Numerical studies on the role of mesoscale convective systems during the formation of Typhoon Ketsana (2003)

In about 60-70% of all tropical cyclones (TCs) in the western North Pacific, occurrence of mesoscale convective systems (MCSs) at multiple times was identified during the 48 hours prior to formation (Ritchie and Holland 1999; Lee et al. 2008). In particular, it was found that co-existence of MCSs during TC formation is more common in the monsoon-related synoptic patterns. Typhoon Ketsana (2003) formed in a reverse-oriented monsoon trough at 1200UTC 18 October. During the 48 hours prior to Ketsana's formation, four MCSs were observed in satellite imageries, and two of them developed almost simultaneously during 1500UTC 17 October and 0400UTC 18 October. Numerical simulations utilizing the Weather Research and Forecasting (WRF 3.1) model with highest resolution of 3 km are performed on Typhoon Ketsana. Whereas early intensification is quite well simulated, the distribution of MCSs does not exactly match with that observed. By assimilating QuikSCAT oceanic winds and SSM/I vertically integrated moisture into the WRF model by using the WRFDA 3.1 data assimilation system, improvements are obtained in terms of the large scale circulation, the evolution processes and structural features of the MCSs. Techniques are being developed to further improve the simulations of MCSs such that realistic convection episodes are reproduced in the model. Sensitivity tests are carried out to further explore the role of MCSs during the formation of Typhoon Ketsana, and the possible physical mechanisms of how the MCSs influence the formation.