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

Six non-dimensional control parameters were proposed by Lin et al. (2005, JAS) to explain the deflection of a vortex tracking near a mesoscale mountain: basic-flow Froude number (U/Nh), vortex Froude number (Vmax/Nh), aspect ratio of vortex and mountain scales (R/Ly), nondimensional vortex vorticity, mountain slope, and basic-flow Rossby number. It was proposed by Lin et al. that the first three control parameters are dominant for track deflection. According to observations, however, the track of a vortex impinging on a finite-length mountain range is also strongly influenced by landfall location and the approach angle. This problem is studied by performing systematic numerical modeling simulations using a simple mesoscale model. The control parameters are fixed with U/Nh=0.4, Vmax/Nh=0.8, and R/Ly=0.75.

For a vortex approaching the central portion of the mountain range from east (CNTL), the vorticity center is deflected to the north upstream of the mountain. A secondary vorticity center forms at a low level to the lee of the mountain range and to the south of the deflected parent vortex, which coexists with the parent vortex for at least 2 hours. The surface secondary vortex then replaces the blocked low-level parent cyclone. A vorticity budget analysis indicates that the upstream rightward deflection is affected by the vorticity advection, while the secondary lee vorticity center derives its vorticity from vorticity stretching. A vortex approaching the southern portion of the mountain range from east is deflected to the south upstream and crosses the southern end of the mountain range. The southward deflection is dominated by vorticity advection, rather than by vorticity stretching. For a vortex approaching the northern portion of the mountain range from east (case N), the deflection of the vorticity center is similar to the CNTL case, as is the generation of vorticity around the mountain range. The deflection in cyclone motion and the vorticity generation exhibited in a case of a vortex impinging on the mountain range from southeast is similar to cases CNTL and N. For a cyclone impinging on the mountain range from the northeast, the deflection of cyclone motion and the generation of vorticity are dominated by both vorticity stretching and advection. In summary, the deflection of a cyclone encountering a mountain range is largely controlled by vorticity advection and stretching, depending upon the approach angle and landfall location of the cyclone.