广东高空槽后暴雨的多尺度天气特征及概念模型

2010年5月6—7日广东出现了罕见的高空槽后大暴雨过程。利用华南区域气象观测站、加密自动站和NCEP分析资料等,从大尺度环流背景、天气系统特征和雷达回波特征等方面进行分析,并利用局地经向环流模式和尺度分析技术诊断探讨其可能形成机制。结果表明,南亚高压西移配合高空急流北退为大暴雨的发生提供高层辐散条件。在前倾槽形势下(500 hPa高空槽移出广东、低层切变线和地面冷锋仍在南岭附近),低层暖湿不稳定配合高空槽后干冷空气的下传,对广东上空不稳定能量释放起到增强作用,强降水主要发生在锋区南侧。数值诊断结果进一步表明,激发暴雨过程的主要物理因子为潜热加热和温度平流。潜热加热是暴雨的正贡献和反馈最直接因子,而北方冷空气一方面通过温度平流直接激发广东地区的上升运动,同时通过促进水汽辐合抬升区南移并释放潜热,是暴雨过程的重要触发机制。此外,空间滤波分析表明,弱冷空气越过南岭后扩散南下和形成的中小尺度天气系统对强降水落区和增幅起关键作用,暴雨业务预报对此类弱冷空气在地形作用下扩散南下的影响需要更加关注。

THE STUDY OF MULTI-SCALE SYSTEM AND CONCEPTUAL MODEL OF THE TORRENTIAL RAIN UNDER THE FORWARD-TILTING TROUGH

Guangdong suffered a rarely observed torrential rain on 7 May,2010,which occurred behind an upper-level trough.Based on meteorological observation data and NCEP analysis data,the characteristics of large-and synoptic-scale circulation and the mechanisms of the unusual torrential rain are analyzed with a linear,diagnostic model for local meridional circulation and a scale separation method.The results show that the westward movement of South Aisa High with the northward movement of an upper-level westerly jet was in favor of the upper-level divergence over Guangdong.Guangdong was controlled by the northwest airflow in the rear of the upper-level trough at 500 hPa and a shear line and a surface cold front were to the north of Nanling Mountains.Under this forward-tilting trough circumstance,it would probably reinforce the release of unstable energy.Numerical quantitative diagnosis shows that the contributors to the local meridional circulation associated with the torrential rain are mainly latent heating and horizontal temperature advection.Latent heating is the major contributor and provides positive feedback to the torrential rain,while horizontal temperature advection plays an important role in triggering the torrential rain.Further analysis with the scale separation method shows that the impact of weak cold air that crosses Nanling Mountains is important to the location and enhancement of the torrential rain.Therefore,more attention needs to be paid to activities of the weak cold air and the topographic effects in forecasting the torrential rains during the annually first rainy season of South China.