Dual-Doppler Radar Observation Of Precipitation Formation Mechanism Of The Beijing Extreme Rainfall Event On July 21, 2012

The extreme rainfall event hited Beijing on 21 July 2012 with a record-breaking amount of 350 mm in 20h from 08:00LST 21 to 04:00LST 22. This study examines the mesoscale structure and formation mechanism of the precipitation system in extreme rainfall event, using the measurements from two nearby ground-based Doppler radars (37km) located in Beijing, the Beijing Radar (S band) and the Airport Radar (C band). An advanced variational-based multiple-Doppler radar synthesis technique particularly designed for dealing with non-flat surfaces is applied to analyze the three-dimensional wind fields over the terrain.According to the radar and rain gauge observation, the rainfall process consists of two stages:the precipitation over the terrain (08:00LST-18:00LST) and the precipitation over the flat region (18:00LST-04:00LST 21). In the earlier stage, the distribution of the surface rainfall focuses on the southwest orography region of Beijing. The distribution of the convection frequency has the similar pattern with the surface rainfall; the southwest orography region is the most vigorous convection region in the terrain stage. The height of the convection is low in the terrain stage, with only 3 km height of 35 dBZ echo top. The retrieved wind field and the surface observation show that southeasterly winds prevail at low-level, and impinge on the southwest-northeast oriented mountains of Beijing at a nearly perpendicular angle which the perpendicular wind component exceeded 7ms-1. And southwest winds prevail in the middle level (3-5 km), which enhance the rainfall over the terrain region. An analysis of the Froude number (0.12) revealed that the upstream atmosphere was statically unstable and the lifting by the topography was favorable for the convection in the upslope.In the later stage, the distribution of the rainfall was mainly over the southeast plain of Beijing, which was similar to the convection frequency pattern. Associated with the heavy precipitation, the strong downdraft and the cold pool on the ground developed in the later stage. Subsequently, the precipitation system organized into a southwest-northeast-oriented quasi-linear convective system, with the new convective cells triggered at the boundary between the cold pool and the southeasterly flow at low level. The retrieved three-dimensional wind revealed that there were two mainly airflows in the quasi-linear convective system:the front-to-rear (FTR) airflow and the rear-to-front (RTF) airflow. The warm moist FTR generating from the front low level of the linear system rose tilting. The RTF generating from the middle level sunk into the surface and resulted in the dynamic high pressure. The interaction of the cold pool and environmental wind lead to strong updraft motion, which triggered the new precipitation system.