Optical Measurements of Capillary-Gravity Wave Spectra Using a Scanning Laser Slope Gauge

Abstract

Capillary-gravity wave spectra are measured using a scanning laserslope gauge (SLSG), and simultaneously by X and K band Doppler radarsoff the Chemotaxis Dock at the Quissett campus of the Woods HoleOceanographic Institution at Woods Hole, Massachusetts. Wave spectraldensities estimated from the radar measurements using the Bragg theoryagree with those measured using the SLSG at the Bragg wavenumberto within a few decibels, suggesting that Bragg scattering theoryis valid for the conditions of this experiment. The observed degreeof saturation of capillary-gravity waves is in reasonable agreementwith measurements by Jahne and Riemer (1990) obtained from measurementsin a large wind-wave flume at intermediate wind speeds, but the authors'data indicate a higher degree of saturation at very low wind speeds.The rate at which the slope-frequency spectrum falls off, however,is much lower in the field than in laboratories, even at moderatewinds, suggesting long waves are responsible for a large Dopplershift of capillary-gravity waves. Close examination of combined wavenumber-frequencyslope spectra also reveals significant smearing of the spectra inthe frequency domain due to long waves. These observations confirmthat spatial measurements (wavenumber spectra measurements) are essentialfor characterizing short capillary-gravity waves, since this strongDoppler shift will dramatically change apparent frequency spectra.