气象2012，Vol.38Issue(10)：1267-1277,11.
北京721特大暴雨极端性分析及思考（二）极端性降水成因初探及思考
Analysis and Thinking on the Extremes of the 21 July 2012 Torrential Rain in Beijing PartⅡ: Preliminary Causation Analysis and Thinking
摘要
Abstract
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.关键词
极端强降水/雨强/降水持续时间/整层可降水量/MCSs/后向传播/列车效应/低涡切变线Key words
extreme torrential rain/rainfall intensity/duration of precipitation/whole atmospheric precipi-table water/MCS/backward propagation/train effect/low vortex shear line分类
天文与地球科学引用本文复制引用
孙军,谌芸,杨舒楠,代刊,陈涛,姚蓉,徐珺..北京721特大暴雨极端性分析及思考（二）极端性降水成因初探及思考[J].气象,2012,38(10):1267-1277,11.基金项目
国家自然科学基金面上项目（41175048）及公益性行业（气象）科研专项（GYHY201206004、GYHY201006005和GYH200906001）共同资助 致谢：感谢数值预报中心陈静提供了有关集合预报极端天气预报指数方面的资料. （）
