Control Parameters for the Influence of a Mesoscale Mountain Range on Cyclone Track Continuity and Deflection

Abstract

In this study prospective control parameters are identified for diagnosing the continuity and deflection of cyclone tracks across a mesoscale mountain range. Based on idealized simulations of a westward-moving cyclone, it was found that the cyclone track becomes a discontinuous (continuous) track and the cyclone experiences more (less) deflection with a combination of small (large) values of Vmax/Nh, U/Nh, R/Ly, U/fLx, and Vmax/fR, and large (small) value of h/Lx. The symbols are defined as follows: Vmax the maximum tangential wind, N the Brunt?Väisälä frequency, h the mountain height, U the basic wind speed, R the radius of Vmax, f the Coriolis parameter, and Lx and Ly the horizontal scales of the mountain in x and y directions, respectively. A conceptual model is proposed to explain track deflection and continuity for a westward-moving cyclone encountering idealized topography representative of the Central Mountain Range of Taiwan. With weak orographic blocking, a cyclone crosses over the mountain range with some northward deflection. With moderate orographic blocking, northward deflection of a cyclone is greater upstream of the mountain range and a secondary, leeside vortex forms to the southwest of the mountain range, indicative of discontinuity in the cyclone track. With strong orographic blocking, a westward-moving cyclone is deflected southward and a secondary cyclone forms to the northwest of the mountain range. The northward or southward deflection of a cyclone track is explained by the orographic blocking on the outer circulation of the cyclone.