Diagnosis and application of potential shear deformation wave-activity density in the torrential rain of Typhoon Morokat (2009)

Abstract

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.