Visual demonstration of three-scale sea-surface roughness under light wind conditions

During the Southern Ocean Waves Experiment (SOWEX) an aircraft carried a down-looking video camera to help document the sea surface. Reflected images of the aircraft were intermittently observed in the video recorded at 15-30-m height under light and variable wind conditions. A numerical simulation was developed to relate image contrast to the gravity-capillary wave contribution to sea-surface mean square slope (mss). "Carnival fun-house" mirror-type distortions of the image in the absence of the gravity-capillary waves relate to intermediate-scale wave persistence when wind forcing stops. Video image estimates of mss correlated better with 36-GHz scanning radar altimeter estimates than with the wind speed measured at 30-m height. When the gravity-capillary waves disappeared in the absence of wind forcing, about one-third of the 0.0015 residual mss was contributed by the dominant waves, and about two-thirds was contributed by the 1-10-m wavelength region. Near the shores of a lake in Alaska, reflected aircraft images were also observed, indicating that the gravity-capillary wave contribution to mss was only about 0.000 001, even though the wind speed at the 160-m aircraft height was 10 m/s.