Numerical Simulation of Typhoon Morakot (2009):The Formation and Development of Topography-induced Secondary Vortex

【摘要】：Using the Weather Research and Forecasting(WRF) model in this study,the evolution and physical processes of Typhoon Morakot(2009) were investigated as it was passing over Taiwan's Central Mountain Range(CMR).Without implanting a bogus vortex,the numerical simulations initialized by the NCEP/GFS analysis underestimate the Morakot's intensity,but reasonably capture its slow motion,landfalling locations,the rainfall distribution over Taiwan,and other interesting physical processes,such as the vertical separation of typhoon centers prior to landfall,interactions between the typhoon circulation and the southwesterly flow,and especially a discontinuous track due to the dissipation of original typhoon center and the formation of secondary vortex.As captured by the numerical model,the secondary vortex is spawned from the convergence zone associated with the interaction of typhoon circulation,monsoon southwesterly jet and topographical effects on the southwestern Taiwan,which differs from those associated with the adiabatic warming on the lee side of terrain in previous studies.The PV budget analysis indicates that the horizontal advection from upstream airstream plays a major role in the subsequent development of the secondary vortex after it departed from the convergence zone.As the mid-level original center passed over the CMR and collocated with the secondary vortex,a newly typhoon has been generated over the western Taiwan.