On the upstream track deflection of tropical cyclones past a mountain range: Idealized experiments

Abstract

Upstream track deflection of a propagating cyclonic vortex past an isolated mountain range is investigated by using idealized simulations with both boundary layer turbulent mixing and cloud effects. The westbound vortex past a shorter mountain range may experience an earlier northward deflection prior to landfall. The vortex then takes a sudden southward turn as it gets closer to the mountain range, in response to the effects of the stronger northerly wind over the mountain due to the effects of channeling flow. The vortex may deflect southward when approaching a longer mountain range and then rebound northward upstream of the mountain ridge. The southward deflection is primarily induced by the convergence (stretching) effect due to the combination of the speedy core at the southwestern flank of the vortex and a northerly jet between the vortex and the mountain. The vortex then rebounds northward to pass over the mountain as the speedy core rotates counterclockwise to the eastern flank of the vortex. The track deflection near the mountain is also affected as either of both physics is deactivated. Sensitivity experiments show that for a given steering flow and mountain height, a linear relationship exists between the maximum upstream deflection distance and the nondimensional parameter Rmw/Ly, where Rmw is the vortex size (represented by the radius of the maximum wind) and Ly is the north-south length scale of the mountain. The southward deflection distance increases with smaller Rmw/Ly and higher mountains for both weaker and stronger steering flow. When the steering-flow intensity is doubled, the southward deflection is roughly reduced by 50%.