No abstract

Shipboard measurements of fractional whitecap coverage W and wind speed at 10-m height, obtained during the 2006 Marine Aerosol Production (MAP) campaign, have been combined with ECMWF wave model and Quick Scatterometer (QuikSCAT) satellite wind speed data for assessment of existing W parameterizations. The wind history trend found in an earlier study of the MAP data could be associated with wave development on whitecapping, as previously postulated. Whitecapping was shown to be mainly wind driven; for high wind speeds (.9 m s 21 ), a minor reduction in the scatter of in situ W data points could be achieved by including sea state conditions or by using parameters related to wave breaking. The W values were slightly larger for decreasing wind/developed waves than for increasing wind/developing waves, whereas cross-swell conditions (deflection angle between wind and swell waves between 6458 and 61358) appeared to dampen whitecapping. Tabulated curve fitting results of the different parameterizations show that the errors that could not be attributed to the propagation of the standard error in U 10 remained largely unexplained. It is possible that the counteracting effects of wave development and cross swell undermine the performance of the simple parameterizations in this study.

[1] Whitecap foam decay times for 552 individual breaking waves determined from digital images of the sea surface are reported. The images had sub-centimeter pixel resolution and were acquired at frame rates between 3 and 6 frames per second at the Martha's Vineyard Coastal Observatory over a 10-day period in 2008, subdivided into 4 observation periods. Whitecap foam decay times for individual events varied between 0.2 s to 10.4 s across the entire data set. A systematic positive correlation between whitecap foam decay time and maximum whitecap foam patch area was found for each observation period. For a given whitecap size within each observation period, the decay times varied between a factor of 2 and 5, with the largest variation occurring during unsteady environmental forcing conditions. Within observation periods, bin-averaged decay times varied by up to a factor of 4 across the range of foam patch areas. Between observation periods, the effective whitecap foam decay time, which we define as the area-weighted mean decay time, varied by a factor of 3.4 between 1.4 s and 4.8 s. We found a weak correlation between decay times and individual event-averaged breaking wave speeds. The variation in the active breaking area across all 4 observation periods was small, indicating relatively uniform surface whitecap area generating potential. We speculate that the variation in the foam decay times may be due to (i) the effect of surfactants on bubble and foam stability, and (ii) differences between bubble plume characteristics caused by a variation in breaking wave type.

A laboratory experiment to quantify whitecap foam decay time in the presence or absence of surface active material is presented. The investigation was carried out in the glass seawater channel at the Hydraulics Facility of Scripps Institution of Oceanography. Whitecaps were generated with focused, breaking wave packets in filtered seawater pumped from La Jolla Shores Beach with and without the addition of the surfactant Triton X-100. Concentrations of Triton X-100 (204 μg L−1) were chosen to correspond to ocean conditions of medium productivity. Whitecap foam and subsurface bubble-plume decay times were determined from digital images for a range of wave scales and wave slopes. The experiment showed that foam lifetime is variable and controlled by subsurface bubble-plume-degassing times, which are a function of wave scale and breaking wave slope. This is true whether or not surfactants are present. However, in the presence of surfactants, whitecap foam is stabilized and persists for roughly a factor of 3 times its clean seawater value. The range of foam decay times observed in the laboratory study lie within the range of values observed in an oceanic dataset obtained off Martha’s Vineyard in 2008.