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在水平风场切变形变的基础上引入物理量——位势切变形变, 把水平风场的垂直分量和切变形变与广义位温的空间梯度结合起来. 该物理量的二阶扰动被定义为波作用密度, 在局地直角坐标系中推导了能够描述中尺度扰动系统发展演变的波作用方程. 对2009年登陆台风莫拉克引发的暴雨过程进行诊断分析, 结果表明, 台风中心区切变形变向涡度的转化显著, 该转化通过纬向风的经向梯度实现. 纬向风速的经向梯度与广义位温垂直梯度的耦合引起位涡和位势切变形变之间的转化, 以至于位势切变形变和位涡的异常值区分别位于台风环流的外侧和内侧. 波作用密度因能够描述水平扰动风场的垂直切变和切变形变以及凝 结潜热函数扰动梯度而与观测降水联系紧密. 相关统计分析表明, 2009年夏季波作用密度与6 h观测降水存在明显滞后相关性, 对未来6 h降水有一定的指示意义. 另外, 波作用密度通量散度和扰动非地转风位涡是影响波作用密度局地变化的主要物理因素. 基于波作用密度与观测降水的良好相关性, 建立了波作用密度暴雨预报方程. 2009年登陆台风"苏迪罗", "天鹅", "莫拉菲"和"巨爵"的暴雨预报个例分析和长时间序列的ETS评分计算表明, 波作用密度对台风暴雨具有较好的预报效果, 其预报降水能力略优于美国全球预报系统(GFS)的直接降水预报.Based on previous studies of the interaction between wave and flow, combining the vertical component of disturbance convective vorticity vector, the disturbance horizontal divergence and the vertical gradient of disturbance generalized potential temperature, We define a new disturbance thermodynamic shear advection parameter in this paper. This parameter means a typical wave action density, its equation of tendency is a typical wave action equation, which could describe the development or evolution of mesoscale disturbance. With the wave action density and the wave action equation, we have conducted a diagnostic analysis of the heavy-rainfall event caused by landfall typhoon "Morakot" in 2009. Results show that the abnormal values of the wave action density (namely the disturbance thermodynamic shear advection parameter) will always change with the development of the observed precipitation regions, both their horizontal distribution and their temporal evolution are quite similar. Statistical analysis reveals a certain correspondence between the wave action density and the observed 6-hour accumulated surface rainfall in the summer of 2009, and they are intimately related to each other. The wave action density may be used to describe the typical vertical structure of dynamic and thermodynamic fields of a precipitation system, so it is closely related to the occurrence and development of the precipitation system and may have certain relation with the surface rainfall regions. The calculation of the wave action flux divergence shows that at the time around the landing of typhoon "Morakot" in the southeastern China coastal region, the potential vorticity of disturbance ageostrophic wind is the main forcing term affecting the local variation of wave action density. The contribution of the coupled term between the first-order disturbance advection and disturbance shear to the local variation of wave action density is secondary. While the exchange of disturbance momentum and disturbance heat between the basic state and disturbance has relatively weak influence on the development and evolution of disturbance. After the landing of typhoon "Morakot" in the southeastern China coastal region, these forcing terms and the wave action itself are apparently weakened.
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Keywords:
- disturbance thermodynamic shear advection parameter/
- wave action density/
- convective vorticity vector/
- generalized potential temperature
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