The Effect of Swell on the Generation and Dissipation of Wind Sea

Laboratory observations by others have shown that a following swell reduces the net growth of wind waves whereas an opposing swell enhances this growth. Computations show that third-generation wave models based on the commonly accepted WAM-III formulations do not reproduce these effects for following swell. For opposing swell they do, but this seems to be fortuitous. With some relatively simple modifications of the formulation for the generation and dissipation of the waves, both observed effects can now be reproduced. The modifications for the generation by wind consist of replacing the driving wind by an apparent wind (which depends on the swell orbital motion) and by scaling the wind input with the swell orbital motion. For dissipation (whitecapping) the modification consists of decoupling the whitecapping at high frequencies from the wave steepness at low frequencies.