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.

http://dx.doi.org/10.1175/mwr-d-17-0170.1

Journal Article

Grant no. NA17OAR4320101