Orographic Influences on Rainfall and Track Deflection Associated with the Passage of a Tropical Cyclone

Abstract

In this study, a nonhydrostatic mesoscale model [Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] was adopted to simulate Supertyphoon Bilis (2000) and investigate the dynamics of orographic rain and track deflection accompanying the storm as it passes the Central Mountain Range (CMR) of Taiwan. Both the storm track and its associated orographic rainfall distribution are well predicted by the numerical model. The intensity of the storm is underpredicted, resulting in a discontinuous track, due to the lack of specifying a ‘‘bogus’ ’ vortex at the time of model initialization. Cyclonic curvature of the storm track over the island topography track as well as major circulation features are similar to previous studies of landfalling typhoons affecting Taiwan. The model overpredicts the total amount of accumulated rainfall. Generalization of the flux model proposed in a 2001 study by Lin and coauthors is used to help predict and understand the observed rainfall distribution by calculating both the orographic and general vertical moisture fluxes from COAMPS model-predicted wind and moisture fields. The vertical moisture flux calculated from the 15-km-resolution simulation compares reasonably well to the actual, storm-observed rainfall distribution. Results of the flux model using 5-km COAMPS model output are not necessarily better than those using the coarser 15-km-resolution results. The overall consistency between the observed rainfall distribution and that predicted by the moisture