Looping tracks associated with tropical cyclones approaching an isolated mountain. Part I: Essential parameters

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, V_max/Nh, U/fL_x, V_max/fR, h/L_x, and R/L_y. Here U is the uniform zonal wind velocity, N the Brunt–Vaisala frequency, h the mountain height, f the Coriolis parameter, V_max the maximum tangential velocity at a radius of R from the cyclone center and L_x 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/fL_x, moderate h/L_x, and large V_max/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 are consolidated into a TC looping index (LI), U2R2Vmax2hLy, which is tested by several historical looping and non-looping typhoons approaching Taiwan’s Central Mountain Range (CMR) from east or southeast. It is found that LI < 0.0125 may serve as a criterion for looping track to occur.