北京“23·7”极端强降雨特征和成因分析

Characteristics and Causes of the July 2023 Extremely Torrential Rain in Beijing

利用北京地区加密气象站、补盲的北京市规划和自然资源委员会雨量站、双偏振雷达、风廓线雷达、GPS水汽等观测数据和ERA5再分析数据,对北京“23·7”极端强降雨阶段性特征和成因进行了分析。结果表明:“23·7”极端强降雨累计降水量(331 mm)和单点最大降水量(1025 mm)均打破历史纪录,最大雨强(126.6 mm·h^(-1))排名历史第二位,具有显著的极端性。强降雨可分为5个阶段,其中第Ⅱ和第Ⅳ阶段降水量分别占过程累计降水量的37.1%和39.7%,第Ⅳ阶段雨强更大,对应急流更强,高温、高湿特征也更明显。地形对降雨的增幅作用显著,降水量在海拔100~300 m山区迅速增加,极大值出现在海拔约400 m的山区,第Ⅱ(第Ⅳ)阶段山区平均降水量和小时雨强分别是平原的2.1(3.0)倍和2.0(2.7)倍;第Ⅱ阶段主要为地形对急流的直接抬升,第Ⅳ阶段为地形绕流辐合和直接抬升共同作用。7月31日上午(第Ⅳ阶段)边界层急流出口区与低空急流入口区耦合导致低层上升运动增强,促使西部山前β中尺度对流系统由块状发展成线状并有γ中尺度涡旋产生,该β中尺度对流系统北上时形成短暂的列车效应,引发了西部山区8个站次100 mm·h^(-1)以上的极端短时强降水。

Based on the data of automatic weather stations in Beijing region,complementary rain gauge stations of Beijing Municipal Commission of Planning and Natural Resources,dual-polarization radar and wind-profiling radar as well as the GPS data and ERA5 reanalysis data,we analyze the spatio-temporal characteristics,and causes of this July 2023 extremely torrential rain in Beijing.The results indicate that the accumulated precipitation and intensity of this event were significant,with an average accumulated precipitation of 331 mm and maximum precipitation 1025 mm at a single station,both of which broke historical records.The maximum hourly rainfall of 126.6 mm·h^(-1) ranked the second in history.This rainfall process can be divided into five stages,of which stageⅡand stageⅣof the rainfall accounted for 37.1%and 39.7%of the total process,being the primary phases.During stageⅣ,the corresponding jet stream intensity got much stronger,and the characteristics of high temperature and high humidity were more obvious.Terrain had a significant impact on the increase in rainfall.The rainfall increased most rapidly in the region between 100-300 m above sea level,and the maximum value appeared near the areas approximately at the altitude of 400 m.The average accumulated rainfall and maximum hourly rainfall in mountainous areas were 2.1(3.0)times and 2.0(2.7)times that of the plains during stageⅡ(stageⅣ),respectively.StageⅡwas primarily influenced by the direct terrain lifting of low-level jet,while stageⅣwas characterized by the combined effect of terrain lifting and blocking convergence.On the morning of the 31(stageⅣ),the coupling of the boundary layer convergence at the terminus and the low-level divergence at its entrance led strong ascending motions,promoting the massiveβ-MCS to develop into a linear shape accompanied by a meso-γscale vortex.Thisβ-MCS propagated northward along the western mountainous regions,forming a short train effect and triggering the short-time extremely severe rainfall over 100 mm·h^(-1) at eight stations.