The Impacts of Uncertainty in Air–Sea Enthalpy and Momentum Exchange Coefficients on Tropical Cyclone Predictability and Intensification

Tropical cyclone predictability is currently limited by both intrinsic, related to the chaotic nature of the atmosphere, and also practical, related to initial condition and model physics uncertainty, limitations. In this study, we focus on the model physics uncertainty associated with the air-sea interactions, which has been previously shown to significantly modulate the development, structure and intensification of tropical cyclones. Uncertainties in the representation and parameterization of the air-sea fluxes exists, at least in part, because of the difficulty in obtaining observations of the air-sea fluxes of enthalpy and momentum at high wind speeds. Using high resolution idealized CM1 simulations and WRF ensemble forecasts of Hurricane Patricia (2015) initialized with initial perturbations from a cycling ensemble data assimilation system, with and without ocean coupling, the impacts of the air-sea fluxes uncertainties on the development, structure and intensification are examined. The physics uncertainties manifested through altering the enthalpy and momentum exchange coefficients in the air-sea flux parameterization scheme results in intensity forecast uncertainty comparable to that from initial condition uncertainty, suggesting realistic physics uncertainty associated with the representation and parameterization of air-sea fluxes can be a significant source of uncertainty limiting the current predictability of intense tropical cyclones. Additionally, the inclusion of ocean coupling is shown to not reduce the intensity uncertainty resulting from uncertain enthalpy and momentum exchange coefficients. The impacts of the air-sea fluxes on the underlying dynamics of tropical cyclone development, structure and intensification are further investigated through application of the self-stratification theory from Emanuel and Rotunno (2011). Differences in air-sea fluxes are shown to significantly modulate M ds^*/dM and, as a result, the intensification rate, peak intensity, and overall storm structure. Additionally, the contribution from the unbalanced flow to the steady state maximum intensity is shown to be modulated by the surface enthalpy and momentum exchange coefficients. Ongoing work hopes to not only improve our understanding of the role of air-sea fluxes on tropical cyclone intensification, but also reduce the physics uncertainty encapsulated within the air-sea flux parametrization through simultaneous state and parameter estimation with a cycling ensemble data assimilation system.