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 flux model of Lin and coauthors indicates that the rainfall occurring in the vicinity of the topography was strongly controlled by orographic forcing, rather than being associated with the original rainbands accompanying the typhoon as it moved onshore. Analysis of simulation control parameters from previous studies of tropical cyclones (TCs) passing over Taiwan's CMR implies that track continuity is strongly linked to Vmax/Nh and Vmax/Rf, where Vmax and R are the maximum tangential wind and radius of the tropical cyclone, N the Brunt?Väisälä frequency, h the maximum mountain height, and f the Coriolis parameter. It appears that track continuity (discontinuity) is associated with higher (lower) values of these two control parameters. Numerical estimates of these two control parameters from observational data and the numerical simulation results for Supertyphoon Bilis produce results consistent with the findings shown here. Physically, Vmax/Nh represents the vortex-Froude number (linearity) of the outer circulation of the vortex, and Vmax/Rf represents the intensity (inertial stability) of the vortex. It is hypothesized that when these two control parameters are small, orographic blocking forces a greater percentage of flow around the mountain, instead of allowing the flow to pass over the topography. The vortex becomes unstable, subsequently resulting in a discontinuous surface and near-surface storm track. Analysis of control parameters from previous studies of landfalling typhoons affecting Taiwan also indicates that a westward-moving TC tends to be deflected to the north (south) when Vmax/Nh is large (small). The dependence of TC track deflection on the basic-flow Froude number (U/Nh) is not revealed by parameter analysis of the previous studies.