Air-Sea Turbulent Fluxes From a Wave-Following Platform During Six Experiments at Sea

Bourras, Denis ; Cambra, Rémi ; Marié, Louis ; Bouin, Marie-Noëlle ; Baggio, Lucio ; Branger, Hubert ; Beghoura, Houda ; Reverdin, Gilles ; Dewitte, Boris ; Paulmier, Aurélien ; Maes, Christophe ; Ardhuin, Fabrice ; Pairaud, Ivane ; Fraunié, Philippe ; Luneau, Christopher ; Hauser, Danièle

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Turbulent fluxes at the air-sea interface are estimated with data collected in 2011 to 2017 with a low-profile platform during six experiments in four regions. The observations were carried out with moderate winds (2-10 m/s) and averaged wave heights of 1.5 m. Most of the time, there was a swell, with an averaged wave age (the ratio between wave phase speed and wind speed) being equal to 2.8 ± 1.6. Three flux calculation methods are used, namely, the eddy covariance (EC), the inertial dissipation (ID), and the bulk methods. For the EC method, a spectral technique is proposed to correct wind data from platform motion. A mean bias affecting the friction velocity (u*) is then evaluated. The comparison between EC u* and ID u* estimates suggests that a constant imbalance term (ϕimb) equal to 0.4 is required in the ID method, possibly due to wave influence on our data. Overall, the confidence in the calculated u* estimates is found to be on the order of 10%. The values of the drag coefficient (CD) are in good agreement with the parameterizations of Smith (1988, <a href="" target="_blank"></a>) in medium-range winds and of Edson et al. (2013, <a href="" target="_blank"></a>) in light winds. According to our data, the inverse wave age varies linearly with wind speed, as in Edson et al. (2013, <a href="" target="_blank"></a>), but our estimates of the Charnock coefficient do not increase with wind speed, which is possibly related to sampling swell-dominated seas. We find that the Stanton number is independent from wind speed.</p>

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