Optical Measurement of Short Wind Waves --- from the Laboratory to the Field

Abstract

With the Reflective Stereo Slope Gauge and the Medium Angle Slope Gauge, two new imaging instruments for the measurement of short wind waves on the ocean have been developed, validated, and deployed to a four week field experiment. Using active illumination with near-infrared LED light sources they are independent of natural light. Unlike other reflection-based techniques, they can be operated day and night under a wide range of environmental conditions. The instruments' performance was carefully validated in the laboratory and in the field. Their unique capabilities of simultaneously measuring the twodimensional slope probability distribution and the mean square slope (mss) of the waves, as well as wave height and a parameter linked to local surface curvature are demonstrated. Extensive measurements of short wave statistics from an air-sea interaction experiment off the coast of Peru are reported. A large variability of surface conditions due to the changing presence of surfactants on spatial scales smaller than one hundred meters was encountered. In a laboratory experiment, the dependence of gas transfer velocities on the suppression of waves by the soluble artificial surfactant Triton X-100 was investigated. It is shown that mss describes gas transfer velocities better than wind speed or the friction velocity. The new instruments can provide robust routine ship-borne measurements of mss, a key component in the effort of replacing wind speed with mean square slope as the standard parameter for gas transfer velocities.