Sensitivity of tropical cyclone simulations to parametric uncertainties in Air-Sea fluxes and implications for parameter estimation

Tropical cyclones (TCs) are strongly influenced by fluxes of momentum and moist enthalpy across the air-sea interface. These fluxes cannot be resolved explicitly by current-generation numerical weather prediction models, and therefore must be accounted for via empirical parameterizations of surface exchange coefficients (C_D for momentum and C_k for moist enthalpy). The resultant model uncertainty is examined through hundreds of convection-permitting Weather Research and Forecasting Model (WRF) simulations of Hurricane Katrina (2005) by varying four key parameters found in commonly used parameterizations of the exchange coefficient formulas. Two of these parameters effectively act as multiplicative factors for the exchange coefficients over all wind speeds (one each for C_D and C_k); the other two parameters control the behavior of C_D at very high wind speeds (i.e., above 33 m s^-1). It is found that both the intensity and the structure of TCs are highly dependent upon the two multiplicative parameters. The multiplicative parameter for C_D has a considerably larger impact than the one for C_k on the relationship between maximum 10-m wind speed and minimum sea level pressure: C_D alters TC structure, with higher values shifting the radius of maximum winds inward and strengthening the low-level inflow; C_k only affects structure by uniformly strengthening/weakening the primary and secondary circulations. The TC exhibits the greatest sensitivities to the two multiplicative parameters after a few hours of model integration, suggesting that these parameters could be estimated by assimilating near-surface observations. The other two parameters are likely more difficult to estimate because the TC is only marginally sensitive to them in small areas of high wind speed.