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Advanced Taiwan Ocean Prediction System

Tseng, Tsu-Lun - ALL News | 2018-02-27 | Count:491


Title: Looping Tracks Associated with Tropical Cyclones Approaching an Isolated Mountain


Essential parameters for making a looping track when a westward-moving tropical cyclone (TC) approaches a mesoscale mountain are investigated by examining several key nondimensional control parameters with a series of systematic, idealized numerical experiments, such as U/Nh, Vmax/Nh, U/fLx, Vmax/fR, h/Lx, and R/Ly.  Here U is the uniform zonal wind velocity, N the Brunt-Vaisala frequency, h the mountain height, f the Coriolis parameter,  the maximum tangential velocity at a radius of from the cyclone center and Lx is the halfwidth of the mountain in the east-west direction.  It is found that looping tracks (a) tend to occur under small U/Nh and U/fLx, moderate h/Lx, and large Vmax/Nh, which correspond to slow movement (leading to subgeostrophic flow associated with strong orographic blocking), moderate steepness, and strong tangential wind associated with TC vortex; (b) are often accompanied by an area of perturbation high pressure  to the northeast of the mountain, which lasts for only a short period; and (c) do not require the existence of a northerly jet.  The nondimensional control parameters were consolidated into a TC looping index (LI), which was tested by several historical looping and non-looping typhoons approaching Taiwan’s Central Mountain Range (CMR) from east or southeast. It was found that LI < 0.0125 may serve as a criterion for looping track to occur.

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