Impacts of Ocean Cooling and Reduced Wind Drag On Hurricane Katrina (2005) Based On Numerical Simulations

Abstract

Tropical cyclone (TC) intensity is strongly influenced by surface fluxes of momentum and moist enthalpy (typically parameterized in terms of “exchange coefficients” Cd and Ck, respectively). The behavior of Cd and Ck remains quite uncertain especially in high wind conditions over the ocean; moreover, moist enthalpy flux is extremely sensitive to sea surface temperature (SST). This study focuses on numerical simulations of Hurricane Katrina (2005) from an atmosphere–ocean coupled modeling system to examine the combined impacts of air–sea flux parameterizations and ocean cooling on TC evolution. Three momentum flux options—which make Cd increase, level off, or decrease at hurricane-force wind speeds—with five different Ck curves are tested. Maximum 10-m wind speed Vmax is highly sensitive to Cd, with weaker sensitivities for minimum sea level pressure Pmin and track. Atmosphere-only runs that held SST fixed yielded TCs with Pmin substantially deeper than observations. Introducing ocean coupling weakens TC intensity with much more realistic Pmin. The coupled run with the flux parameterization that decreases Cd at high wind speeds yields a simulated TC intensity most consistent with observations. This Cd parameterization produces TCs with the highest Vmax. Increasing Ck generally increases surface heat fluxes and thus TC intensity. For coupled runs using the default Ck parameterization, the simulated SST fields are similar (regardless of Cd parameterization) and agree well with satellite observations. The mesoscale oceanic eddies, which are well resolved in the ocean model, contribute to the magnitude of TC-induced SST cooling and greatly influence TC intensity.