The coupling between wind and waves in the WAM model

The reliability of the wave model (WAM, cycle 4) for predicting waves and w ind stress in restricted fetches is investigated using measured data obtain ed during the Riso Air-Sea Experiment (RASEX) at Vindeby, Denmark. The WAM model includes Janssen's theory for calculating sea roughness as a function of wave spectra. RASEX is characterized by being located in relatively sha llow waters (depths of about 3 to 4 m in an area where the waves are predom inantly fetch limited, with a maximum fetch of about 20 km). Comparison between WAM results and measured data (integral wave parameters and friction velocities) shows fair agreement for moderate winds (U-10 simi lar or equal to 10 m s(-1)) but significant overprediction for strong winds . Analysis of the WAM results for sea roughness yields a trend of increasin g dimensionless roughness with inverse wave age, as obtained from field dat a; however, the WAM values are generally higher than that obtained from fie ld data. It is shown that inclusion of depth-induced wave breaking does nor explain the overprediction of measured wind stress and associated wave heights. Fur thermore, it is shown that using the measured wind Friction velocities to f orce the WAM model significantly reduces the wave height overprediction for strong winds. These investigations indicate that further improvements are required before the WAM model can be reliably used in shallow and fetch-limited areas, suc h as Vindeby.