2.1 Potential for Parameter Estimation of Tropical Cyclone Air–Sea Enthalpy and Momentum Exchange Coefficients through Ensemble Data Assimilation

Monday, 13 January 2020: 10:30 AM
104C (Boston Convention and Exhibition Center)
Robert G. Nystrom, NCAR, Bolder, CO; and F. Zhang

The tropical cyclone (TC) surface exchange coefficients of enthalpy (Ck) and momentum (Cd) at high wind speeds have been notoriously challenging to estimate as a result of many factors including the difficulties in collecting observation 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 current uncertainty in the physical representation of the air-sea fluxes of momentum and enthalpy at high wind speeds, we investigate the potential to constrain the surface enthalpy and momentum exchange coefficients 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 suggests potential to use airborne radial velocity and all-sky satellite observations to estimate these unknown parameters. Using a series of observing system simulation experiments (OSSEs) and a known truth we examine the potential for simultaneous state and parameter estimation to converge on the correct known parameter values. We also examine the observations necessary to adequately constrain the surface exchange parameters and potential challenges and limitations. Lastly, we explore future potential benefits from utilizing strongly coupled ocean-atmosphere data assimilation to further constrain the uncertainty in unknown surface exchange parameters by examining ensemble correlations between ocean state variables and the unknown parameter values.
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