MODELING SPECTRAL DISSIPATION IN THE EVOLUTION OF WIND-WAVES .1. ASSESSMENT OF EXISTING MODEL PERFORMANCE

This study examines the performance of a state-of-the-art spectral win d wave model that uses a full solution to the nonlinear interaction so urce term. The situation investigated here is fetch-limited wind wave evolution, for which a significant observational database exists. The authors consider both the evolutionary characteristics such as the pre dicted development of wave energy and peak wave frequency with fetch, as well as the predicted local features of the directional wavenumber spectrum: the spectral shape of the dominant wave direction slice, tog ether with the directional spreading function. In view of the customar y practice of constraining the shape of the spectral tail region, this investigation required relaxing the constrained tail assumption. This has led to new insight into the dynamic role of the spectral tail reg ion. The calculations have focused on the influence of two of the sour ce terms in the spectral evolution (radiative transfer) equation for t he energy density spectrum-those due to wind input and to dissipation predominantly through wave breaking. While the form of the wind input source term exerts some influence, the major impact arises from the di ssipation source term, for which the authors explore a range of varian ts of the quasi-linear form proposed by Hasselmann. Due to the nonline ar coupling of spectral components through the wave-wave interaction t erm, it is only possible to obtain a detailed physical understanding o f spectral evolution through such numerical experiments. The results p oint to basic shortcomings in the present source terms. These lead to predicted local spectral properties and fetch evolution characteristic s that differ significantly from the available observations. It is con cluded that further refinement of the dissipation source term is requi red to improve modeling capabilities for wind sea evolution.