Effects of Impinging Location and Angle of a Long Mountain Range on the Passage of an Idealized Tropical Cyclone

When a tropical cyclone (TC) passes over a mesoscale mountain range, its track is often deflected and may become discontinuous. In this study, a series of idealized numerical experiments is performed to help understand the orographic effects of impinging location, impinging angle and their combination on the track deflection, track continuity, as well as the induced precipitation during the passage of a cyclone vortex over a long mountain range with the latent heating. In these numerical simulations, the TC is spun up by an initial bogus vortex, which is in gradient wind balance, in a conditionally unstable stratified fluid flow. The size and shape of the mountain are set using the dimension of the Southern-Central Appalachians (SCA). It is found that the ideally simulated cyclone vertex tracks compare reasonably well with the observed tracks of TCs over the SCA.

Compared with the mountain on an aqua-planet, the vortex cyclone passing over the mountain on the land is deflected slightly further to the south. In both cases (on aqua-planet and on land), the vortex cyclone keeps going straight to the west without turning back to north after it passes over the mountain. If a coastal line is set to the east foot of the mountain, the vortex cyclone is deflected much further to the south before it impinges the mountain and is slightly turning further south after the mountain. When the mountain is sufficiently wide, the back circulation in the immediate downstream area is able to generate the local vorticity tendency, thus steers the vortex back to its original westward movement. When the cyclone vortex impinges on the mountain at its northern tip, the vorticity advection upstream of the mountain range is able to balance the vorticity stretching causing the cyclone to continue its original westward movement and leaves an almost straight track with no deflections. Analyses of vorticity budget and the precipitation and sensitivity to various impinging angles are performed to support the above mechanisms.