中文摘要：

2012年7月21—22日北京地区发生了1951年以来最严重的特大暴雨,北京地区许多台站日降水量创下1951年以来的历史新纪录。利用NCEP/NCAR再分析资料以及台站降水资料,对引起此次特大暴雨的大尺度条件进行分析,重点研究＂7·21＂大暴雨与赤道辐合带、亚洲夏季风以及菲律宾-南海洋面上热带气旋之间的密切关系。研究表明亚洲夏季风的异常北进作为暖湿输送带所携带的大量偏南暖湿空气为这次特大暴雨的发生提供了充沛的水汽条件,它与同时来自北方低槽冷涡的高空强冷平流相遇,形成了明显的不稳定层结。没有这种强烈的水汽输送,仅靠来自中高纬度南下的冷空气作用,这次暴雨不会达到破纪录的强度。进一步研究表明,季风暖湿输送带的北推与亚洲季风30~60天季节内振荡的活跃期密切相关。在这个时期产生的季风涌触发和维持了季风暖湿带的迅速向北推进过程。另外,通过与另一次严重影响华北地区的1963年8月上旬的＂63·8＂大暴雨的大尺度环流条件的比较发现,这次暴雨过程中也观测到明显的季风暖湿输送带,正是由于它与北方冷空气的强烈相互作用,从而也造成了破纪录的大暴雨发生。因此,强的季风暖湿输送带的北进可以说是北方特大暴雨发生的必要条件。

英文摘要：

On 21 to 22 July 2012 a heavy rainstorm hit Beijing, which was regarded as the most severe rainstorm since 1951. The daily precipitation amount in many stations in Beijing has broken the historical record. Based on the NCEP/NCAR reanalysis data and precipitation observation, large-scale conditions which caused the heavy rainstorm are investigated, with emphasis on the relationship between this heavy rainstorm and equatorial convergence zones, Asian summer monsoon as well as tropical cyclones over the ocean from the Philippines to the South China Sea. The results indicated that a great number of southerly warm and wet air accompanied by northward movement of the Asian summer monsoon provided plenty of moisture for the heavy rainstorm. When the warm and wet moisture met with the strong cold temperature advection induced by the cold trough or vortex, an obvious unstable stratification formed. Without this kind of strong moisture transport, the rainstorm only relying on the role of the cold air from mid- and higher-latitudes could not reach the record-breaking intensity. Further research suggested that the northward movement of monsoonal warm and wet moisture transport was closely related with the active phaseof 30-60 day intra-seasonal oscillation of the Asian summer monsoon. During this time, the monsoon surge triggered and maintained the northward movement of monsoonal warm and wet transport belt. In addition, compared with another heavy rainstorm which occurred in the mid-August in 1963（and seriously affected North China）, it is revealed that a monsoonal warm and wet transport belt was also observed, which interacted with the strong cold air from north, thus resulting in the record-breaking heavy rainstorm.