北京“7.21”暴雨雨团的发生和传播机理

基于京津冀5部新一代天气雷达、区域5min自动站和中尺度数值模式模拟资料,通过雷达资料快速更新循环四维变分同化技术和三维数值云模式对低层三维动力和热动力特征的模拟分析,为北京"7.21"特大暴雨中尺度对流系统(Mesoscale Convective System,MCS)的结构特征和传播机理的分析提供了佐证。结果表明:(1)低层动力场和地形强迫对MCS的触发、增强和维持起到关键作用。在MCS形成阶段,地形强迫有利于低空偏南气流带来的暖湿空气在山前的辐合上升。随着MCS的加强,强降水区域呈现与地形走向接近的"西南—东北"向带状分布,单体移动具有明显的"列车效应",而MCS整体则向东偏南缓慢传播。在MCS传播前沿(山前)形成强的出流风场,低层2 km以下均为深厚的辐合上升区且进一步加强,表明地形强迫和低层风辐合对偏南暖湿空气抬升起到重要作用,有利于MCS长时间"列车效应"的维持和MCS的发展。MCS出流风场与平原地区近地面偏南风交汇,使得在距MCS传播前沿约50 km的、已经存在的一个接近"西西南—东东北"走向的出流边界明显增强。在MCS传播前沿存在较为明显的0-3 km风垂直切变,由MCS出流与低层偏南风形成的风向切变以及地形强迫造成的风速切变构成,切变区域与地形走向及MCS伸展方向密切相关,切变强度达到中纬度低层强切变阈值范围。低层风垂直切变与MCS存在明显的正反馈效应,亦有利于MCS的长时间发展和维持。(2)低层热动力场为MCS的发展、传播提供了重要条件。在MCS传播前方的环境低层是明显的暖湿区,而在传播后方的低层则是由于地面冷锋及MCS降水造成的冷区,冷暖空气交汇对MCS的高度组织化和强降水的持续起到重要作用。低层的热力层结不稳定区域主要分布在MCS的南部到西南部地区,为MCS尾部风暴单体的不断新生和移动传播创造了良好的热力条件。最后,通过观测和模拟结果综合分析,初步得出了与此次强降水MCS发展演变密切相关的低层热、动力配置的概念模型,为MCS"列车效应"和后向传播特征的机制分析提供了依据。

Initiation and propagation mechanism for the Beijing extreme heavy rainstorm clusters on 21 July 2012

Simulation and analysis of low-level 3D dynamical and thermo-dynamical characteristics of the extreme-rain-associated mesoscale connective system (MCS) occurred in Beijing on 21 July 2012 were performed using the radar data 4D Var and 3D numerical cloud model under rapid refresh cycling mode, with the data from five CINRAD radars,auto weather stations with 5min observational interval in the Beijing-Tianjin-Hebci region,and the a mesoscale numerical model,which offer analysis evidenec for attributes and the propagation mechanism for the MCS.The simulation results arc as follows:(1)The low-level dynamical fields have high impact on the initiation,intensification and evolution of the extrem-rain-associated MCS.During the MCS initiation period,low-level convergence and updraft over the foothills arc very obvious due to the terrain forcing to the low-level southerly wind,which is very advantageous to converge and raise warm,moist air brought by the low-level southerly airflow.During the MCS intensification period,the extreme rain extent shows oriented southwest-northeast belt shape that is parallel to the orientation of the mountainous terrain,and storm cells embedded in the MCS show distinct training connective clements while the MCS propagates cast-southeastward.A strong outflow is formed in the front of the MCS along the propagation direction,and remarkable and enhancing low-level convergence and updraft arc in existence up to 2 km altitude over the foothills,which indicate that terrain forcing and low-level wind convergence play significant roles in raising southerly warm and moist airflow for keeping the durative echo training and promoting the MCS development.The interaction between the MCS outflow and southerly wind near the surface over the plains has a high effect on intensifying a pre-xistcnt outflow boundary that is near oriented west-southwest-cast-northeast and about 50 km apart from the MCS propagation frontage.The 0-3 km vertical wind shear is remarkable in the front of the MCS, which is comprised of both directional shear produced by the MCS outflow and the low-level southerly wind,and the speed shear resulted from the variation of low-level wind speed with height due to terrain forcing and blocking.The strong shear extent is high correlated to the orientation of terrain and the MCS range, and the shear intensity is corresponding to the low-level strong shear threshold in the midlatitudes.The significant positive feedback effect between the low-level vertical wind shear and the MCS evolution is very propitious to the development and maintenanec of the long-lived MCS.(2) The low-level thermo-dynamical fields provide important conditions for the development and propagation of the MCS.Low-level warm and moist environment is remarkable in the front of the MCS propagation while distinet cold environment behind the MCS propagation which results from the cold front and precipitation.The interaction between the clod and warm air has significant influence on high organization of the MCS and the persistence of the extreme rain.The arcas of low-level thermal stratification instability mainly distribute south and southwest of the MCS activity area, which supplies advantageous thermal conditions for the regeneration and movement of connective cells in the rear of the MCS.Finally, a conceptual model of the low-level dynamical and thermo-dynamical collocation that significantly affects the development and evolution of the extrem-rain-associated MCS is derived from composite analysis of the simulation results and observational data, which provides a basis of mechanism analysis of the MCS echotraining and back-propagating.