北京“7.10”暴雨β-中尺度对流系统分析

The Meso-β Scale Convective System of a Heavy Rain Event on July 10,2004 in Beijing

采用NCEP/NCAR再分析资料、自动站加密观测资料、逐时云顶亮温TBB资料、多普勒雷达资料以及成功模拟基础上的高分辨率模式输出资料,对2004年7月10日北京突发性暴雨过程β-中尺度对流系统的发生发展、结构与成因进行了综合分析。结果表明:此次过程影响系统为β-中尺度对流系统,它发生在大尺度暖脊之中,对流层中层的短波槽以及低层西风槽前西南气流与暖切变线北侧东南气流的汇合为其发生提供了良好的环境条件;该β-中尺度对流系统由两个对流云团合并而成,具有椭圆形结构特征,其水平尺度为150 km×100 km,时间尺度约为5 h;低层流场上它表现为中尺度辐合线或强辐合中心,雷达回波和径向速度场所反映的中尺度回波带和辐合线与它的演变有密切关系;在发展强盛期,β-中尺度对流系统具有较强的斜压性特征,垂直倾斜的上升气流及其两侧有明显的下沉补偿气流,显示它具有对流型风暴结构特征;在强对流不稳定层结条件下,700 hPa以下对流层低层具有明显的假相当位温θse暖湿舌,近地面层偏南风与偏东风两支气流的辐合及冷空气的侵入,导致行星边界层内能量锋区的加强,从而有利于β-中尺度对流系统发生发展。

An analysis on the meso-β scale convective system of formed by using the special observational data, including a heavy rain event on July 10, 2004 in Beijing is perautomatic meteorological stations data, radar images and satellite images and NCEP/NCAR reanalysis data on the basis of successful simulation. It is found that the heavy rainfall process is generated by a meso-β convective system which is produced in a large scale warm area. The short wave trough in the mid level of troposphere, the convergence between the southwest air current from west wind trough and the southeast air current from the north part of warm shear line in low troposphere provide a good background condition. The meso-β scale convective system is formed by the mergence of two meso-scale convective clusters, it shows an ellipse-shape structure of a horizontal scale of 150 km × 100 km and the time scale of about 5 hours. It shows the features of the meso-scale convergence line （or convergence center） in low levels stream fields during its occurrence and development stage, and the strong meso-scale convective clouds echo band and meso-scale convergence line exhibited in radar reflectivity image and in radar velocity fields are often related with the occurrence and the development of the meso-β scale convective system. During the stage of strong development, the meso-β scale convective system shows strong baroclinity perpendicular features and has a similar structure of slantwise updraft current to convective storm. Its occurrence and development are forced by the meso-scale convergence line of low troposphere in strong convective instability condition and a warm tongue below 700 hPa. The convergence between the southern air current and eastern air current and the invading of the cold air in the boundary layer lead to the strengthen of the energy front, which is helpful to induce the generation of the meso-β scale convective system. In addition, the cloud-top infrared brightness temperature （TBB） of the meso-β scale convective system that induces the heavy rain on July 10, 2004 in Beijing is at - 45 ℃or so, and the updraft airflow reaches the height of around 300 hPa, which means in this case the convection is only activating in the low level of the tro posphere in contrast with the deep convective systems of meso-β scale convective system in the mid and lower reaches of Yangtze River and South China, which symbolize with infrared brightness temperature between - 70 and - 85 ℃ , and the updraft airflow reaches the top of troposphere. Future research is needed on whether this conclusion is characteristic for popular convective heavy rain process in North China.