2002年南海季风建立及其雨带变化的天气学研究

利用南海海 气通量观测试验资料结合NCEP ,GPCP以及GMS - 5云图资料 ,综合分析了 2 0 0 2年 5～ 6月南海西南季风建立过程及其雨带变化 ,确定 5月 14日西沙及北部海区西南季风爆发 ,5月 15日整个南海季风爆发 ,季风爆发时间属于正常年 ;季风爆发时风向、风速、云量、降水、湿度、辐射及海面温度等要素都发生突变。这种突变是由大气环流的突变造成的。季风爆发前后大气环流变化过程是 :80～ 90°E越赤道气流加强 ,同时印缅低压加深 ,孟加拉湾南北向气压梯度增大 ,而后东亚大陆上气旋发展东移 ,副热带高压东撤 ,孟加拉湾低压槽前的赤道西风突然加强越过中南半岛 ,南海北部首先出现强西南风 ,继而南海季风迅速地全面爆发。孟加拉湾西南风加强到南海季风爆发是一个连续的过程 ,大陆冷空气南下起了重要的作用。南海季风爆发时呈现单雨带型 ,而后由单雨带型转变为双雨带型 ,雨带受副热带高压和季风系统共同影响 ,并且随着副热带高压移动位置变化。

A SYNOPTIC STUDY ON ESTABLISHMENT OF THE MONSOON AND ASSOCIATED VARIATION OF RAIN BELT OVER THE SOUTH CHINA SEA IN 2002

Based on air-sea flux in-situ observation data in South China Sea (SCS), NCEP reanalysis dataset,GPCP, OLR and GMS-5 cloud image, this study synthetically analyzed establishment process of SCS southwest monsoon and related changes of rain belt in May and June 2002. Evidence shows that May 14 is the onset date of Xisha and north SCS, and May 15 for the whole SCS monsoon as a normal year. Remarkable changes of variables, such as wind direction, wind speed, cloud, precipitation, humidity, moisture flux, radiation and seasurface state, et al, were observed during the period of the monsoon onset. In the radiation components, variations in short-wave radiation and net radiation budget of ocean are especially significant. Abrupt variations in the lower and upper-level air structure are responsible to large-scale adjustment of the general circulation. The physical evolution of general circulation prior to and after the onset are as follow: firstly,cros-sequatorial flow in 80-90°E intensified, so did India-Burma low and air pressure gradient in Bay of Bengal, then the cyclone over East Asian developed and moved eastward, accompanied by the withdrawal of subtropical high. At some time equatorial westerly associated with the low pressure trough in Bay of Bengal enhanced and crossed Indochina Peninsula, leading to the buildup of strong southwesterly in north SCS, then finally SCS monsoon commenced in whole SCS. It is a cont inuous process from the intensificaion of southwesterly in Bay of Bengal to the onset of SCS monsoon. The southw ard movement of continental cold front plays an important role in triggering the onset of SCS monsoon. The earlier onset of the SCS monsoon in the northern part is due to the interaction of mid-latitude and low-latitude systems.At the time of SCS monsoon onset, it appeared single rain belt in the northern part of SCS, and then transited from single to double ones. The belts were affected by subtropical high and monsoon system, often following the movement of subtropical high. Onset periods of the monsoon (east ward withdrawal subtropical high) were associated with substantial rainfall over central and northern part of SCS. Break periods of the monsoon (westward shift of subtropical high) wererelated with minimal rainfall over northern part of SCS but substantial rainfall over the Bay of Bengal, Indian and Yangtze River basin. There were two different kinds of precipit ation patterns. Their variation period was about 30 days, which may reflect the intraseasonal oscillations of 30-40 days time scale.