多单体雷暴的形变与列车效应传播机制

The Configuration Change and Train Effect Mechanism of Multi-Cell Storms

本文利用变分多普勒雷达分析系统(VariationalDopplerRadarAnalysisSystem,简称VDRAS)基础上构建的四维变分(4DimensionalVariationalassimilation,简称4DVar)低层热、动力反演系统,针对发生在北京地区的几次多单体雷暴系统演变过程,研究了线性多单体对流系统传播过程中发生的形变过程、强弱变化和"列车效应"等现象的物理机制。结果表明:(1)对于雷暴单体的传播方向与雨带的移动方向基本一致的多单体雷暴系统(如飑线系统),单体的传播速度不同最终造成多单体雷暴在形态上发生变化(如由直线形回波演变为"弓"形回波等),以及雷暴单体传播过程中的强弱变化等,是雷暴单体传播过程与低层环境大气相互作用的结果:如果雷暴前端的入流本身是暖湿的,并存在较强的水汽辐合现象时,雷暴单体发展更旺盛,传播速度更快,反之则趋于减弱,传播速度减慢。因此,对飑线系统的临近预报而言,需要特别关注多单体雷暴系统传播方向与近地面层中尺度水汽通量辐合带的交叉区域,该区域的雷暴单体"移动"速度更快、发展更为强烈。(2)对于"列车效应"多单体雷暴而言,其传播环境、传播机制与上述多单体雷暴系统几乎完全不同:"列车效应"一般发生在低空暖湿气流或低空急流附近,环境大气表现为条件性静力不稳定。雷暴单体的传播机制可能是惯性重力波的激发、传播的结果,由于西南暖湿气流或急流是一支暖湿气流输送带,惯性重力波由假相当位温θse的高值区向低值区传播,重力波将从背景场中不断获得能量而发展。因此,雷暴传播过程中不断增强的现象造成波动排列的多单体雷暴形成的最大降水中心往往出现在波列的前端。

Several multi-cell storm systems occurring in the Beijing area have been investigated by using the low-level thermal and dynamical retrieval system of four-dimensional variational assimilation （4Dvar） based on the Variational Doppler Radar Analysis System （VDRAS）. This paper examines the physical mechanism of configuration processes, intensity changes, and train-effect phenomena that occur during the transmission of linear multi-cell storms. The following results are reported： （1） In a multi-cell storm system in which the transmission direction of the cells is constant with a convective band such as a squall line, the configuration changes such that linear reflectivity becomes arched, and intensity changes of the cells are related to interaction of cells’ transmitting process and environmental low-level air flow. If the frontal inflow is a warm and moist air stream, and stronger vapor convergence is present, the cell flourishes more easily and transmits faster; otherwise, the cell weakens and transmits slowly. Therefore, in squall-line system nowcasting, the intersection of the multi-cell transmitting direction and the environmental vapor convergent line should be closely followed because the cell will travel faster, and the most severe effects of convection will occur. （2） In a multi-cell system that shows characteristics of a train effect, the cell’s transmitting feature and developing environment differ almost completely from those of the multi-cell previously mentioned. Train-effect phenomena generally occur in low-level warm and moist air streams or neighboring low-level jets in unstable ambient atmospheric conditions. The transmitting mechanism of the storm cell can be connected with the propagation and stimulation of an inertial gravity wave such that the warm and moist air stream or low-level jet is a transmitting band for warm vapor advection. When inertial gravity wave propagates from high to low θse, the wave receives energy from the ambient atmosphere and develops constantly. Therefore, during storm cell transmission, cells gradually enhance and become arranged in a row to produce the torrential rain center in the leading end of the wave train.