Impact of ENKF Surface and Rawinsonde Data Assimilation on the Simulation of the Extremely Heavy Rainfall in Beijing on July 21, 2012

MENG Zhiyong1,†, TANG Xiaojing1, YUE Jian2, BAI Lanqiang1, HUANG Ling3

1. Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871; 2. National Meteorological Center, Beijing 100081; 3. Chinese Academy of Meteorological Sciences, Beijing 100081; † E-mail: zymeng@pku.edu.cn

Abstract Regarding the forecasting errors of operational models for the high-impact extremely heavy rainfall event in Beijing on July 21, 2012, this work examines the impact of assimilating surface and rawinsonde observations using ENKF data assimilation system on the simulation of rainfall distribution and the surface features in the initiation period of the rainfall in Beijing, and reveals the possible reasons for the forecasting errors. Results show that data assimilation significantly improves the simulation of rainfall distribution, confirming that the cyclonic vortex is the key influencing system of the heavy rainfall event, which was proposed by previous researchers based on observations and sensitivity analyses. This work also reveals that the surface low and its associated inverted trough are the direct producers of the rainfall in Beijing. These results indicate that the reason of the failure of the operational models in this extremely heavy rainfall is the large forecasting errors in the strength and location of the cyclonic vortex and the associated inverted trough eastward of the surface low. Key words 7·21 heavy rainfall; cyclonic vortex; inverted trough; extremely heavy rainfall; Beijing

1 北京 7·21 暴雨过程天气回顾

2012年7月21日, 北京地区发生一次极端暴雨事件(简称7·21暴雨), 出现大暴雨(大于100 mm/24 h)的范围占全市总面积的86%以上[1](图1(a))。房山

区河北镇的总雨量最大, 达541 mm (数据来自北京市防汛办公室)。这次暴雨过程共造成79人死亡,经济损失达116.4亿元。

此次暴雨过程具有多尺度特征, 表现为横贯我国西南到东北的弱雨带和京津冀地区的局地极端暴