Numerical Study on the Formation of Typhoon Ketsana (2003) in the Western North Pacific

Lu et al. (2011) simulated the formation of Typhoon Ketsana (2003) in the western North Pacific using the Weather, Research and Forecasting (WRF) model. In particular, the mesoscale dynamics associated with the generation of mesoscale convective vortices (MCVs) and the roles of mesoscale convective systems (MCSs) during the formation process were investigated. Lu et al. concluded that with the successive occurrence of MCSs, midlevel average relative vorticity was strengthened through generation of MCVs via mechanisms such as vertical stretching and eddy fluxes. Through sensitivity experiments to modify the vertical humidity profile in each MCS, it was found that the development of a MCS depends substantially on that of the prior ones through remoistening of the midtroposphere, and thus leading to different scenarios of system intensification during the typhoon formation. This study further analyzes the formation mechanisms of this typhoon case by examining the warm core formation process and contribution from the convective-scale vertical hot towers (VHTs). For the warm core generation, it was identified in the WRF simulations that every MCS was associated with strong mid- to upper-level heating as typically found in stratiform clouds. However, how these heating associated with the MCSs lead to the warm core when the tropical cyclone forms was not extensively discussed in previous studies, and will be one of the key questions to answer. On the other hand, scale separation showed that the activity of the VHT-type vortices correlated well with the development of the MCSs. These VHTs have large values of positive relative vorticity induced by intense low-level convergence. A quantitative comparison between the relative contributions from these VHTs and the MCSs to the typhoon formation will be performed, and mechanisms by which the VHTs can get organized to form the system-scale surface vortex will be identified.

Reference: Lu, X., K. K. W. Cheung, and Y. Duan, 2011: Numerical study on the formation of Typhoon Ketsana (2003). Part I: Roles of the mesoscale convective systems. Mon. Wea. Rev. (in press).