Development of a coupled hurricane-wave-ocean model toward improving air-sea flux parameterization in high wind conditions

This study aims to improve hurricane forecasts of the operational GFDL/URI model by introducing a new coupled hurricane-ocean-wave system. The coupled system consists of the 2005 high-resolution GFDL hurricane model, the Princeton Ocean Model (POM), the WAVEWATCH III model, the URI wave boundary layer (WBL) model, and the URI equilibrium spectrum model.

The key component of the coupled system is a newly-developed air-sea momentum flux parameterization based on the wave fields. In the coupled system, the complete wave spectrum is constructed by merging the WAVEWATCH III spectrum in the vicinity of the spectral peak with the spectral tail parameterization. The result is incorporated into the WBL model to explicitly calculate the wave induced stress, the mean wind profile, and the roughness length (z0). In the coupled system, the estimated wave-field dependent roughness length is transferred into the GFDL model, replacing z0 of the current GFDL hurricane model based on the Charnock relation. For computational efficiency, we introduce a movable grid mesh configuration for the wave model as well as the MPI parallel computing system for the coupled model.

We have tested and evaluated the new coupled system for hurricanes in the 2004 and 2004 season by comparing the storm intensity forecasts and the spatial distributions of the surface wind with and without the wave coupling. The wind structure with the wave coupling is evidently in a better agreement with the HRD wind analysis and its intensity forecast is improved, in some cases significantly. This is accomplished by the use of more realistic wave-dependent surface momentum fluxes and their spatial distribution.

The new GFDL coupled system is planned to be implemented operationally at NCEP in 2006 and some elements to be transitioned to the Hurricane WRF.