Interaction of Typhoons with the Taiwan Orography. Part I: Upstream Track Deflections

Abstract

A combined observational and numerical modeling approach is used to study the upstream track deflections of westward-moving tropical cyclones 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 are 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 decelerations and southward track deflections are explained by an enhanced blocking and deflection of the environmental flow advecting the cyclone. The larger effect for southern-approaching storms compared to northern storms is because the stronger winds on the right side impinge more directly on the barrier. AS the inner circulation of the southern-approaching cyclone interacts with the barrier, the track deflections become northward relative to the expected blocking flow around the island, which thus changes the landfalling point. Decomposing the cyclone circulation 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 deflection of the southern-approaching cyclone. The numerical model sensitivity study demonstrates that the storm structure, including size as well as intensity, and translation speed are important factors in the upstream track deflections.