A UNIFIED DIRECTIONAL SPECTRUM FOR LONG AND SHORT WIND-DRIVEN WAVES

Review of several recent ocean surface wave models finds that while co mprehensive in many regards, these spectral models do not satisfy cert ain additional, but fundamental, criteria. We propose that these crite ria include the ability to properly describe diverse fetch conditions and to provide agreement with in situ observations of Cox and Munk [19 54] and Jahne and Riemer [1990] and Hara et al. [1994] data in the hig h-wavenumber regime. Moreover, we find numerous analytically undesirab le aspects such as discontinuities across wavenumber limits, nonphysic al tuning or adjustment parameters, and noncentrosymmetric directional spreading functions. This paper describes a two-dimensional wavenumbe r spectrum valid over all wavenumbers and analytically amenable to usa ge in electromagnetic models. The two regime model is formulated based on the Joint North Sea Wave Project (JONSWAP) in the long-wave regime and on the work of Phillips [1985] and Kitaigorodskii [1973] at the h igh wavenumbers. The;omnidirectional and wind-dependent spectrum is co nstructed to agree with past and recent observations including the cri teria mentioned above. The key feature of this model is the similarity of description for the high- and low-wavenumber regimes; both forms a re posed to stress that the air-sea interaction process of friction be tween wind and waves (i.e., generalized wave age, u/c) is occurring at all wavelengths simultaneously. This wave age parameterization is the unifying feature of the spectrum. The spectrum's directional spreadin g function is symmetric about the wind direction and has both wavenumb er and wind speed dependence. A ratio method is described that enables comparison of this spreading function with previous noncentrosymmetri c forms. Radar data are purposefully excluded from this spectral devel opment. Finally, a test of the spectrum is made by deriving roughness length using the boundary layer model of Kitaigorodskii. Our inference of drag coefficient versus wind speed and wave age shows encouraging agreement with Humidity Exchange Over the Sea (HEXOS) campaign results .