Sensitivity of Simulated Tropical Cyclones to Air-Sea Flux Parameterizations

Tropical cyclones (TCs) are highly influenced by fluxes of momentum and moist enthalpy across the air-sea interface. These small-scale fluxes cannot be explicitly resolved in numerical weather prediction models and thus must be parameterized. The most widely used parameterization incorporates exchange coefficients for momentum (C_D) and moist enthalpy (C_k). Previous studies on the sensitivities of TC intensity and structure to the parameters C_D and C_k assumed them to be independent and constant. Nevertheless, present-day models such as the Weather Research and Forecasting (WRF) parameterize C_D and C_k as functions of wind speed and (in agreement with similarity theory) express C_k as a function of C_D.

This study uses hundreds of high-resolution WRF simulations of Hurricane Katrina (2005) to investigate the sensitivity of TC intensity and structure to systematic changes in these more sophisticated parameterizations of C_D and Ck. It is found that increased C_D (surface drag) yields not just lower near-surface wind speed, but also a smaller radius of maximum winds and lower minimum central pressure. The latter result is a secondary effect because of the dependence of C_k on C_D. Changes to C_k only, however, do not impact the radius of maximum winds or the relationship between minimum central pressure and maximum near-surface wind speed; instead, increased C_k yields higher near-surface wind speed and lower minimum central pressure. In addition to providing insight into TC dynamics, the present results also have implications for reducing model error.