Observational Analysis On Mesoscale Convective System And Discussion On The Reason Of Development In Regional Torrential Rain Event Over Beijing

Many high spatial-temporal resolution data of Beijing, such as auto-weather station (AWS) network data, Doppler radar data, wind profile data, and Variational Doppler Radar Analysis System(VDRAS) are used to analyze the structure and developing characteristic of the meso-scale convective system, and the generating circumstance, in a strongest torrential rain event in Beijing on 10th and 14th August 2008. It is revealed that:1. There was a torrential rain that influenced mainly city occurred on 10th Aug. 2008, which was last about 24hr and two short but strong rain events took place respectively in the afternoon whose raininess once can be 45mm/h and 12mm/5min and in the evening. Conversely, the torrential rain that occurred on 14th August 2008 and also mainly influenced city only lasted 8hr, and average precipitation was 25~40mm, which local area may be 50mm and more, and raininess can also be 51.1mm/h, and 8mm/5min. Although the influencing domains and time, and precipitation of the two rain events were not the same, yet there was short but strong convective rain occurring in them.2. the torrential rain of 10th Aug. 2008 occurred at the advantaged background of air circulation, such as middle-level eastward trough moving, low-level convergence coupled with upper-level divergence, confliction between warm and cold air, and conveyor of abounding water vapor from southeastern sea; but the condition of the one of 14th was almost conversed, whose the dominant synoptic system was a cut-off low of 500hPa, and which was lack of abundant vapor and advantaged dynamic condition, however, the latter case had rich CAPE energy. By comparison of the two torrential rain events, it is revealed that the rain band and rainfall are about low-level meso-βshear and surface pour slot, that when southeast wind on 850hPa covers Beijing where is on the top surface pour slot, the rainfall and strength of convective rain become stronger.3. It was observed that it was 2 hours before the beginning of raining which there was a soaring increase in the CAPE(about 2000J/kg beneath 300hPa) and low-level wind shear over main city of Beijing in 10th ; was the same as 14th . Increase in CAPE and low level east wind of local area was the key environmental condition of Beijing.4. it is revealed from radar data that (1) there were stratified and convective radar reflection in 10th , which before 14:00 the main category was stratified and after that was convective. Especially after 14:00, with the rain band closing, some irregular meso-γconvective cells appeared in the junction between Fang Shan of Bejing and He Bei, which organized and developed to obvious meso-βconvective line. (2) The development of convective cells in 14th was faster than the former. Before 11:00, there was no obvious radar reflection over Beijing; but after 11:00, some increasing intensely convective cells appeared in Da Xing and northeast of Beijing, especially the strength of radar reflection's core of the one in Da Xing can increase from almost 0 to 50dbz within 30 minutes with the characteristic of super cell and hook-like echo. This supercell moved to Chang Ping and increased again. (3) The life stage of meso-scale convective system and raininess can be reflected better by vertical structure of its radar echo than radar echo of only a single layer, for example, at developing stage, the vertical structure of radar echo of convective cell is similar to chair, at developed stage, the structure circular arc, and at decreasing stage, the structure saw tooth.5. Analyzing on data fo AWS and VDRAS, it is revealed that the development of torrential rain in the afternoon in that day was about the low-level meso-βshear where is in the front of stratified precipitation system; the reason of the creation of meso-βshear is the conflict between cold out-flow from stratified rain system in the northwest of Beijing and southeast wind from southeast of Beijing beneath 0.5km. The upward motion due to the convergence of the two air-flow trigger the organizational development of convective cells.6. The development of convective cell in Da Xing of 14th was resulted by a cyclone whose diameter was 20km. And due to the east environmental wind, the cell moved northwestward and lightly weaked, but while closing to the windward slope, because of the convergence of the east environmental wind within 0.4 to 0.9km, the convective cell's strength increased again.7. According to these above analysis, some key points are illustrated as the following:(1) The area which is at east wind of shear on 850hPa and the top of surface pour slot is the potential place of raining.(2) Another key place is in the front of stratified precipitation system, where conflict between cold horizontal flow of and environmental wind before the system may be triggers the organizational development of convective system.(3) Additionally, as these cells closing, the local total vapor water and liquid water will soar, which are good indices of rain forecast; and change in some traditional meteorological fields(wind and press) can foretell the weakness of rain.