This paper is the second part o~ "Analysis and thinking on the extremes of the 21 July 2012 torrential rain in Beijing". The first part o~ "Observation Analysis and Thinking" has analyzed the precipitation features, water vapor conditions, environmental conditions, generation and development of the mesoscale convective systems (MCSs) during the process of the severe torrential rain. As the second part of the series studies, this paper mainly focused on the impact factors of the extreme rainfall, i. e. , precipitation efficiency, water vapor, ascending motion and duration and so on, to explore the causes of the extreme torrential rain by using various observation and model data that are often used in operations. The results showed that this extreme severe rainfall event happened under a typical circulation pattern of North China torrential rains. The interaction of high-level wind divergence with the low-level vortex and wind shear directly triggered the extreme rain event. The comparatively high relative humidity and thick humid layer,low lifting condensation height and free convection height as well as the characteristcs of tropical torrential rains improved the precipitation efficiency of this severely heavy rain process. The extreme high water vapor content (with the atmospheric precipitable water vapor getting to 60--80 mm) and the abnormity of associated physical quantity can be judged as the extreme severe precipitation event. The middle- and low-level troposphere was instable conditionally, while the upper-level had the characteristics of humid neutral stratification. With a medium CAPE value and convective instability enhanced by upper-level dry intrusion, the upward motion was intensified. MCS was generated and developed in the warm areas due to the combined actions of vortex wind shear and the terrain in the North China region. The northward jump of vortex and the backward propagation of MCS made the warm-area MCS more slowly eastward forming the "train effect" and resulting in the long-lasting precipitation. Finally, this paper discussed the methods of forecasting extremely severe rainfalls.%本文是“北京7.21特大暴雨极端性分析及思考”的第二部分，第一部分“观测分析及思考”对此次过程的降水特点、水汽特点、中尺度对流系统（MCS）的环境场条件和发生发展过程进行了分析，指出这是一次极端降水过程。本文进一步从影响降水的因子：降水效率、水汽、上升运动、持续时间等方面进一步探讨极端性降水的成因，所用资料为业务中常用的模式分析和各种观测资料。分析表明，本次过程为典型华北暴雨环流形势，其中高层气流辐散区与低层低涡切变线的耦合是直接诱因；较高的环境相对湿度和湿层较厚，较低的抬升凝结高度和自由对流高度以及热带降水性质提高了本次过程的降水效率；异常大的水汽含量（可降水量达60－80mm）及与其相关的物理量异常，可作为判断极端降水的重要因子；环境大气具有中下层条件性不稳定，上层湿中性层结特性，CAPE值中等，同时上层干侵入增加了对流不稳定，有利于上升运动发展；低涡切变线及华北地形共同触发了MCS的在暖区生成发展；低涡北跳、MCS后向传播特性使暖区MCS东移速度慢，形成“列车效应”，造成降雨持续时间长。本文最后探讨了极端降水的预报思路。
展开▼
