Capillary rollers and bores

A very intriguing phenomenon, which throws much light on the small-scale structure of the sea surface, is the occurrence of “parasitic capillaries” on the forward face of moderately short gravity waves, especially those with wavelengths 5 to 50 cm; see Figure la. These capillary waves were first studied experimentally by Cox (1958). Evidently their existence depends on the fact that a gravity wave and a much shorter capillary wave may have the same phase speed. The dynamical theory of the generation of parasitic capillaries has been developed by Longuet-Higgins (1963), Crapper (1970) and Ruvinsky et al. (1981, 1985, 1991). This so far takes into account only the first-order effects of viscous damping.