资料和NCEP/NCAR全球再分析资料,对2016年8月21日和2018年7月22日2次贺兰山东麓特大致洪暴雨进行数值模拟和地形影响对比分析。结果表明:(1)2次暴雨过程发生时的大气环流形势基本相似,西北地区东部处于高温、高湿、高能环境下,宁夏东侧有副热带高压阻挡,西侧有弱冷空气东移影响,中低空受偏南和偏东暖湿气流控制;(2)在高湿高能和极不稳定的大气层结条件下,当宁夏北部出现风速达12 m·s-1以上的低空急流,且风向与贺兰山东麓迎风坡夹角达60°~90°,因地形阻挡触发强对流,使得暖湿不稳定能量强烈聚集与快速释放,低空急流的维持时间决定了暴雨的持续时间;(3)贺兰山苏峪口的喇叭口地形和其北侧的贺兰山主峰对低空急流的共同作用,造成水汽在此处大量聚集并强烈抬升,决定暴雨的中心位置。

Based on the conventional observations data and NCEP/NCAR reanalysis data,the numerical simulation and comparative analysis of the topographic effects on two severe flood rainstorms which occurred on 21 August 2016 and 22 July 2018 in Helan Mountain were carried out by using the weather research and forecasting(WRF)model.The results are as follows:(1)The atmospheric circulation situation during the two heavy rainstorm was basically similar.The eastern part of Northwest China was under the control of air with high temperature,high humidity and high energy.There was subtropical high blocking in the east and cold air moving eastward in the west side of Ningxia.Further more,the lower layer was controlled by southerly and easterly airflow.(2)Under the conditions of high humidity,high energy and extremely unstable atmospheric stratification,when a low-level jet with a wind speed more than 12 m·s-1 appeared in northern Ningxia,and the angle between the wind direction and the windward slope of the eastern foot of Helan Mountain reached 60°-90°,the strong vertical motion was triggered by the terrain blocking,which made the warm and humid unstable energy accumulate and release rapidly.The duration of the rainstorm depended on the duration of the low-level jet.(3)The joint effect of the trumpet-shape topography of Suyukou and the main peak of Helan Mountain in the north side of Suyukou on the low-level jet led to the massive accumulation and strong uplift of water vapor,which determined the location of the rainstorm center.