华南前汛期的锋面降水和夏季风降水 I.划分日期的确定

前汛期暴雨常常引发华南地区的洪涝,但是前汛期降水的预报能力却相当低。降水的预报在很大程度上依赖于对降水性质的理解,而华南前汛期降水通常被认为只是锋面性质的降水。事实上,南海夏季风在6月(甚至5月)就可以影响到华南地区并产生季风对流降水。因此,华南前汛期包含了两种不同性质的降水,即锋面降水和夏季风降水,如何区分它们是非常重要的。为了区分它们,利用NCEP/NCAR再分析资料、CMAP资料和中国730站降水资料,分析气候平均(1971~2000年)状态下锋面降水和季风降水期间大气性质和特征的差异,得到华南前汛期夏季风降水开始的基本判据:100 hPa纬向风由西风转为东风并维持5天以上。利用该判据得出气候平均条件下的华南夏季风降水开始于5月24日,并得到1951~2004年逐年华南前汛期锋面降水和季风降水的划分日期。合成分析的结果表明,得到的划分日期是基本合理的,因为它将锋面降水和季风降水期间大气特点的显著差别区分开来。

Frontal Rain and Summer Monsoon Rain During Pre-rainy Season in South China.Part I: Determination of the Division Dates

Heavy rain during pre-rainy season usually causes a flood in South China,but the prediction for the rain during pre-rainy season is still a challenge for meteorologists.To a large degree,the rain prediction depends on understanding of its characteristics.Generally,the rain during pre-rainy season is considered as frontal rain.But,the South China Sea summer monsoon(SCSSM) can invade South China and dominate rain there in June,even early May.Since the characteristics of rain during pre-rainy season are so different,it is important to distinguish SCSSM rain from frontal rain.In order to distinguish them,NCEP/NCAR reanalysis data,the Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP) and data of 730 surface stations of China are used to analyze the differences of frontal rain and monsoon rain during pre-rainy season under the mean climatological condition(1971-2000),and the basic criterion of beginning of summer monsoon rain in South China is made as follows: zonal wind at 100 hPa shifts from westerly to easterly and easterly persists for exceedingly five days.An important feature of SSCSM onset is upper tropospheric zonal wind shift from westerly to easterly,corresponding the northward move of South Asia High.To make sure that the upper troposphere over South China is dominated steadily by easterly,easterly wind must persist for over five days. Accompanying the zonal wind shift,lower tropospheric temperature and moisture change abruptly.Therefore,zonal wind shift at 100 hPa can be used to determine the start of summer monsoon rain in South China,and the changes of lower tropospheric temperature and moisture are used to validate and revise the start date determined by zonal wind shift at 100 hPa.By the criterion,beginning date of summer monsoon rain in South China has been derived as 24 May under the mean climatological condition.Through the analyses,the beginning dates of that from 1951 to 2004 are obtained too.The results of composite analysis indicate that the derived dates distinguish tropospheric characters during frontal rain from that during monsoon rain.