Structural Asymmetries and Track Changes of Tropical Cyclones during Landfall: Beta-Plane Simulations

Numerical simulations are performed using the Weather Research and Forecasting (WRF) Model to investigate the effects of coastline orientation, land-surface moisture availability and friction on the motion and structure of a tropical cyclone (TC) on a beta-plane during landfall. Experiments are carried out by inserting a land surface with a north-south or west-east oriented coastline at ~525 km west or north respectively from a spun-up TC undergoing a northwestward beta drift. The results suggest that the pattern of secondary circulation within TC is modified over most of the troposphere prior to landfall, and such modification is most evident if the land surface friction is strong. Also, when the land is rough, different modifications of the secondary flow are found for different coastline orientations as a result of differential frictional convergence/divergence along the coast induced by both the symmetric TC flow and the asymmetric beta gyres. The southeasterly ventilation flow through the TC center created by the beta gyres is thus modified as a result and the vertical shear profile over the TC is also changed. For most of the rough-land cases, differential vertical wind shear apparently plays an important role for the change of convective structure of the TC during landfall. Together with the effect of enhanced frictional convergence created by onshore wind and/or convective instability due to three-dimensional advection of dry air when the surface is also dry, a wavenumber-2 rainfall asymmetry is established. These changes in the flow and convective asymmetries modify the potential vorticity tendency and thus affect the motion of the TC.