Whitecaps in deep water

Abstract

Wavenumber-frequency spectra of space-time images of radar backscatter from the ocean exhibit a low-frequency feature which is nearly linear and passes through zero. This feature cannot be explained by second-order wave-wave interactions, shadowing, or wind turbulence. It can, however, be reproduced in simulations of sea surfaces in which breaking occurs at locations of maximum slope on the wind-wave interference pattern. Using this finding, we varied the peak of the wind-wave spectrum to produce a plot of the speed (slope) of the low-frequency feature, ie, the speed of the wind-wave interference pattern, as a function of the speed of the dominant wave on the surface. We show that the speeds of whitecaps in deep water that have been measured by various means are very close to the speed of the interference pattern. Three conclusions are drawn: 1) whitecaps are produced when gravity waves in the range of 1 to 12 m, much shorter than dominant wavelengths, are steepened by the interference pattern of the dominant waves, and 2) video, microwave, acoustic techniques measure the same breaking wave speeds, and 3) interference patterns of wind waves move more slowly than the dominant wave group speed. © 2012 IEEE.