Numerical study on the formation and early developmentof tropical cyclones

This study investigates the formation mechanisms and early development of tropical cyclones (TCs) through both observational data analysis and numerical model simulations. Typhoon Ketsana (2003) in the western North Pacific is the major case study and has been simulated using the Weather Research and Forecasting Model. First, the roles that mesoscale convective systems (MCSs) play in determining the rate of TC formation is investigated by comparing the simulation of Typhoon Ketsana, which involved multiple MCSs during its formation, with that of Typhoon Dan (1999) with only one MCS involved. Diagnoses of the convection activity, convective energy consumption and recovery show that the longer the convective heating supply from the convective clouds is, the longer is the preconditioning period before TC forms and starts to intensify. In addition, this study further analyses the contribution from the convective-scale vortical hot towers, or, in more general convectively induced vorticity anomalies (CVAs), to the formation of Typhoon Ketsana. Scale separation showed that the activity of the convective-scale vortices correlate well with the development of the MCSs. These convectively induced vorticity anomalies have large values of positive relative vorticity induced by intense low-level convergence but at the same time large downdrafts associated with negative relative vorticity. The system dynamics through which the CVAs can get organised to form the system-scale surface vortex is analysed with several measures, such as merger index and OWZP. Observed TC warm-core structure is also examined in this study. The warm core structures of 17 TCs in monsoon trough of the western North Pacific are analysed using the AMSU-A microwave brightness temperature data. Although there is limitation of the data such as signal scattering by ice, with proper interpretation of the limitations on applications, the temperature retrieval from passive microwave data is able to indicate the warm-core development processes in TCs well. On the other hand, the WRF simulations indicate that every MCS was associated with strong mid- to upper-level heating as typically found in stratiform clouds. However, the simulated warm core development of Typhoon Ketsana shows some interesting development with variation in the warm core height and potential double warm core development. The relationship of such development to MCS activity is discussed.