中文摘要：

利用S波段测雨雷达、毫米波测云雷达、风廓线雷达等观测资料对2011年6月23日和2012年7月21日（以下简称7·21暴雨）发生在北京地区的两次中尺度对流系统（MCS）引起的大暴雨观测资料进行分析,结果表明：导致两次暴雨的回波均具有线状对流特征,其组织形态为平行层状（PS）降水中尺度对流系统;两次暴雨中PS型MCS与长江中下游地区此类MCS移动方向偏东或东北略有不同,而是向偏东或东南方移动,但7·21暴雨过程中最后阶段影响北京形成的PS型MCS却是向东北移动的.毫米波测云雷达可以观测到测站上空及周边云的形态特征及发展情况,回波强度及速度可以展现出云的演变、内部结构特征以及降水状态.而风廓线资料表明两次暴雨在风场变化中均存在着相似的特征,两次暴雨前期低层均存在东风层,这与北京特殊地形的共同作用可能是导致夏季强降水的因素之一;同时,暴雨发生之前存在对流层低层风速明显增大的过程.低层扰动的加强、风向之间的切变以及超低空急流均有可能是导致强降水的原因.

英文摘要：

By using S-band weather radar, 8.6 mm wave-length cloud radar and wind proifle radar data, the two heavy rainfalls in June 23, 2011 (hereinafter referred to as the 6·23 rainstorm) and July 21, 2012 (hereinafter referred to as the 7·21 rainstorm), which occurred in Beijing area, are caused by a meso-scale convective system (MCS). Observation results show that the characteristics of linear convection were found in two rainstorms, and its organization mode was classiifed as parallel stratiform MCS (PS). Such MCSs in the middle and lower reaches of the Yangtze River move to the east or northeast, but these two MCSs move to the east or southeast. However, the PS which affect Beijing during the ifnal stages is moving to the northeast. Millimeter wave cloud radar could observe the morphological characteristics and development of the cloud over the station. The intensity and velocity of the echo show the evolution, internal structure of the cloud and precipitation state. The wind proifler data indicate that similar changes of the wind ifeld existed in two rainstorms. The interaction of the east wind in the boundary layer during its early stages and the special terrain might be one of the factors leading to the heavy rain in that summer, Beijing. Meanwhile, the velocity in low-troposphere increases signiifcantly before rainstorm. The strengthening of low-level disturbances, wind direction shear and the ultra low-level jet are likely to be the causes of heavy precipitation.