Tuesday, 30 June 2015
Salon A-3 & A-4 (Hilton Chicago)
When a tropical cyclone (TC) passes over a mesoscale mountain range, its track is often deflected by the orography, which may enhance the precipitation. 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 during the passage of a tropical cyclone over an idealized Southern-Central Appalachian Mountains (SCA). In these numerical simulations, the TC is spun up by an initial bogus vortex in a conditionally unstable stratified fluid flow with both the moisture and planetary boundary layer included. 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. Detailed budget analyses of the vorticity are performed to verify the above mechanisms for case studies with various impinging locations and angles. When the TC track impinges the mountain center in 90 degrees, the track deflection is mainly controlled by the vorticity stretching, i.e. deflected to south upstream and then turns back north downstream. When the TC track impinges the northern tip of the mountain in 90 degrees, the vorticity advection compensates the vorticity stretching making the track less southward. When the TC track impinges the southern tip of the mountain in 90 degrees, there is less vorticity stretching near the southern tip. The cases with an impinging angle of 45 degrees are essentially similar to the cases with an impinging angle of 90 degrees. When the TC track is parallel with the mountain, however, the relative vorticity stretching is mainly contributed by the convection/latent heating, instead of the downslope column stretching.
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