On the directional distributions of the observation-based wind input and dissipation terms in a third-generation wave model

Abstract

New observation-based directional spreading functions for the wind input term $S_{in}$ and the dissipation term $S_{ds}$ have been investigated in third-generation wave models (WAVEWATCH III, ST6 version). In this study we evaluate bimodal directional distribution for $S_{ds}$ and narrower (${cos}^2\theta$ and ${cos}^4\theta$) distributions for $S_{in}$. The impact of these new directional functions on the integral wave parameters, as well as the nondirectional and directional spectra, is analyzed through a set of numerical experiments and their results are compared against buoy observations in the Caribbean Sea. Improvements in the estimation of $H_s$ and $T_p$ under wind-sea dominant conditions are presented. For the shape of frequency spectra, the bimodal distribution of $S_{ds}$ causes a weaker energy enhancement by comparison with the narrower distribution of $S_{in}$, but it generally shows a better fit than the default functions in the model. The combined effect of both modifications brings larger energy allocation around the main wave propagation direction, generating both an energy increase and a downshift at the spectral peak. These findings suggest the relevance of including realistic directional distributions and reveal the need for further studies of directional distributions of the source terms.