Dynamics of orographic rain associated with the passage of a tropical cyclone over a mesoscale mountain

In this study, the Penn State/NCAR Mesoscale Model Version 5 (MM5) was used to simulate Supertyphoon Bilis (2000) in order to improve the prediction and the understanding of dynamics of orographic rainfall associated with the passage of typhoons over the Central Mountain Range (CMR) of Taiwan. The rainfall prediction is significantly improved by bogussing a vortex, compared to a previous study (Lin et al. 2002). We identified several common ingredients present in this case which are responsible for producing heavy rainfall. The most important ingredients appear to be the presence of convective available potential energy (CAPE), a potentially unstable layer, a very moist airstream impinging on the CMR, and the presence of a low level wind maximum associated with the outer circulation of the typhoon. A simple moisture flux model was also used to estimate rainfall, which compares well with observations. In addition, we also found that the orographic rainfall is dominated by moisture convergence and the contribution of moisture advection in the vicinity of the mountain is negligible.