In this study,the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) is used to simulate Typhoon Mindulle (2004) at high resolution (3-km grid size).The data from measurements show that in the upper atmosphere the existence of an upper jet is important to the transition cyclone.When Mindulle moved to the area of the upper jet entrance,where high-altitude divergence existed, the pumping of the high-altitude divergence would enhance the vertical motion and low-level cyclone convergence. The enhanced vertical motion was confirmed by the simulation results and indicated that the existence of upper divergence enhanced the vertical motion which was favorable for the maintenance of Typhoon Mindulle.The process of extratropical transition (ET) and re-intensification always accompanies the process of cold air invasion. This process enhances the baroclinicity of the atmosphere and the formation of front at high altitudes, which converts baroclinic potential energy into kinetic energy and strengthens the cyclone vortex.The distributions of equivalent potential temperature (θe) and temperature anomalies show that the warm-core of the typhoon at the tropopause aids the re-intensification of the system. As the typhoon reenters the ocean, latent heat flux (LHF) increases in the north and west and the strong reflectivity and vertical motion occur in the east and southeast,and the west.With the re-intensification of the typhoon the wind field evolves from an oval to a circle at the lower atmosphere, the area coverage by high winds increases, and the distribution of the tangential wind shows an asymmetric pattern.
The polar low and tropical cyclone type vortices over topography are assumed to be the axisymmetrical and thermal-wind balanced systems, which are solved as an initial value problem of a linearized vortex equation set in cylindrical coordinates. The roles of the sensible and latent heating, friction, and topography in the structure and intensification of the polar low and tropical cyclone type vortices are analyzed. The radial velocity, vertical velocity, azimuthal velocity, and the unstable growth rate including the topography effects are obtained. It is shown that the interaction between the flow and the topography plays a significant role in the structure and intensification of the polar low and tropical cyclone system. The analysis of the topography term indicates that, in the up-slope side of the mountain, the radial inflow and the vertical ascent forced by the mountain can intensify the polar low and tropical cyclone type vortex and increase the unstable growth rate. However, in the lee side of the mountain, the radial inflow and the vertical descent forced by the mountain can weaken the polar low and tropical cyclone type vortex and decrease the unstable growth rate of the polar low and tropical cyclone system. In addition, the evolutionary process and the spatial structure of the polar low observed over the Japan Sea on 19 December 2003 are investigated with the observational data to verify this theoretical result.
