Effects of Taiwan orography on the motion and structure of typhoons
Elsberry, Russell L.
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Effects of steep island terrain on typhoon motion and structure changes are studied by both observational analyses and numerical simulations. The analyses involve calculation of center position deviations from a second- order polynomial curve fitted to the best tracks, and an empirical orthogonal function analysis to determine the terrain-induced surface structures. The simulations use a limited-area model, with a elliptical-shaped barrier and a 'spun-up vortex' to represent the typhoon, to demonstrate the sensitivity on track and structure relative to vortex intensity, approach direction and translation speed. Both observational and simulation studies show that the orography induces significant track deflections and structure distortions, especially for a weaker and slowly moving typhoon approaching central or southern Taiwan. As the typhoon approaches, the outer edge of the circulation accentuates the blocking effect of the barrier. Asymmetric gyres are induced by a vortex-terrain interaction as the vortex moves closer to the barrier, which accounts for the significant track deflections from a simple barrier effect. The simulations suggest that a center-following algorithm based on the wind field would be more representative than on the pressure field. A track discontinuity occurs when the secondary vortex created from flow deflections around the barrier becomes the primary center downstream.
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