Two-dimensional wave number spectra of small-scale water surface waves

Two-dimensional wave slope spectra have been measured in the large Delft wind-wave facility using an imaging optical technique and digital image processing. The data cover wavelengths from 0.4 to 24 cm and wind speeds (U 10) from 2.7 to 17.2 ms-1. The spectral densities of small gravity waves at higher wind speeds are proportional to k ^-3.5 and u*. Capillary-gravity and capillary waves show features which clearly manifest that the energy balance for these waves is much different from that for gravity waves. The degree of saturation is approximately constant at a given wind speed, but strongly increases with friction velocity (proportional to u*^2.5). A sharp cutoff, which is almost independent of the wind speed, occurs at a wavelength of about 7 mm.