Observations of whitecap coverage and the relation to wind stress, wave slope, and turbulent dissipation

Abstract

Shipboard measurements of whitecap coverage are presented from two cruises in the North Pacific, and compared with in situ measurements of wind speed and friction velocity, average wave steepness, and near‐surface turbulent dissipation. A threshold power law fit is proposed for all variables, which incorporates the flexibility of a power law with the threshold behavior commonly seen in whitecapping. The fit of whitecap coverage to wind speed, U10, closely matches similar relations from three recent studies, particularly in the range of 6–14 m/s. At higher wind speeds, the whitecap coverage data level off relative to the fits, and an analysis of the residuals shows some evidence of reduced whitecapping in rapidly developing waves. Wave slope variables are examined for potential improvement over wind speed parameterizations. Of these variables, the mean square slope of the equilibrium range waves has the best statistics, which are further improved after normalizing by the directional spread and frequency bandwidth. Finally, the whitecap coverage is compared to measurements of turbulent dissipation. Though still statistically significant, the correlation is worse than the wind or wave relations, and residuals show a strong negative trend with wave age. This may be due to an increased influence of microbreaking in older wind seas.

Estimation of bubble-mediated air–sea gas exchange from concurrent DMS and CO<sub>2</sub> transfer velocities at intermediate–high wind speeds

“…2a). The whitecap vs. wind speed relationship for Knorr_11 is similar in shape to recently published windspeed-based whitecap parameterisations (Callaghan et al, 2008;Schwendeman and Thomson, 2015). At intermediate wind speeds the Knorr_11 whitecap data are lower than the parameterisations (Fig.…”

“…Callaghan et al, 2008;Schwendeman and Thomson, 2015). Bubble-mediated gas transfer is the only viable explanation for the magnitude and wind speed dependence of k w .…”

“…Both k bub,CO 2 models depend on the choice of wind speed versus whitecap parameterisation. Using the Schwendeman and Thomson (2015) whitecap parameterisation instead of the Knorr_11 best fit makes some difference to the model output, but not enough to adequately fit to the data (Fig. 8).…”

“…2) and mean SST (Woolf, 1997: black;Asher et al, 2002: red). Model calculations were also performed using the Schwendeman and Thomson (2015) wind speed versus whitecap areal fraction relationship (dashed lines). they reach the sea surface.…”

“…Whitecaps were observed during Knorr_11 when wind speeds exceeded 4.5 m s −1 , a typical wind speed threshold for whitecap formation in the open ocean (Callaghan et al, 2008;Schwendeman and Thomson, 2015). Whitecap areal fraction is a strong, non-linear function of wind speed (Fig.…”