IDEALIZED SIMULATIONS OF THE INNER CORE BOUNDARY LAYER STRUCTURE IN A LANDFALLING TROPICAL CYCLONE. PART I: KINEMATIC STRUCTURE

The effects of coastal topography and coastal location in the distribution of boundary layer winds in the inner core of mature tropical cyclones are examined using a high-resolution multi-level model. In these numerical simulations, the evolution of the tropical cyclone boundary layer (TCBL) is studied in storm-relative coordinates, and in lieu of an actual steering current moving the model vortex, the position of the land-sea interface was shifted through the grid domain at a constant speed with separate surface boundary conditions specified over the land and ocean areas. It is shown that the presence of a coastal boundary produces land-induced asymmetries (along with an internal boundary layer) due to the asymmetric structure of surface drag. This land-induced asymmetry is found in both the azimuthal and radial wind field at landfall. For a moving storm, nonlinear advective interactions between storm-induced asymmetries and land-induced asymmetries can generate a low-level vorticity band ahead of the hurricane. When the storm motion vector has a component that is perpendicular to the coastal boundary, the interaction between this band and the mean vortex leads to a temporary weakening and re-intensification cycle. Furthermore, it is shown that the relative magnitude of the land-induced asymmetry depends upon the terrain slope and the terrain height such that the land-induced asymmetry dominates over the motion-induced asymmetry for elevated terrain. These results underscore the specific differences in boundary layer evolution and intensity evolution for hurricanes interacting with complex topographical features.