The vortex moving toward Taiwan and the influence of the central mountain range

Wen-Yih Sun

doi:10.1186/s40562-016-0052-5

Abstract

Surface friction is important to a vortex moving toward Taiwan but was ignored in several previous studies. The change of the potential vorticity comes from friction in the shallow-water equation, hence, it was applied to study the westbound vortex influenced by the central mountain range (CMR) blocking and surface friction, which is defined as friction coefficient multiplied by the square root of topographic elevation. Without surface friction, the movement of vortex is not affected by the CMR due to the conservation of potential vorticity. With small surface friction, the simulated vortex first deflects southward slightly, then rebounds gently north due to the effect of channel flow, as the previous studies. With moderate or large surface friction, when the vortex approaching Taiwan, it deforms and creates two wind maxima; one due to effect of channel flow and the other on the east of the vortex, because the slowdown vortex is pushed by the mean easterly flow behind. Meanwhile, the vortex and two wind maxima rotate cyclonically. Hence, the vortex can deflect north or south, or form a loop, that depends on the strength and location of the wind maxima. If the circulation of the vortex moves around the northern tip of Taiwan, it can induce a significant secondary vortex on the lee side. On the other hand, the secondary vortex, triggered by the flow passing over the CMR, is rather weak. This paper may provide the formation of asymmetric inner flow and the deflection of the vortex, which may be difficult to define in a more complicated atmospheric model.

Highlights

The central mountain range (CMR) has a significant impact on the movement of the front (Sun and Chern 2006) and typhoons near Taiwan (Wang 1980; Shieh et al 1998, etc.)
The vortex continues to rotate and moves west–northwest; it induces a weak, secondary cyclonic circulation at 124,800 s near the southwestern coast, triggered by the flow passing over the CMR (Fig. 6e)
A shallow-water model was applied to study the deflection of a westbound vortex approaching Taiwan and the induced circulations on the lee side

Summary

Introduction

The CMR has a significant impact on the movement of the front (Sun and Chern 2006) and typhoons near Taiwan (Wang 1980; Shieh et al 1998, etc.). The vortex passed an isolated island, has been widely investigated using observations and numerical models (Brand and Blelloch 1974; Wang 1980; Chang 1982; Bender et al 1987; Yeh and Elsberry 1993; Lin et al 2005; Huang and Lin 2008; Huang et al 2011; Tang and Chan 2013, etc.). Observations showed that westbound typhoons tend to deflect northwestward around the northern side of the CMR (Wang 1980; Chang 1982).

Methods

Results

Conclusion