中文摘要：

采用1954—2010年NCEP/NCAR逐日再分析资料和中国753站逐日站点降水资料,定义了华北雨季的开始日和结束日,发现华北夏季存在单峰和双峰两种降水过程,选取典型单峰和双峰降水过程,对比分析这两种降水过程的特征和机理。结果表明：1）在华北雨季期间,单峰降水过程中的副高脊线没有中断的现象,而双峰降水过程中的降水中断现象是因副高的东退南压所致。2）华北夏季单峰降水年的季风中断现象不明显,季风的影响能一直持续至雨季结束,而双峰降水年的季风加强（减弱中断）对应着降水峰值（中断）。3）滤波后的华北夏季降水时间变化显示,当30-60 d和7-14 d振荡的波峰相对时,则出现降水峰值;当30-60 d振荡的波谷和7-14 d振荡的波峰相对时,则出现降水中断。4）华北夏季双峰降水年的低频纬向风传播与双峰降水的时间演变具有较好的对应关系。

英文摘要：

Based on the NCEP/NCAR daily reanalysis data during 1954-2010 and the daily precipitation data at 753 stations in China,the beginning and end dates of rainy season over North China are defined.Analysis shows that there are two kinds of precipitation processes, one is unimodal precipitation process, the other is bimodal precipitation process.Meanwhile,the paper selects the two kinds of typical precipitation processes and studies their characteristics and mechanisms.Results show that： 1 ）During the rainy season of North China, the ridge of subtropical high are not interrupted during the unimodal precipitation process, but there is precipitation break phenomenon during the bimodal precipitation process, which is caused by the eastward retreat and southward downshift of subtropical high.2）In the unimodal precipitation years, the phenomenon of monsoon break is not obvious and the influence of monsoon can continue to the end of the rainy season. However, in the bimodal precipitation years, the strengthening （weakening） of monsoon corresponds to the peak（break） of precipitation.3 ）Temporal variation of the filtered precipitation in North China shows that a precipitation peak will occur when there is a good corresponding relation between the peaks of 30-60 d and 7-14 d oscillations, and a rainfall break will happen when there is a good corresponding relation between the trough of 30-60 d oscillation and the peak of 7-14 d oscillation.4）The evolution of bimodal precipitation in North China obviously has a close relation to the low frequency oscillation of zonal wind in summer.