Modeling the Effects of Land–Sea Roughness Contrast on Tropical Cyclone Winds

Abstract

Abstract The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate tropical cyclone (TC) wind distribution near landfall. On an f plane at 15°N, the effects of the different surface roughness between the land and sea on the wind asymmetry is examined under a strong constraint of a dry atmosphere and time-invariant axisymmetric mass fields. The winds are found to adjust toward a steady state for prelandfall (50, 100, and 150 km offshore), landfall, and postlandfall (50, 100, and 150 km inland) TC positions. The TC core is asymmetric even when it lies completely offshore or inland. The surface (10 m) wind asymmetry at the core for pre- (post) landfall position is apparently related to the acceleration (deceleration) of the flow that has just moved over the sea (land) as a response to the sudden change of surface friction. For prelandfall TC positions, the resulted strong surface inflow to the left and front left (relative to the direction pointing from sea to land) also induces a tangential (or total) wind maxima at a smaller radius, about 90° downstream of the maximum inflow, consistent with the absolute angular momentum advection (or work done by pressure). The surface maximum wind is of similar magnitude as the gradient wind. There is also a small region of weak outflow just inside the wind maxima. For postlandfall TC positions, inflow is weakened to the right and rear right associated with the onshore flow. Both onshore and offshore flows affect the surface wind asymmetry of the core in the landfall case. Above the surface and near the top of the planetary boundary layer (PBL), the wind is also asymmetric and a strongly supergradient tangential wind is primarily maintained by vertical advection of the radial wind. Much of the steady-state vertical structure of the asymmetric wind is similar to that forced by the motion-induced frictional asymmetry, as found in previous studies. The associated asymmetry of surface and PBL convergences has radial dependence. For example, the landfall case has stronger PBL convergence to the left for the 0–50-km core region, due to the radial inflow, but to the right for the 100–500-km outer region, due to the tangential wind convergence along the coastline. The strong constraint is then removed by considering an experiment that includes moisture, cumulus heating, and the free adjustments of mass fields. The TC is weakening and the sea level pressure has a slightly wavenumber-1 feature with larger gradient wind to the right than to the left, consistent with the drift toward the land. The asymmetric features of the wind are found to be very similar to those in the conceptual experiments.