中文摘要：

本文针对2008年上海8月25日暴雨过程,利用WRF中尺度数值模式对其较好模拟的结果,应用EOF分析方法对其做了动能偏差场的统计动力诊断。探讨了该暴雨过程中各种尺度天气系统,特别是β中尺度雨团,其有效位能向动能的转化机制。所得主要结论有：对该暴雨过程,动能偏差场EOF分析的第一、二、三模态可分别称为暴雨背景模态、暴雨系统模态和暴雨雨团模态。在该暴雨落区附近,低层各模态动能偏差场水平梯度的绝对值均较大,系统演变也较剧烈,且尤以暴雨雨团模态为甚;这表明低层暴雨雨团模态该处风场具有强烈的非地转性,有着强辐合,运动是非平衡的,其性质为重力惯性波（含涡旋—重力惯性混合波）。在某等压面上,有效位能的时间变化与位势偏差的时间变化相同,而前者的水平梯度则与后者的空间梯度大致相同,这样讨论某层前者的变化就归结于讨论该层后者的变化。暴雨来临前在该暴雨落区附近,前三个模态都有有效位能向动能的转化,其表现为动能的增长和有效位能的下降,且以暴雨雨团模态表现更突出。暴雨雨团模态的尺度为β中尺度,这表明该尺度的系统在有效位能向动能转换中起着关键作用,且扮演着有效位能与涡旋场动能之间转换的中介角色。

英文摘要：

Using empirical orthogonal function （EOF） expansion, this paper analyzes Weather Research and Forecasting （WRF） simulative data of the torrential rain occurring in Shanghai on August 25 of 2008 to diagnose the deviation field of kinetic energy. The conversion mechanism from the available potential energy to kinetic energy is discussed for various weather system scales, and meso-β scale rain clusters in particular. The results show that the preceding three EOF expansion modes can be individually called the ambient mode, the torrential rain system mode, and the rain clusters mode. Around the precipitation area, the absolute value of the horizontal gradient in the low-level kinetic energy deviation field is bigger, especially in the meso-13 scale rain clusters mode. This indicates that the wind field of this mode is ageostrophic with strong convergence, and its motion is a non-equilibrium inertial-gravity wave （containing a mixed eddy-inertial gravity wave）. On a certain isobaric surface, the available potential energy is homological with geopotential deviation in the time-variation. The horizontal gradient of the available potential energy is homological with the spatial gradient of the geopotential deviation. So the variation of geopotential deviation can represent a change in the available potential energy. Before a rain occurrence, the available potential energy converts to kinetic energy for all three modes around the precipitation area. For the rain clusters mode, especially, the change is more obvious. This mode is a meso-β scale system, so this scale system is key in the conversion course from available potential energy to kinetic energy, playing an intermediary role in the eddy field＇s conversion mechanism of available potential energy to kinetic energy.