江淮流域梅雨环流结构特征及其演变分析

ANALYSIS OF THE CIRCULATION STRUCTURE DURING MEIYU AND ITS EVOLUTION CHARACTERISTICS

通过对比1954—2001年江淮流域各省入梅日与国家气候中心划定的入梅日,选取入梅一致年份进行合成,分析了入梅前后大尺度环流场的变化特征及梅雨期间的典型环流结构,并用多年平均的逐候资料追踪了典型环流结构的演变过程。研究发现:(1)入梅后,低空急流向西伸展至长江以南地区,高空急流北抬,有利于江淮流域垂直运动的发展。中、低层高度差值场出现"印度-我国东海-日本岛以东洋面-北太平洋中部"的波列,其中印度半岛、江淮流域及东部沿海、北太平洋中部变化显著。入梅前、后,低层辐合中心均与雨带配合,经向风散度的变化在总散度中起主导作用;(2)梅雨期典型环流结构在我国东部经向剖面图上,表现为"双脚型"涡度场和"鞍型"温湿场及高而窄的θse密集带(锋面/锋区),密集带与雨带对应。入梅前此典型结构已存在并与江南雨带相伴随,入梅后典型结构与雨带一致北推影响江淮。追踪其演变过程发现,梅雨期典型环流结构于3月底4月初开始建立,与东亚-西太平洋海陆温度热力差异的转向一致,可认为是东亚副热带季风开始的表现。

Using the data of the beginning date of Meiyu decided by each province in Jianghuai Basin and National Climate Center from 1954--2001, the composite analysis is applied to detect the variation characteristics of the large scale circulation fields before and after the begining of Meiyu, and the typical circulation patterns during Meiyu. The evolution of the typical patterns is tracked by the 5-day mean data. The results show that： the westward extension of low level jet to south of the Yangtze River, accompanied by the northward shift of the high level westerly jet is favourable for the development of the vertical motion. A wave train from India to the eastern East China sea, to the east of Japan, and finally to the central North Pacific as indicated by difference of height fields between lower and middle layers shows remarkable variations in the height field in Indian peninsula, the Jianghuai Basin, the coast of East China, and the central North Pacific. The rain belts are always matching the lower level convergence centers before and after Meiyu. Meanwhile, the variation of meridional convergence plays a dominant role in the whole convergence field. The typical circulation pattern in meridional cross-section in eastern China during Meiyu season is characterized by the ＂double feet＂ shape in the vorticity field, and the ＂saddle-like＂ shape in the fields of temperature and humiudity, as well as a high and narrow but dense zone of the pseudo-equivalent potential temperature field （frontal zone）, which corresponds to the rain belt. The typical pattern of the large scale circulation field has been built up before the start of Meiyu together with the rain belt located at the south of the Yangtze Rive. This typical pattern and the rain belt both advance northward to the Jianghuai Basin after the beginning of Meiyu. By tracing the evolution process of the typical circulation pattern, it is found that the typical circulation pattern is initially established from late March to early April, consistent with the reverse of the land-sea thermal contrast between East Asia and western North Pacific. This evolution process of typical circulation pattern can be recognized as the establishment of East Asian Summer Monsoon.