对流层低层偏东风对北京局地暴雨的作用

本文利用风廓线仪、地面自动站观测资料及NCAR/NCEP 1°×1°分析资料等,对北京地区两次局地暴雨天气过程的对流层低层偏东风进行了对比分析研究,重点分析了浅薄和深厚两次偏东风的形成机制、偏东风的垂直结构特征和温、湿特性,以及偏东风在北京局地暴雨中的作用等。主要结论如下:(1)浅薄偏东风活动在距地面高度500 m 以下,水平尺度约250 km,时间尺度约12 h, 地面风速平均约1 m/s;深厚偏东风活动在距地面高度3000 m 以下,水平尺度大于600 km,持续时间大于24 h,地面风速平均约4 m/s。(2)浅薄偏东风由边界层内浅薄的次天气尺度暖性低涡引起,深厚偏东风由天气尺度地面暖性低压倒槽的发展引起。(3)偏东风具有高相当位温的属性,其源地是北京东部或东南部的暖湿气团;在暖湿偏东风上方800～600 hPa 存在干冷空气活动,形成了有利的对流不稳定层结;浅薄偏东风暖湿能量的局地集中特征更为显著,而深厚偏东风在水汽和能量的持续输送方面,以及与500 hPa 偏西风形成较强的、有利于强对流风暴发展的低层垂直风切变方面作用更为显著。(4)浅薄偏东风在时间和空间上与近地面层辐合中心对应较好,与中高空辐散有较好配合,动力作用明显,直接起到了对流风暴的触发机制作用;深厚偏东风与辐合中心对应较差,与垂直运动及上层辐散也没有很好的配合,与对流风暴的触发没有直接的关系。但是深厚偏东风在对流层低层对应厚达3000 m 的潮湿空气层,削弱了雨滴下落过程中产生的蒸发降温作用,有利于对流的发展和维持。

Roles of Low-Level Easterly Winds in the Local Torrential Rains of Beijing

Using data from wind profile radar and automatic surface weather observation stations, and NCEP 1°×1° analysis data, a comparative analysis of two cases of low-level easterly winds (LLEWs) associated with two local rainstorms in Beijing was performed. The investigation concentrated on the formation mechanism, the features of the vertical structure and thermodynamics, and the role in local rainstorms. The results showed: 1) a shallow low-level wind (SLLEW) existed below 500 m with a ground wind speed of about 1 m/s, horizontal scale of about 250 km, and time scale of 12 h, and a deep low-level wind (DLLEW) existed below 3000 m with a ground wind speed of about 4 m/s, horizontal scale greater than 600 km, and time scale longer than 24 h. 2) The SLLEW was caused by a sub-synoptic-scale warm vortex generated in the boundary layer, whereas the DLLEW was caused by a synoptic-scale inverted surface warm trough. 3)Both the SLLEW and DLLEW were accompanied by high equivalent potential temperature, and their sources were warm moist air masses located east or southeast of Beijing. There was a cold, dry air intrusion over the LLEWs that could result in convective instability. The SLLEW supported a significant local concentration of air with high equivalent potential temperature, and the DLLEW contributed to the transportation of moisture and energy and the formation of stronger low-level wind shear, which was favorable to the development of severe convective storms. 4) The SLLEW was consistent with the low-level convergence in time and space, and interacted strongly with the upper divergence, thus acting as a triggering factor; however, the DLLEW could not act as a triggering factor owing to its inconsistency.