中文摘要：

2012年7月21日10时至22日02时，北京经历了自1963年8月8—9日极端降水事件以来最强的一次降水过程，导致人民生命和财产重大损失。文章对此次极端降水过程进行了详细分析和探讨，主要结论是：（1）高空低槽伴随地面冷锋东移，在华北遇到副热带高压和山西地形阻挡移动缓慢；另外2012年第8号台风韦森特登陆前，台风低压和副热带高压之间形成强气压梯度，导致通向华北地区的东南风／南风低空急流建立并加强，为华北地区输送了充分的水汽，为北京特大暴雨的发生提供了极为关键的条件。（2）导致北京极端暴雨的中尺度对流系统MCS起源于河套地区低层涡旋的发展。河套地区在7月20日20时左右类似热带气旋形态的α中尺度涡旋的形成有可能与涡旋自组织机制有关，而上述MCS系统是20日20时河套地区类似热带气旋形态的α中尺度涡旋的主要降水部分，该MCS系统从形成到消散历经44小时，其超长的生命史的主要原因包括：（a）其始终具有的明显的正垂直螺旋度（由正的垂直涡度和垂直上升气流组合而成）阻止了其动能向较小尺度串级输送；（b）低层暖湿平流、对流云团云顶辐射降温、下游地区正的对流有效位能这些使对流维持和加强的因素强过由对流垂直混合和非绝热加热等导致的对流消散的因素，或二者处于大致动态平衡的状态；（c）该MCS位于地面冷锋之前的暖区和地面的低压槽内，也始终位于500hPa低槽前的正涡度平流区，那里盛行天气尺度上升气流。（3）21日北京及周边环境非常有利于大暴雨发生，包括500hPa明显的正涡度平流、1000-2000J·kg^-1的CAPE值、深厚的湿层、强的低空急流、高的地面露点温度和异常大的可降水量。（4）21日08～20时，MCS主轴的走向与太行山和风暴承载层平均风（西南偏南风）大致平行，加上东部副热带高?

英文摘要：

On 21 July 2012 Beijing experienced the most severe rainfall event since August 1963. The ex- treme rainfall induced flooding killed over 100 people and the property damage is over 11.64 billion RMB yuan （about 2 billion U. S. dollars）. Based on the routine upper-level and surface observation, the satellite and radar data, a detailed analysis and investigation have been done on this event. The major results are as following： （1） The upper-level trough accompanied by surface cold front moving toward east blocked by the subtropical high provides favorable synoptic scale conditions for torrential rain event in Beijing area. （2） The presence of a tropical cyclone over the South China Sea near the coastline led to the establishment and enhancement of southeastward and southward low-level jet toward Beijing area, providing plenty of water vapor to Beijing area. （3） The development of Hetao vertex on 20 July led to the formation of a me- so-a scale MCS over that area, and its high value of vertical helicity made it well organized and longlived. This MCS moved with the upper-level trough eastward, and was over Huabei region （including Beijing are- a） on the second day （21 July）, producing extreme rainfall over Beijing area. （4） The south-east low-level jet constantly triggered new convective cells on the east slope of the Taihang Mountain Range, then moved to northeast direction into Beijing area, leading to the torrential rainfall and severe flooding there.