Swell and Whitecapping—A Numerical Experiment

Swell enhances wind-induced wave growth in the third-generation wave models of the WAM family (e.g., WAM, SWAN, TOMAWAC). This is contrary to observations in laboratory conditions where swell reduces wind-induced wave growth. To improve the performance of these models, it is hypothesized here that high-frequency whitecapping is increased by surface-straining near the crests of low-frequency waves. The pulse-based parameterization of whitecapping in these models is accordingly scaled at every frequency with the ratio of high-frequency steepness over total wave steepness (in an experimental version of SWAN). A generic laboratory experiment which illustrates the phenomenon can now be reproduced. The growth of young wind sea in idealized conditions is not affected. Moreover, the spectral shape (both in frequency and direction) improves. However, in the one field case that is considered here, the effects are marginal.