Effects of landfall location and the approach angle of a cyclone vortex encountering a mesoscale mountain range

The orographic effects of landfall location, approach angle, and their combination on track deflection during the passage of a cyclone vortex over a mesoscale mountain range are investigated using idealized model simulations. For an elongated mesoscale mountain range, the local vorticity generation, driving the cyclone vortex track deflection, is more dominated by vorticity advection upstream of the mountain range, by vorticity stretching over the lee side and its immediate downstream area, and by vorticity advection again far downstream of the mountain as it steers the vortex back to its original direction of movement. The vorticity advection upstream of the mountain range is caused by the flow splitting associated with orographic blocking. It is found that the ideally simulated cyclone vortex tracks compare reasonably well with observed tracks of typhoons over Taiwan's Central Mountain Range (CMR). In analyzing the relative vorticity budget, the authors found that jumps in the vortex path are largely governed by stretching on the lee side of the mountain. Based on the vorticity equation, this stretching occurs where fluid columns descend the lee slope so that the rate of stretching is governed mostly by the flow speed and the terrain slope. In other words, the maximum stretching and associated track jump are located on the faster side of the vortex. In the type E and N landfalling tracks, the faster winds are well north of the mountain crest, and the vortex track has very little change across the mountain. For the S case, however, the stronger winds are near the center of the ridge, and the track jump is much larger. For the NE case, the jump in the vortex track occurs once the vortex center shifts south of the ridge. For the SE case, there is considerable stretching, but it is aligned with the original track, so there is no jump in track.