Due to the high elevation of the central mountain range of Taiwan, a typhoon often experienced great changes in its circulation and convective features when it moved across Taiwan. Observations have shown that some typhoons had significant upstream deflection in their moving direction before they made landfall on Taiwan. However, this upstream deflection appeared not significant for other landfall typhoons. To understand the mechanism that leads to the upstream deflection of typhoon track is an important issue to the typhoon operation in Taiwan. Unfortunately, because of the lack of observation, such issue can only be addressed using numerical model simulation at the present time.
The PSU/NCAR mesoscale model MM5 is used to study the effect of Taiwan topography on typhoon track. The EC/TOGA advanced grid point data was used as initial condition. However, this data set can not resolve typhoon circulation properly. To overcome this problem, our approach was to spin up the typhoon vortex at the time period twelve hours before the desired initial time period. The generated typhoon vortex was then used as the bogus vortex to replace the weak vortex resolved by the original grid-point data. Results showed that this bogusing approach could provide an initial model vortex with reasonable intensity for typhoon Marian (1990) that made landfall on Taiwan on May 19. The simulated typhoon track and the intensity changes also agreed well with observations except a slight shift in the landing point. The effect of Taiwan topography on the track of Marian and the interaction between Marian and a Mei-yu front will be discussed in the presentation