城市边界层过程在北京2004年7月10日局地暴雨过程中的作用

The Role of Urban Boundary Layer in Local Convective Torrential Rain Happening in Beijing on 10 July 2004

从大量的观测事实入手,结合简要的理论分析,揭示发生在北京城区的一次相对孤立的β中尺度对流暴雨系统的启动机制以及城市边界层在这次局地暴雨过程中的作用。研究结果表明:(1)2004年7月10日北京暴雨前后,局地水汽和天气尺度低空水汽输送条件是非常有利的,但是天气尺度系统的垂直运动对这次北京局地暴雨是存在抑制作用,至少没有明显的帮助。这一点与区域性降雨带中出现的中尺度暴雨中心具有明显区别。(2)暴雨发生在偏南暖湿气流中,首先,可能是位于北京西南的河北涞水和易县附近强雷暴群激发的重力波传播,触发对流不稳定能量释放,形成线形分布、孤立的γ中尺度对流单体,这些对流单体在城市中尺度的风场辐合线的组织下,形成β中尺度对流系统。这一对流系统激发的重力波,再次形成一系列γ中尺度对流单体,当这些单体又一次被组织成β中尺度对流系统时,由于对流不稳定能量减弱,北京城区出现的第二个降水峰值明显减弱。(3)城市与郊区下垫面物理属性造成的热力差异,不仅形成城市中尺度的低空风场辐合线,它的存在对对流单体具有明显的组织作用。同时,这种热力差异还可能造成边界层内中心城区风场垂直切变加强,郊区低空风速加大,这种强迫有利于暴雨中心区强烈的上升运动得以维持,保证了低空水汽在较大范围内向对流体中流入,维持对流降水的持续。

An isolated mesoscale convective torrential rain which happened in Beijing urban on 10 July 2004 （＂7·10＂） made a great traffic trouble because of serious inundation cross the urban areas and caught various social attention. The triggering mechanism of the convective torrential rain and the reason that the downpour occurred only in urban center are studied by analyzing a large number of observation data sets, such as observational data with high spatial-temporal resolution based on auto-weather station network, Doppler radar observational products, avail able vertical distribution of wind detected by a boundary wind profiler, TBB data from GOES and conventional weather observational data sets. Based on a simple mesoscale theoretical analysis and detail observational investigation, the spatial structure of the weather system is proposed. The research results indicate that （1） the local vapor condition and the large-scale vapor transportation are favorable during the torrential rain. However, the large-scale descending area has been keeping inhibition during the weather event, and this is a great difference between the isolated meso-βscale convective storm system （MCSS） and other mesoscale torrential rain events which happen in regional precipitatiom （2） The convective activities in Beijing area are closely related to gravity wave. At the initial stage of ＂7·10＂ local torrential rainfall, the local convective instable energy is possible to be triggered by gravity wave which is motivated by the stronger convective activities in Laisbui and Yixian counties of Hebei Province to the southwest of Beijing and a series of relatively isolated meso-γ scale convection cells （MCCs）, which appear to be linear, develop in Beijing. Finally, a meso-β scale convective storm system is organized by urban mesoscale convergence line. The MCSS not only causes the heaviest rain intensity in the urban center, but also excites gravity wave and brings forth the similar meso-γ scale convection cells again. When the meso-γ scale convection cells are reorganserved not only in urban surface but also in total boundary layer above, which plays a key role in organizing the isolated meso-γ scale convection cells （MCCs）. The research confirms that the thermodynamic forcing caused by the temperature difference between urban areas and suburbs, is a fundamental factor in the development of the convergence line. On the other hand, because of the thermodynamic difference between urban areas and suburbs, the vertical wind shear is strengthened in the urban center, and the horizontal flow in lower layer is accelerated in suburbs, in other words, the thermodynamic forcing is advantageous to keeping the stronger convergence motion toward cen- tral convection area and providing enough compensated moisture current around a relative large field.