异常环流背景下贺兰山地形对8.21特大致洪暴雨的影响分析

利用常规气象观测资料、NCEP再分析资料对2016年8月21日傍晚到夜间贺兰山沿山特大致洪极值暴雨展开研究,分析了异常大气环流形势及其影响,并利用天气研究和预报模式WRF(Weather Research and Forecasting model)进行数值模拟和地形敏感性试验,研究了贺兰山地形对暴雨过程的影响。结果表明:超强厄尔尼诺结束后的盛夏,大气环流形势发展异常,8月南亚高压和副热带高压异常偏强,西北地区东部处于高温、高湿、高能控制,副高的快速进退和冷平流的入侵,触发暖湿不稳定能量强烈聚集与快速释放,导致特大暴雨的爆发。其发生在200hPa高空急流分流区即强辐散区、中空西南气流的高温高湿区、低空偏南急流轴左侧流场最大弯曲处的强暖平流区、850hPa偏东大风速轴南侧的风速辐合区,天气尺度强迫作用相对较弱的环境中,500hPa短波槽与700hPa、850hPa低涡切变线和偏南偏东急流以及地面气旋式切变辐合线共同作用是其发生的主要影响系统。贺兰山地形对特大暴雨的发生有明显的增幅效应,主要是贺兰山地形阻挡与强迫抬升作用,促使低涡切变强烈发展从而影响了降水范围、强降水落区及其中心位置等。

Analysis on influence of Helan Mountain topography on extraordinary severe flood-causing rainstorm under abnormal circulation background occurring on 21 August

By the use of conventional meteorological and NCEP reanalysis data,the abnormal circulation and its effects on an extraordinary severe flood-causing rainstorm which occurred in the evening of August 21,2016,along the Helan Mountain,was analyzed.Based on simulation of Weather Research and Forecasting model and sensitivity studies,the impacts of topography on rainstorm were explored.The results show that:the development of atmospheric circulation during midsummer appeared abnormal features after the strong El Ni o.The South Asian high and subtropical high were abnormally strong in August.The eastern part of the Northwestern China was under the control of air with high temperature,high humidity and high energy.In this background,the rapid advance and retreat of the subtropical high,and the invasion of cold advection triggered the intense accumulation and rapid release of warm and wet unstable energy,which ultimately led to the outbreak of extraordinary severe rainstorm.The rainstorm occurred in the diversion area of high-level jet at 200hPa,which was correspond to the divergence zone,and in the southwest airflow region with high temperature and high humidity at middle atmosphere,in the strong warm advection area at the maximum bend of the left-side of the low-level south jet,and in the convergence zone at the south side of axis of easterly wind speed at 850hPa.The weather scale forcing of this rainstorm is relatively weak,with the combined action of the short wave trough at 500hPa,the low vortex shear at 700hPa and 850hPa,the southerly jet at 700hPa and easterly jet at 850hPa,and cyclonic shearing convergence line on the ground,played the main role in occurrence of this rainstorm.On the other hand,the terrain obstruction and forced uplift function of Helan Mountain promoted the intensify of vortex shear at low level,which enhanced the development of rainstorm,affected the precipitation range,strong precipitation area and central position through the influence of the weather system,etc.