Bound and free surface waves in a large wind-wave tank

Microwave and acoustic systems operated in the large wind-wave tank in Luminy, Marseille, France, show that most small-scale waves produced at large angles to the wind are products of breaking events bound to longer waves in the tank. These longer waves propagate at the dominant wave phase speed for fetches near 7 m but travel at speeds corresponding to the phase speed of a wave half as long as the dominant wave at fetches near 26 m. The microwave and acoustic systems operated at both 8 mm and 2 cm wavelengths. They were set to look at the same surface spot simultaneously at the same incidence and azimuth angles. Measurements were made at seven wind speeds, five incidence angles, seven azimuth angles, and two nominal fetches. Two peaks were found in either the microwave or acoustic Doppler spectrum when looking upwind or downwind but never in both. The low-frequency peak is due to Bragg scattering from freely propagating short waves, while the high-frequency peak is a result of Bragg scattering from short waves bound to longer waves. At azimuth angles not aligned with the wind direction the high-frequency peak was found to move lower until it merged with the low-frequency peak at azimuth angles around 60degrees. Fitting the first moments of these Doppler spectra along with the backscattering cross sections to a model of free wave/bound wave scattering showed that the intensity of bound and free short waves generally decreased with azimuth angle but that free wave spectral densities decreased more rapidly. Differences in microwave and acoustic cross sections confirmed that the bound waves were tilted by their parent waves. Spectral densities of bound and free waves were estimated individually by fitting the data to the model. The sum of these spectral densities, the total short-wave spectral density, was similar to, but lower than, previous measurements. The nature of millimeter-length bound waves was found to be different at long fetches than at short fetches, a feature not observed in centimeter-length bound waves.