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

Abstract

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, https://doi.org/10.1029/JC093iC12p15467) in medium‐range winds and of Edson et al. (2013, https://doi.org/10.1175/JPO-D-12-0173.1) in light winds. According to our data, the inverse wave age varies linearly with wind speed, as in Edson et al. (2013, https://doi.org/10.1175/JPO-D-12-0173.1), 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.

Formation and Evolution of a Freshwater Plume in the Northwestern Tropical Atlantic in February 2020

“…Between the two crossings of the plume, the two vessels surveyed a salinity-stratified area on the shelf. During this survey, the free-drifting platform Ocarina (Bourras et al, 2019) was deployed for a little more than 12 h. This platform was equipped with a 1,200 kHz ADCP to measure current profiles at 0.5 m resolution between 0.29 and 17.29 m from the sea surface. These data are combined with temperature and salinity profiles collected from the RV Atalante MVP to estimate the gradient Richardson number (R i ) in this stratified near-surface layer at different times of the day.…”

Saildrone Direct Covariance Wind Stress in Various Wind and Current Regimes of the Tropical Pacific

“…We feel that this condition is overly strict, especially in tropical regions where swell-dominated light-wind conditions commonly occur, causing wave-coherent wind fluctuations (Prytherch et al 2015;Sullivan and McWilliams 2010). In summary, the motion correction seems to be sufficient to correct the wind spectra in the inertial subrange, but has a relatively light touch, compared to approaches that remove all wave-coherent wind oscillations (e.g., Bourras et al 2019) for lower-frequency oscillations. We suggest this is appropriate for swell conditions.…”

“…Therefore, the wind can include genuine wave-coherent perturbations (Högström et al 2015). This means that the criterion used by Bourras et al (2019) to compare motion correction methods}that they should remove all wave-coherent motions from the wind spectrum}is not suitable in our case. Despite the absence of a clear indicator of accuracy for motion correction, investigations of the Saildrone motion correction are presented below.…”