INTERACTION OF TYPHOONS WITH THE TAIWAN OROGRAPHY .1. UPSTREAM TRACK DEFLECTIONS

A combined observational and numerical modeling approach is used to st udy the upstream track deflections Of westward-moving tropical cyclone s approaching the mountainous terrain of Taiwan. Although the standard deviations are large, the mean track deflections are larger and have a different orientation for storms approaching the southern end of the island from those approaching the northern end. The deflections tend to be larger for weaker and slower-moving storms. Two distinct regions of upstream track deflections that are detected in the observations a re explored in a sensitivity study with a 45-km horizontal resolution and 16-layer numerical model. As the outer circulation of the tropical cyclone begins to interact with the mountain barrier, the zonal decel erations and southward track deflections are explained by an enhanced blocking and deflection of the environmental flow advecting the cyclon e. The larger effect for southern-approaching storms compared to north ern storms is because the stronger winds on the right side impinge mor e directly on the barrier. As the inner circulation of the southern-ap proaching cyclone interacts with the barrier, the track deflections be come northward relative to the expected blocking flow around the islan d, which thus changes the landfalling point. Decomposing the cyclone c irculation into symmetric and asymmetric circulations about the center indicates that the asymmetric flow has a component across the center that is consistent with the zonal deceleration and northward deflectio n of the southern-approaching cyclone. The numerical model sensitivity study demonstrates that the storm structure, including size as well a s intensity, and translation speed are important factors in the upstre am track deflections.