Whitecap Coverage Dependence on Wind and Wave Statistics as Observed during SO GasEx and HiWinGS

Abstract

Concurrent wavefield and turbulent flux measurements acquired during the Southern Ocean (SO) Gas Exchange (GasEx) and the High Wind Speed Gas Exchange Study (HiWinGS) projects permit evaluation of the dependence of the whitecap coverage W on wind speed, wave age, wave steepness, mean square slope, and wind-wave and breaking Reynolds numbers. The W was determined from over 600 high-frequency visible imagery recordings of 20 min each. Wave statistics were computed from in situ and remotely sensed data as well as from a WAVEWATCH III hindcast. The first shipborne estimates of W under sustained 10-m neutral wind speeds U10N of 25 m s−1 were obtained during HiWinGS. These measurements suggest that W levels off at high wind speed, not exceeding 10% when averaged over 20 min. Combining wind speed and wave height in the form of the wind-wave Reynolds number resulted in closely agreeing models for both datasets, individually and combined. These are also in good agreement with two previous studies. When expressing W in terms of wavefield statistics only or wave age, larger scatter is observed and/or there is little agreement between SO GasEx, HiWinGS, and previously published data. The wind speed–only parameterizations deduced from the SO GasEx and HiWinGS datasets agree closely and capture more of the observed W variability than Reynolds number parameterizations. However, these wind speed–only models do not agree as well with previous studies than the wind-wave Reynolds numbers.

Wave‐Related Reynolds Number Parameterizations of CO2 and DMS Transfer Velocities

“…They were supplemented by a cruise‐specific WW3 hindcast for the entire period of the cruise. The WW3 hindcast, described in Brumer et al (), matches the buoy measurements well and provide the most complete data set. The model directional spectra along the cruise track will be used for the results reported here.…”

“…The latest direct eddy correlation fluxes of CO 2 and DMS were made during the High Wind Gas exchange Study (HiWinGS) (Blomquist et al, ; Brumer et al, ; Yang et al, ) at wind speeds up to ~25 m s −1 . The wavefield was monitored with a Riegl laser altimeter and a Datawell DWR‐4G Waverider buoy.…”

“…Similarly to k , W is typically modeled as a nonlinear function of wind speed. However, several studies have shown that W can be better constrained by a function of breaking ( R B ) and wave‐wind ( R H ) Reynolds numbers (Brumer et al, ; Goddijn‐Murphy et al, ; Zhao & Toba, ). Norris et al () provided further evidence that wave‐breaking‐related processes may be better constrained by Reynolds numbers than by wind speed alone, showing R H to be a more adequate predictor for sea spray aerosol flux, which results from breaking waves, than wind speed.…”

“…These relationships were determined for growing wind‐sea conditions. Recent studies (Brumer et al, ; Goddijn‐Murphy et al, ) showed that statistics computed from the total wave spectra, including swells, captured observed variability in W as well as wind‐sea‐only statistics suggesting an extended applicability of Reynolds number parameterizations. To date, few gas transfer measurements have been made with concurrent wave physics observations, and the universality of these parameterizations has yet to be assessed.…”

Constraining the Surface Flux of Sea Spray Particles From the Southern Ocean

“…In order to assess the conditions under which the parameterization presented above may be applicable, we analyzed global whitecap data from the literature. In Figure b we have amalgamated 527 in situ ship‐borne and tower observations of the surface coverage of whitecaps as a function of the 10‐m wind speed, (Callaghan et al, ; Bell et al, ; Brumer et al, ; Jia & Zhao, ; Norris et al, ; Schwendeman & Thomson, ; Stramska & Petelski, ; Sugihara et al, ; Xu et al, ). Studies published after the year 2000 were used since they all employed some form of automated image processing.…”