Impacts of ocean cooling and reduced wind drag on Hurricane Katrina (2005) based on numerical simulations

Tropical cyclone (TC) intensity is strongly influenced by surface fluxes of momentum and moist enthalpy (typically parameterized in terms of "exchange coefficients" C_d and C_k, respectively). The behavior of C_d and C_k 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 C_d increase, level off, or decrease at hurricane-force wind speeds-with five different C_k curves are tested. Maximum 10-m wind speed V_max is highly sensitive to C_d, with weaker sensitivities for minimum sea level pressure P_min and track. Atmosphere-only runs that held SST fixed yielded TCs with P_min substantially deeper than observations. Introducing ocean coupling weakens TC intensity with much more realistic P_min. The coupled run with the flux parameterization that decreases C_d at high wind speeds yields a simulated TC intensity most consistent with observations. This C_d parameterization produces TCs with the highest V_max. Increasing C_k generally increases surface heat fluxes and thus TC intensity. For coupled runs using the default C_k parameterization, the simulated SST fields are similar (regardless of C_d 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.