Orographic Effects on Track Deflection of an Idealized Tropical Cyclone

Idealized numerical experiments are performed to help understand orographic effects on the tropical cyclone (TC) track deflection. For an idealized TC passing over a mesoscale mountain from the east with a moderate basic-flow Froude number (~ 0.5), it is first deflected to the south upstream of the mountain, crosses the mountain anticyclonically, and then resumes its westward movement downstream. The TC is able to keep its integrity upstream, but is decelerated and becomes asymmetric over the upslope. In addition, the storm becomes significantly asymmetric and weakened after passing over the mountain due to orographic blocking. A vorticity budget analysis indicates that the TC's track deflection over the mountain is due to both vorticity stretching and advection, but is mainly due to vorticity advection away from the mountain. The major difference of the moist flow from the dry case is that the local rate of change of relative vorticity generated by the latent heating is significantly larger than that by the orography. Similar to the corresponding dry case as found in a previous study, the southward TC deflection upstream of the mountain is influenced by advection from the orographic-influenced basic flow.