1998年夏季长江流域梅雨期环流演变的特殊性探讨

A Study of Special Circulation During Meiyu Season of the Yangtze River Basin in 1998

利用 1 998年南海季风试验 (SCSMEX)和华南暴雨试验 (HUAMEX)的同化资料和 2 0 0 1年“973”中国暴雨试验 (CHeRES)的T1 0 6背景场资料 ,对 1 998、 2 0 0 1年夏季 6～ 7月环流和降水的分析表明 :(1 ) 1 998年中高纬度双阻形势有利于冷空气南下 ,对“二度梅”的形成有重要作用 ;2 0 0 1年副高偏东 ,中纬度槽脊的位置与典型梅雨相反 ,没有形成典型的梅雨。 (2 )东亚沿岸大槽的形成以及相应西风急流的南移、东伸和北风(冷空气 )的南进 ,对“二度梅”开始时副高的突然南压有重要作用 ,这些因子可以作为1 998年 7月下旬“二度梅”形成的前期预报因子。 (3)对降水与夏季风关系的分析表明 ,季风的短期变化与雨带的进退和维持有密切关系 ,特别是 2 0～ 30 oN较大范围的低空急流对梅雨锋降水有重要作用 ;1 998年梅雨期风速和湿度场有明显的日变化 ,且与强对流系统发生的时间一致。 (4)对历史上“长梅”和“二度梅”年情况进行了分析 ,并将 1 998年的环流与 1 950年以来造成严重洪涝灾害的“长梅”和“二度梅”的环流作了对比 ,再次确认这几次梅雨期的中高纬度均为双阻或单阻形势 ,而 1 954、 1 980、 1 991年的副高为带状 ;1 954、 1 980年副热带高压属于西进稳定型 ,1 991、 1

Data from HUAMEX and SCSMEX experiments in China in 1998 and heavy rainfall experiment of '973'(CHeRES) are employed to analyze the general circulations, the variation of summer monsoon during the summer of 1998 and 2001. The analyses suggest some marked characteristics in these two years. In 1998, two blocking highs in middle-high latitudes induce to the invading of cold air, of which are very favorable to the generation of heavy rainfall in the Yangtze River basin. However, the distribution of trough and ridge of 2001 is opposite to that of 1998 and the western Pacific subtropical high locates easterly, which induce to the relatively less precipitation during Meiyu season. The most important reasons of occurrence of Meiyu during later July in 1998 are the retreating of subtropical high suddenly.The further analysis shows that the build of east Asian trough and southern invading of westerly jet and cold air result in the retreating of western Pacific subtropical high. The study on the relationship between precipitation and summer monsoon indicates that the low-level jet of 20～30 oN is important to rainfall of Meiyu front and diurnal variation of wind and moisture is consistent to that of intense convective systems. The circulations of typical 'long Meiyu' and 'second Meiyu' are analyzed and compared with that of 1998. The results show that two blocking highs or one blocking high is the circulation in middle-high latitudes. The subtropical high in 1954, 1991, and 1998 are different, but they settle to the south of the Yangtze River and are favorable to the precipitation of Meiyu.