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ABSTRACT
High-resolution simulations for Typhoon Krosa (2007) and a set of idealized experiments are conducted using a full-physics model to investigate the eminent deflection of typhoon track prior to its landfall over mountainous island topography. The terrain height of Taiwan plays the most important role in Typhoon Krosa's looping motion at its landfall, while the surface properties, details in the topographic shape of Taiwan, and the cloud microphysics are shown to be secondary to the track deflection. A simulation with 3-km resolution and realistic model settings reproduces the observed Krosa's track, while that with 9-km resolution fails, suggesting that high resolution to better resolve the typhoon-terrain interactions is important for the prediction and simulation of typhoon track deflection prior to landfall. Results from idealized experiments with model configurations mimicking those of Supertyphoon Krosa show that vortices approaching the northern and central topography are significantly deflected to the south before making sharp turns to the north, forming a kinked track pattern prior to and during landfall. This storm movement is consistent with the observed looping cases in Taiwan.
Both real-case and idealized simulations show strong channel winds enhanced between the storm and the terrain when deflection occurs. Backward trajectory analyses support the concept of the channeling effect, which has been previously found to be crucial to the looping motion of Typhoon Haitang (2005) as well. However, the inner-core asymmetric ventilation flow does not match the movement of a deflected typhoon perfectly, partly because the steering flow is not well defined and could not completely capture the terraininduced deflection in the simulation and in nature.
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1. Introduction
Located in the western North Pacific, Taiwan suffers from 3 to 6 typhoons each year. The severe wind gusts and heavy rain associated with a typhoon often lead to major catastrophes, making typhoons one of the most serious meteorological phenomena. Taiwan is a mountainous area with complex terrain features, including plains in the west and rugged mountains running in five ranges from the northern to the southern tips of the island. Particularly noteworthy is the Central Mountain Range (CMR) with an average elevation of 2000 m and a peak close to 4000 m. These exceptionally high ranges can cause significant changes in typhoon tracks...