梅雨锋暴雨中尺度对流系统的组织特征和触发条件分析

利用常规气象观测、地面加密自动站和多普勒天气雷达资料,结合WRF(Weather Research and Forecast)模式模拟资料,对2015年6月26—28日长江中下游的一次梅雨锋暴雨过程中尺度对流系统(Mesoscale Convective System,MCS)的组织特征和对流触发条件进行分析。结果表明:1)暴雨过程线状MCS在发展初期表现为东西向雨带不断的"后部建立"以及随后对流单体的"列车效应";在发展成熟期,对流单体向东北—西南向发展,形成多个近乎平行的东北—西南向短雨带。呈现2种尺度的对流组织方式:新生对流单体沿着单个雨带向东北方向的"列车效应"和短雨带沿着线状M CS向东平流的"列车带"效应。2)低空急流的持续加强为对流的发生发展提供了条件性不稳定和对流有效位能,偏南暖湿气流在向东北推进的过程中,在风速辐合处被强迫抬升至自由对流高度,释放不稳定能量,触发对流。3)对流雨带内近地面向南的冷出流与低层西南暖湿气流的持续交汇和相互作用有利于新单体生成发展,使雨带得以维持。

Analysis of organization modes and initiation conditions of a heavy-rain-producing mesoscale convective system along a Meiyu front

A Meiyu front torrential rain event occurred over the middle and lower reaches of the Yangtze River on 26-28 June 2015.The organization modes and initiation conditions of Mesoscale Convective Systems(MCS) are analyzed by using the conventional meteorological observation data,the dense surface automatic meteorological stations data,Doppler weather radar data and the simulated data from WRF(Weather Research and Forecast) model.The results are as follows:(1)many types of organization modes are found in the development of the linear MCS.Constantly back building of the east-west oriented rainbands and subsequent echo-training of convective cells are the main development modes of the MCS in the initial stage.The convection cells develop in the direction of NE-SW and form several short parallel convective rainbands in a nearly NE-SW direction in the mature stage.Two scales of convective organization modes are found:one is northeastward echo-training of new convective cells along individual rainbands,and the other is eastward band-training of the NE-SW oriented short rainbands along the linear MCS.(2)The continuous enhancement of low-level jet provides the conditional instability and convective available potential energy for the generation and development of convections.The southerly warm and moist air is lifted to the level of free convection,releasing the unstable energy and triggering convection at the convergence of wind speed.(3)The consistent interactions between the cold outflow of convective cells and the low-level southwest warm and moist air are favorable for the generation and development of new cells,maintaining the rainbands.