Orographic Influence on Basic Flow and Cyclone Circulation and Their Impacts on Track Deflection of an Idealized Tropical Cyclone

Abstract

Abstract A series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range. When an idealized tropical cyclone (TC) is embedded in a uniform, easterly flow and passes over a mountain with a moderate Froude number, it is deflected to the south upstream, moves over the mountain anticyclonically, and then resumes its westward movement. The vorticity budget analysis indicates that the TC movement can be predicted by the maximum vorticity tendency (VT). The orographic effects on the above TC track deflection are explained by the following: 1) Upstream of the mountain, the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic, which advects the TC to the southwest, analogous to the advection of a point vortex embedded in a flow. The VT is primarily dominated by the horizontal vorticity advection. 2) The TC passes over the mountain anticyclonically, mainly steered by the orographically generated high pressure. This makes the TC move southwestward (northwestward) over the upslope (lee slope). The VT is mainly contributed by the horizontal vorticity advection with additional contributions from vorticity stretching and the residual term (which includes friction and subgrid turbulence mixing). 3) Over the lee slope and downstream of the mountain, the northwestward movement is enhanced by asymmetric diabatic heating, making the turning more abrupt. 4) Far downstream of the mountain, the VT is mainly contributed by the horizontal vorticity advection.