用双多普勒雷达分析华南一次飑线系统的中尺度结构特征

飑线是中国春、夏季常见的一种天气现象.2007年4月23-24日华南地区的一次强飑线过程,因强度强、持续时间长,给广东部分县市造成巨大的经济损失.利用广州和深圳的新一代多普勒天气雷达的探测资料,采用双多普勒雷达风场反演方法和热力动力反演方法,研究此次飑线内部的三维风场、动力和热力结构.在此基础上,通过质量、动量诊断探讨其维持机制.同时,结合地面、高空常规观测及自动站资料分析系统伴随的环境和地面中尺度特征.结果表明,系统发生于中等强度垂直风切变(0-3 km,18 m/s)和中等强度对流不稳定度的环境中,其移动速度最快达17 m/s,属快速移动的飑线.飑线过境时,地面测站呈现风向突变、风速骤增、气压上升、温度与露点骤降等现象.其成熟期的雷达回波和风场结构与过去观测的热带、副热带快速移动飑线系统特征相似.由前(前方指系统移动方向)向后,降水结构依次为对流区、过渡带和层云区.其中,对流区由多个对流单体组成,对应强回波和上升运动,而单体之间为弱下沉运动.气流特征呈准二维结构,包括系统前方深厚的从前向后气流和系统后部低层的从后向前气流.这两支气流在系统前缘低层辐合形成动力高压,触发新对流单体,是系统长时间维持的主要机制.在对流区倾斜上升气流的下方,存在一凝结潜热释放增暖引起的低压区.另外,在对流系统西侧不断有新对流单体形成,并被平流至系统中,促进对流维持.进一步对对流区的质量和动量通量诊断揭示,垂直于飑线系统的水平动量的垂直输送有逆梯度输送的特性,将增加垂直飑线方向的环境垂直风切变,有利于系统维持.相对的,平行飑线方向的水平动量具有顺梯度输送特征,使该方向上风切变分量有被均滑趋势.

Dual-Doppler analysis of a squall line in southern China

In this study, the kinematic and thermodynamic structure of a subtropical squall line on 24 April 2007 is investigated using the Dual-Doppler Radar data collected from the South China Heavy Rainfall Field Experiment (SCHeREX). The synoptic environment of the squall line was characterized of a moderate CAPE and strong vertical shear (0-3 km, 18 m/s). The results show that the squall line has a nearly east-west orientation, and propagates south-eastward rapidly with the peak speed of about 17 m/s. The kinematic, dynamic and thermodynamic structures of the squall line at the mature stage are similar to those of the previously observed fast moving tropical and subtropical squall lines. The flow structure is almost two-dimensional, including a deep storm relative front to rear flow on the front of the squall line, and a shallow rear to front flow on the back. From front to rear, the features in precipitation structure include the intense convective band, the transition zone, and the trailing stratiform region. The convective region is composed of many individual cells characterized by intense updrafts and high reflectivity values, with relatively weak convective downdrafts between and behind the cells. The convective updrafts are warmed by the release of latent heat, while cooling prevails in the convective downdraft. In the low level, the cold rear to front flow collides with the advancing environmental warm air at the leading edge to form a dynamic high pressure, which in turn forces the formation of new cells. A buoyancy-induced low-pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. It is also noted that the new convective cells successively form and move into the squall line from its west side, which is favorable to the maintenance of the squall line. The mass and momentum budget calculations in the convective region reveal that the vertical transportation of the horizontal momentum flux perpendicular to the squall line is of counter gradient while the momentum flux parallel to the line is of down gradient.