Both Typhoons Soudelor (2015) and Megi (2016) exhibit a great similarity on the track and accumulated rainfall over Taiwan, which provides a motivation of this study using an idealized WRF model to investigate the behaviors of the tropical cyclone (TC) and the associated potential-vorticity (PV) vortex dynamics. To understand the sensitivity of the track and rainfall to the potential factors, a series of sensitivity experiments are conducted, including the changing parameters in the steering flow, the landfall angle and the initial position of the vortex.
The idealized model results indicate that a double-peak rainfall distribution (over the northeastern and southwestern slopes) can be produced over a Gaussian mountain island when both the factors of steering wind and impinging direction are close to the observed. However, the accumulated rainfall can be greatly changed when both factors considerably deviate from the real conditions. The TC tends to move to the region of maximum wavenumber-one component of the net PV tendency, at a translation speed closely following the calculation of regression on wavenumber-one PV tendency budgets within the TC. The TC will be deflected southward as closing to the mountain, due to the large positive tendency from diabatic heating in PV budgets. The TC will slow down after passing over the mountain and moving to the ocean, as a result of large positive tendency from the vertical advection of potential vorticity over land.
Sensitivity tests show that the southwestern (northeastern) rainfall peak intensity will be reduced if the initial vortex position is farther southward (northward), and the total rainfall on the mountain will be less (more) if the steering flow becomes stronger (weaker). The potential vorticity advection term will be different pattern in different terrain angles, which cause the vortex deflects to the north before landfall. It also shows the southward deflection before crossing the mountains will be changed if the strength of the steering flow is changed.We found that when the initial position of vortex is more norther, the southward deflection displacements before crossing the mountains will increase. On the contrary, the southward deflection displacements before crossing the mountains will decrease.||en_US|