Characteristic Analysis Of Summer Unimodal And Bimodal Precipitation Process In North China

Based on the NCEP/NCAR daily reanalysis data during1954-2010and the daily precipitation data from the China Meteorological Administration, beginning and end date of rainy season over North China is defined. By defining, there are two kinds of precipitation process be found, one is unimodal precipitation process, another is bimodal precipitation process. By analysis of the characteristics on unimodal and bimodal precipitation, the main conclusions are as follows:(1) that rainy season in North China starts in the38th pentad and ends in the45th pentad from climatology aspect. It has an average duration of7pentads, the longest duration is12pentads and the shortest one is empty flood season. The start time difference of rainy season is significant, and the first appeared in mid-June, the latest in mid-August. For end time, the earliest time appeared in mid-July, the latest in the first10days in July. Though linear-trend evaluation, the variations trend of start and end date over North China are analyzed. The beginning of rainy season in North China has a delaying trend while the ending shows a trend of ahead over the past57years.(2) By comparative analysis of the characteristics on unimodal and bimodal precipitation over North China, we discussed that rain belt located in the Yangtze-Huaihe valley before the rainy season, during the rainy season, the rain belt reach the North China, and then the rainy season return to the Yangtze-Huaihe valley and the South China region in unimodal precipitation. But in bimodal precipitation, the rain band activities vary. When the rain belt reach the North China, there will be a precipitation interrupt and then the rain belt is located in the northeastern and the South China. At that point, In the middle troposphere, the subtropical high eastward retreat, the trough in the east of it and near of Lake Baikal and Indian Peninsula high were weakened, which all destroyed the circulation Situation of heavy rain in North China. In the high troposphere, Divergence characteristics have good correlationship with precipitation in North China. When precipitation interrupted, the large divergence center is located in the northeast and southern China.(3) The rainfall break during the bimodal precipitation process maybe related to the eastward retreat and southward downshift of the subtropical high in summer. Meanwhile, the strengthening of the monsoon corresponding to the peak of precipitation and the monsoon weakening lead to the precipitation interrupt. Besides, based on the series of precipitation after filtering in North China, a peak of precipitation occurs over the correspondence between the peak of30-60days and the peak of7-14days oscillation; and a rainfall break will happen when it’s correspondence between the trough of30-60days oscillation and the peak of7-14days oscillation. Furthermore, bimodal precipitation in North China obviously connects with low frequency oscillation of west wind.