Bound waves and sea-surface slopes

For several years, suggestions have been made that sea surface roughness cannot be adequately explained in terms of weakly interacting, wind-generated waves. Rather, ripples that are generated by longer waves, are tilted by them, and are moving at nearly the speed of the longer wave have been postulated to coexist with waves that are directly generated by the wind, that is, with free waves. Effects of these bound waves have been detected in microwave backscatter on the ocean and in wind-wave tanks, and in surface-slope probability density functions (PDFs) in wind-wave tanks. Here we show that Cox and Munk's sea-surface slope PDFs are fully consistent with this bound wave/free wave model. From the fits of these PDFs to the model, we conclude that probabilities of finding bound waves are nearly the same in wind-wave tanks and on the ocean, and both are much higher than the probability of observing whitecaps. Variances of both bound and free waves are larger on the ocean than in tanks.