梅雨锋的典型结构、多样性和多尺度特征

在天气尺度梅雨锋的天气学定义基础上,利用GMS-5静止卫星红外云图、常规气象探空资料、NCEP再分析与最终分析资料对2002年长江流域典型梅雨期6月26—28日和二度梅期间7月23日、1998年5月梅雨与7月二度梅共4个梅雨锋个例进行了分析与比较,归纳了梅雨锋结构多样性;并着重对典型梅雨期的梅雨锋发展过程、水平以及垂直结构进行了多种物理量场(包括风场、温度场、急流、锋区、假相当位温、散度、垂直速度、静力稳定度等)的综合分析。结果表明,不同的个例,不同的地区和时期,一次梅雨过程的不同阶段,梅雨锋的结构和性质都有可能不同,它可以从比较接近极锋的性质过渡到接近赤道锋的性质。在水平结构上梅雨锋是在高、低纬度不同尺度的环流系统共同作用下形成的,从而造成了梅雨锋结构具有丰富的多样性。对典型梅雨锋结构进行综合分析表明梅雨锋对流层中下层锋面由强假相当位温水平梯度形成;梅雨锋南侧为暖湿气团、北侧为变性气团;梅雨锋南面为西南季风、北面为偏东气流;梅雨锋的上升运动和强降水主要发生在梅雨锋的前沿;梅雨锋上方对流层上半部存在与副热带高空急流相配合的高空副热带锋;对流层上部的高空热带东风与副热带高空西风急流构成了梅雨锋降水的高空辐散流场。根据典型期梅雨锋以及二度梅倾斜型梅雨锋的对流层上、中、下水平环流特征,给出了梅雨锋的多尺度概念模型,主要包括中低纬度系统相互作用、对流层高层的行星尺度的环流系统副热带高空西风急流、高空热带东风急流与南亚高压、对流层中层的副热带高压与北方的短波槽以及对流层低层的行星尺度季风和切变线。

TYPICAL STRUCTURE, DIVERSITY AND MULTI-SCALE CHARACTERISTICS OF MEIYU FRONT

Meiyu front plays an important role in summer precipitation in Central China. Based on the GMS-5 satellite images, NCEP reanalyses (2.5°×2.5°) and final analyses (1°×1°) data, and meteorological conventional sounding observations, the horizontal and vertical structures of meiyu front are summarized using multiple diagnostic variables (including wind, temperature, jet streak, frontal area, pseudo-equivalent potential temperature, divergence, vertical motion, static instability, etc). In this paper, four cases of meiyu front were selected and analyzed, two of them are on 26-28 June and 23 July 2002 during the Experiment on Heavy Rain in meiyu Front in the Downstream of Yangtze River, and the other two casesare in May and July 1998. The two cases in July of 1998 and 2002 are the secondary meiyu front cases. The results show that the structures and characteristics of the meiyu front are different for various cases, or at various places and time, or at variious stages of one case, and the frontal characteristics can be transited from the polar front to the equatorial front. Due to the interaction of the different scale circulations in the high and low latitudes, the horizontal structures of the meiyu front are plenty of various forms.The results in this paper also show that the typical meiyu front consists of a narrow band with high gradient of equivalent potential temperature below 500 hPa, south of which is warm and moist air mass, and north of which is the indifferent air mass from the mid-latitude ocean or the polar continent. Below the mid troposphere, south of the front blows southwest wind, and north of the front the easterly. The ascending motion and precipitation usually occur ahead of the meiyu front. In the upper troposphere, the subtropical front is above the meiyu front, but they are not connected. In addition, the upper westerly jet and the easterly to the south of the meiyu front consist of the upper divergent flow field.The multi-scale characteristics of the horizontal structure of meiyu front can be summarized as follows: in the upper troposphere, there exist the subtropical westerly jet, the easterly to the south of meiyu front, and the South Asia high; at the mid tropospheric 500 hPa, the subtropical high over the west Pacific is the main weather system, and there are also some short wave troughs in the north; in the lower troposphere, the planetary scale southwest monsoon, the large scale low level southwest jet, and the meso-scale vortex or waves near the shear line are associated with the meiyu front.