Potential for new constraints on tropical cyclone surface-exchange coefficients through simultaneous ensemble-based state and parameter estimation

Abstract:

The tropical cyclone (TC) surface-exchange coefficients of enthalpy (C_k) and momentum (C_d) at high wind speeds have been notoriously challenging to estimate. This difficulty arises from many factors, including the difficulties in collecting observations within the turbulent TC boundary layer, and the complex coupled physical interactions between the TC boundary layer and ocean surface, which are challenging to accurately model. Motivated by recent studies highlighting the limited practical predictability of TC intensity as a result of uncertainty in the physical representation of the air-sea fluxes of momentum and enthalpy at high wind speeds, the potential to estimate the surface enthalpy and momentum exchange coefficients are investigated through ensemble data assimilation. Significant ensemble correlations between tangential wind, radial wind, and simulated infrared brightness temperatures with parameters controlling the enthalpy and momentum exchange coefficients suggest potential to use all-sky satellite and/or airborne radial velocity observations to estimate these unknown parameters. Using a series of observing system simulation experiments (OSSEs), simulated infrared brightness temperature observations, and a known truth, the potential for simultaneous state and parameter estimation with an ensemble-based data assimilation system to converge toward the correct known parameter values is demonstrated. In all OSSEs with either one or multiple unknown parameters, the initial parameter estimates are improved through simultaneous model state and parameter estimation. Lastly, we demonstrate that the successful parameter estimation related to the surface-exchange coefficients found in this study can significantly improve the ensemble prediction of TC intensity and structure.