Generation of short waves by wave-current interaction

Measurements of blocking of gravity waves by inhomogeneous currents are presented. In the wave tank at the NASA Air-Sea Interaction Research Facility, Wallops Island, VA, mechanically-generated periodic waves (1-3 Hz) were blocked upstream from the wave paddle by an opposing current (20-45 cm s(-1)). A false bottom was used to obtain a spatially-varying flow field in the measurement section. For a given current speed, the frequency of the waves was adjusted so that the blocking occurred within the observed footprint. An imaging slope gauge (ISG) was used to measure short waves with wavelengths from 1 mm to 25 cm and frequencies up to 50 Hz, as well as the surface flow field. Time series of wave elevation were recorded with wire wave gauges at three locations in the wave tank. The captured image sequences show the blocking of the gravity waves and the coincidental generation of steep capillary wave trains in the current gradient zone. Profiles derived from the individual wave slope images show that the generated capillary waves have wavelengths as short as 3 mm.