2007年济南“7.18”大暴雨的持续拉长状对流系统研究

A study of the permanently elongated convective system during the heavy torrential rain event over Jinan on 18 July 2007

利用常规观测、中尺度自动站、卫星和雷达等资料,分析了2007年7月18—19日造成华北南部强降水的持续拉长状对流系统(Permanent elongated convective system,简称PECS)的发生和发展过程。结果表明:持续拉长状对流系统的发生发展与低层来自东北方向冷空气和正在减弱的中尺度对流复合体尾部的向外气流有关。云图分析表明:这次持续拉长状对流系统先后由10个对流单体复合而成,其中2个α中尺度的对流单体复合时造成了济南市北部的商河县强降水,并且强降水发生在持续拉长状对流系统发展阶段,位于持续拉长状对流系统的后部,与低于201K的冷云盖相对应;当对流单体复合时,在可见光云图上出现云线(即飑锋),并且在窄小的云线中又包含4个γ中尺度的对流单体,其中一个产生高达13 km的降水回波;当云线出现时,地面冷锋后部出现γ中尺度雷暴高压,随后逐渐发展成为α中尺度的雷暴群,降水产生在雷暴高压前和冷锋后。济南市强降水过程则是由合并后的对流单体内部的2个强度不同的上冲云顶合并成一个更强的上冲云顶时造成的。雷达回波分析进一步显示,这次持续拉长状对流系统降水回波表现出后部扩建类和回波交叉类的特征,其中商河县的降水由主冷空气涌产生,而济南市强降水可能与主冷空气涌与次冷空气涌合并所形成的2个β中尺度和1个γ中尺度的多单体雷暴有关。

Using the conventional observations,the satellite infrared and visible cloud images,the Doppler weather radar data, and the meso-scale automatic weather station data in the Jinan city,we investigate the formation and development processes of a permanently elongated convective system（PECS）that leads to severe rainfall over the southern part of north China on 18-19 July 2007.The results indicate that the formation and development of the PECS was associated with the intrusion of the lowlevel cold air mass from northeast China and the outward flows from the rear of the weakening meso-scale convective complex. The invaded cold air mass behind a cold front and the southwesterly warm/wet flow in front of the cold front strengthened the convergence of low-level wind over the southern part and triggered the local frontogenesis.The interactions of the frontogenesis with the outward flow from a weakened MCC produced the PECS.The analysis of cloud images shows that the PECS was a meso-α-scale convective system with a long life,persisting for 18 h from its formation to extinction.This PECS was successively composed of 10 convection cells,in which a combination of twoα-scale convective systems resulted in the severe rainfall at the Shanghe county in the north of the Jinan city.The severe rainfall occurred at the developing stage of the PECS and in the rear of PECS,corresponding to a cold cloud lid with a temperature of lower than 201 K.At the same time,a narrow cloud line （also called the squall front）that contained 4γ-scale convective cells was seen on visible cloud images.Of them,one cell produced the radar echo up to the height of 13 km above the ground surface.The PECS is accompanied by the formation and development of the meso-scale systems associated with the squall that contain a thunderstorm high,a wake low,and a squall front. A meso-γ-scale thunderstorm high occurred behind the cold front with the emergence of the cloud line and then developed into the meso-α-scale thunderstorm complex,with rainfall appearing between the cold front and the thunderstorm high.The severe rainfall at the Shanghe county began with the emergence of the thunderstorm high and terminated with the emergence of the wake low.Differing from the severe rainfall in the Shanghe county,the severe rainfall in the Jinan city occurred when two over- shooting cloud tops in a meso-scale cloud cluster merged into one stronger overshooting cloud top.The rainy echo associated with the development of the PECS showed the features of rear extending and intersecting.The Shanghe heavy rainfall was produced by a main cold intrusion associated with thunderstorm high,while the Jinan heavy rainfall was possibly with the formation and development of twoβ-scale convective cells and oneγ-scale convective cell in a multi-cell thunderstorm that was produced by the combination of a main outflow surge and a sub-outflow surge.