一次暴雨过程的EOF分析

EOF Expansion in One Rainstorm

在用21层η坐标细网格模式对1998年7月21-22日发生在武汉地区的一次持续性特大暴雨过程数值模拟的基础上，利用较高时空分辨的模式输出结果对暴雨过程做经验正交函数分解（EOF分析）尝试。结果表明：EOF第1主分量的空间分布代表典型的暴雨环境背景场的低值系统；EOF第2主分量的空间分布与人字形切变线（西部为冷式切变，东部为暖式切变）相联系，是影响这次暴雨的重要形势场；利用相轨线分析方法发现，暴雨过程中EOF第1主分量和EOF第2主分量的时间系数在暴雨临近阶段，两者正相关，激发暴雨，在暴雨后期两者负相关，促使这次暴雨趋于减弱结束。

Empirical Orthugonal Function （EOF） expansion had been most used in the research of climatic scale, but not in that of synoptic scale. This is because the observational data of synoptic scale in temporal distribution are not dense enough, and the data set is not abundant enough, and it is very difficult to use the EOF expansion. With the using of automated surface meteorological station and the development of numerical model, it is practicable to use the results of model outputs to diagnose EOF expansion. The new attempt is made in this field in this paper. A 21-layer fine grid model and η coordinate is used to simulate a continuing heavy rain in the Wuhan region during 21 - 22 July 1998 in this paper. The results of model outputs are used to diagnose EOF expansion for showiug its mesoscale characteristics of spatial and temporal distribution, and to reveal the physical mechanism of the genesis and development of rainstorm. And the conclusions are as follows： （1） The spatial distribution of the first principal component of EOF （EOF-1） stands for the low-pressure system. And it is a typical background field of the rainstorm, which increases persistently throughout the whole rainstorm. （2） The second principal component of EOF （EOF-2） is relevant to the lambdoidal shear line, the west of which is cold shear and the east of which is warm shear. It is an important posture field of the rainstorm. Before the outbreaks of rainstorm, the cold air comes south, the low pressure of shear line reinforces, and the southwest warm and wet air transports water vapor northward, which are propitious to the development of rainstorm. Then the system weakens evidently, and the rainstorm turns to disappear quickly. （3） By analyzing the path line in phase space, the characteristics of evolvement of principal modes can be compactly expressed on a two-dimensional phase space chart. It can be seen that during the course of rainstorm, the transformation points of EOF-1 and EOF-2 spread clockwise following the path line of parabola. In the immediate phase of the rainstorm, the two principal components are positively correlated, and motivate the rain. At the later phase of rainstorm, they are negatively correlated, and make heavy rain tend to be extinct.