IN-SITU MEASUREMENTS OF CAPILLARY-GRAVITY WAVE SPECTRA USING A SCANNING LASER SLOPE GAUGE AND MICROWAVE RADARS

Capillary-gravity wave spectra are measured using a scanning laser slo pe gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanogr aphic Institution at Woods Hole, Massachusetts. Wave spectral densitie s estimated from the radar measurements using the Bragg theory agree w ith those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of c apillary-gravity waves is in reasonable agreement with measurements by Jahne and Riemer (1990) obtained from measurements in a large wind-wa ve flume at intermediate wind speeds, but our data indicate a higher d egree of saturation at very low wind speeds. The rate at which the slo pe-frequency spectrum falls off, however, is much lower in the field t han in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary-gravity waves. Clo se examination of combined wavenumber-frequency slope spectra also rev eals significant smearing of the spectra in the frequency domain due t o long waves. These observations confirm that spatial measurements (wa venumber spectra measurements) are essential for characterizing short capillary-gravity waves, since this strong Doppler shift will dramatic ally change apparent frequency spectra.