Laboratory study of the fine structure of breaking waves for scatterometry applications

Laboratory measurements of the fine space-time structure of short gravity-capillary waves, as well as Ku-band scattering at grazing and moderate incidence from spilling and plunging breaking waves in a wave channel are presented. Unsteady breaking waves are generated by focusing the waves in space and time. A scanning laser slope gauge was used for measuring short waves of wavelength 0.2-20 cm, and frequencies up to 150 Hz. A dual polarized (VV,HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to simultaneously obtain Doppler spectra of the scattered signals from the breaking area. It was found that the breaking splash is the main source of small-scale surface-wave generation. The short surface wave slope field produced by the breaking could be separated into regular short (5-10 mm wavelength), fast, bound waves, and free gravity-capillary waves. Both types of waves were found to co- and counter-propagate relative to the direction of the dominant wave propagation. Measurements of the Doppler frequency of the scattered signal are consistent with this description of the slope gauge measurements.