The EUREC4A experiment was conducted in January and February 2020 off the North-Eastern coast of the Barbados Island. Air-Sea fluxes and bulk variables were derived from in situ measurements performed at the top of a bow mast on the Ifremer Genavir R/V Atalante and with a wave-following platform. Comparisons between the collected in situ data will be presented first. Second, a spatial view of the flux fields will be given, with model output fields and with satellite sensor data. Third, the spatial products will be compared to the in situ data in order to get a first order estimate of the accuracy of the spatial fields. Last, the focus will be laid on the effect of the proximity of the coast, both in terms of wind and wave fetches and of model Land-Sea Mask (LSM) and blind costal satellite zones

With the flourishing of offshore wind projects there is a new socio-economic interest to better our knowledge and forecasting ability of winds within the coastal marine atmospheric boundary layer (MABL). Air-sea fluxes of enthalpy and momentum greatly influence the turbulent and mean winds in the MABL. Already at moderate but certainly at high winds, wave breaking is a key driver of air-sea fluxes and the sea spray generated by whitecaps is thought to be a crucial component when modelling air-sea interactions. Most studies so far have focused on the role of sea spray in enhancing tropical cyclone intensity. Here we investigate its impacts on the MABL under strong orographic wind forcing. A coupled model framework was developed within the scope of the CASSIOWPE project aiming at characterizing the physical environment in the Gulf of Lion (NW Mediterranean Sea) in the prospective of future floating wind farms development. It consists of the non-hydrostatic mesoscale atmospheric model of the French research community Meso-NH, the 3rd generation wave model WAVEWATCH III®, and the oceanic model CROCO. Sea-spray physics were incorporated into the Meso-NH’s surface model SURFEX. Added parametrizations will be detailed and a series of test cases will be presented to illustrate how sea spray alters the MABL under Mistral and Tramontane winds. Several sea-state dependent sea spray generation functions (SSGF) are considered in the present study. The variability in simulated fields linked to the choice of wave forcing or coupling will be showcased to evaluate their suitability in varying fetch conditions. Sea spray production remains to be adequately quantified. Existing measurement derived SSGFs span several orders of magnitude resulting in uncertainties in simulated fields which will be discussed.

Wave breaking is the main mechanism that dissipates energy input into ocean waves by wind and transferred across the spectrum by nonlinearity. It determines the properties of a sea state and plays a crucial role in ocean-atmosphere interaction, ocean pollution, and rogue waves. Owing to its turbulent nature, wave breaking remains too computationally demanding to solve using direct numerical simulations except in simple, short-duration circumstances. To overcome this challenge, we present a blended machine learning framework in which a physics-based nonlinear evolution model for deep-water, non-breaking waves and a recurrent neural network are combined to predict the evolution of breaking waves. We use wave tank measurements rather than simulations to provide training data and use a long short-term memory neural network to apply a finite-domain correction to the evolution model. Our blended machine learning framework gives excellent predictions of breaking and its effects on wave evolution, including for external data.

This paper reports on laboratory experiment results on wind-driven surface waves in finite depth and their comparison with theoretical predictions and experimental in-situ studies. We introduce the Miles theory's extension to the case of finite depth, as well as the rules transforming theoretical expressions to formulae commonly used in experimental routines, in particular the important rules transforming theoretical growth rates to experimental ones. Wind waves depend strongly on boundary marine layer parameters, namely, the aerodynamic roughness length, the Charnock constant, as well as the drag coefficient. Consequently, this work gives detailed measurements of these parameters in finite depths. Experiments conducted in the IRPHe/Pytheas wind-wave tank (Marseille, France), reveal that for a given wind speed, these values are higher in finite depth than in deep water. In the limit case of fully developed surface, due to depth, theoretical and empirical formulas relating the critical values of wave age to the non-dimensional depth have been experimentally verified. Plots of non-dimensional peak frequency, non-dimensional energy, and the inverse of wave age, against non-dimensional depth are presented. The plots clearly show that these quantities admit a limit due to the depth influence. All data obey the range of empirical values established in field experiments. Experimental data, obtained in the facility, agree with the theoretical family of depth-dependent wave growth rate as a function of wave age in finite depth. The non-dimensional growth-rate data obtained in our laboratory, as a function of the inverse of wave age, are consistent with the theoretical predictions of Miles theory in finite depth (Montalvo et al., 2013a,b, Latifi et al.. 2017), as well as measurements from other laboratories.

The accurate modelling and prediction of bimodal sea states, combining swell and wind waves, is of upmost importance for many applications such as wave overtopping of coastal protections. Yet, the discrepancy in the field observations and the wave model limitations make the modeling of this common sea state condition rather complex. The question guiding this paper is: are wind waves generated the same way with and without pre-existing swell? The approach we chose starts with laboratory measurements in a wind-wave facility showing that wind-sea growth is modified in the presence of long waves (representing swell). To upscale this observation to open oceans, a numerical spectral wave model is firstly validated by comparison with laboratory results, and then at coastal scale using in situ bimodal sea-state observations collected during the SHOWEX campaign. By a separation of the physical processes involved in wind-wave generation, numerical simulations allow to assess the role each physical process plays in the wind-wave growth when a swell system is present.

In February 2020, a 120-km-wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7 degrees N/54 degrees W-56 degrees W). It was initially stratified in the upper 10 m with a freshwater content of 2-3 m of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13-16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off-shelf flow lasted 10 days, contributing to a plume area extending over 100,000 km(2) and associated with a 0.15 Sv (10(6) m(3) s(-1)) freshwater transport. The off-shelf plume was steered northward by a North Brazil Current ring up to 12 degrees N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favorable wind direction closer to the Amazon estuary, which contributed to north-westward freshwater transport on the shelf. Other such events of freshwater transport in January-March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favoring the north-westward freshwater transport toward the shelf break.

Abstract. The science guiding the EUREC4A campaign and its measurements are presented. EUREC4A comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic – eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice.

We investigate the effects of wind–wave interactions on the surface sea-spray-generation flux. To this end, the Marine Aerosol Tunnel Experiment (MATE2019) was conducted at the Pytheas Institute large wave–wind facility in Luminy (Marseille, France) over the period June–July 2019. A unique range of air–sea boundary conditions was generated by configuring the laboratory with four types of wave forcing and five wind speeds spanning 8–20 m s $$^{-1}$$ . Young and developed waves were included, with wave ages between 1.3 and 9.5 (defined in terms of phase speed and friction velocity). Vertical sea-spray-concentration profiles measured over the 0.1–47.5 $$\upmu $$ m radius range and a flux–profile method allowed estimation of the sea-spray-generation flux. Results show that the flux increases for increased wind-induced wave breaking, and is highest for steep and heavily-breaking waves. Scaling analysis shows that the sea-spray generation is best correlated with the wave-slope variance for larger droplets (20 $$\upmu $$ m and above, assumed predominantly spume droplets generated by surface tearing). For smaller droplets (7–20 $$\upmu $$ m, presumed predominantly jet droplets generated by bubble bursting), the highest correlation is found with a non-dimensional number combining the wave-slope variance with the friction velocity cubed. This is reflected in the formulation of two wave-state-dependent sea-spray-generation functions, each valid for wind speeds 12–20 m s $$^{-1}$$ and droplet radii 3–35 $$\upmu $$ m, thereby covering jet and spume droplet production.

A comprehensive analysis of the wavefield evolution under accelerated wind conditions provides an essential contribution to understanding the wind-wave generation process. A set of experiments was carried out in a large wind-wave facility where it is possible to reproduce high wind speed conditions to study the wind acceleration effect in the wind-wave development. The facility was equipped with high-frequency sampling devices that provide accurate air turbulence and water surface displacement measurements. From this deployment, it was possible to describe the evolution of the wave characteristics under different magnitudes of constant wind acceleration in detail. This study analyzes the wind acceleration effect in the early stages of wind-wave generation and evolution. The increase of spectrum energy saturation level and the downshift of the peak frequency are processes associated with the spectral shape evolution under low acceleration wind conditions. Under high wind acceleration conditions, the spectral shape did not vary with wind speed and fetch. Despite under low acceleration wind conditions, the wavefield is more developed than under high acceleration wind conditions; there was no direct relation between wind acceleration and the wavefield efficiency to grow. Besides, during these early instants, it was observed that a more developed wave field, associated with low acceleration wind conditions, could slow down the increase of drag coefficient with wind speed.

The statistical design load from waves on offshore structures is often estimated by aid of experimental studies of irregular waves in wave flumes and basins. When going to the laboratory things are either added or omitted. One of the latter being the direct effect of an instantaneously wind field above the waves, and there is a concern that it can alternate the steepness of the waves. More recently experimental investigations of wave load statistics generated by irregular waves, points at an increase in wave-induced pressure and dynamic loading, when the direct effect of a wind field above the waves was taken into account. The investigations moreover, shows an increase in the number of breaking waves, which leads to the question if the extreme wave load events were increased due to more breaking waves or if the load from the individual breaking wave was increased, when wind was introduced in the tank. An attempt is carried out to answer if the direct effect of wind increases the wave load of an individual breaking wave event. Therefore, breaking waves both with and without wind, nevertheless with a quite similar wave profile, are investigated experimentally. Deterministic, focused waves are then generated with different distances from breaking point to a model. The point at which the wave is focusing is pushed further down tank, when the wind is introduced. Therefore, the input focus point and thereby the resulting breaking point is adjusted to achieve the same distance from breaking point to model, whether wind is present or not. The dynamical force and pressure are in this study highly dependent on the distance from the breaking point to the model, where the optimal breaking point here is detected around 0.15 meters before or after the model. The front steepness of the waves is increased, when wind is introduced. The maximum generated wave load and dynamic force are occurring in a case with wind. The picture for pressure is less clear, and is in one case largest for a case with wind, and in another largest for a case without wind. Due to the low number of repetition tests, the small differences observed between the wave-induced load in the case with and without wind and the intrinsically, stochastic nature of breaking waves, there is no basis for neither confirming nor disconfirming that the direct effect of wind above a breaking wave increases the load or the pressure.

A series of experiments were conducted in a wind-wave tank facility in Marseilles (France) to study the effects of preexisting swell conditions (represented by long mechanically-generated waves) on wind-wave growth with fetch. Both monochromatic and irregular (JONSWAP-type) long wave conditions with different values of wave steepness have been generated in the presence of a constant wind forcing, for several wind velocities. A spectral analysis of temporal wave signals combined with airflow measurements allowed to study the evolution of both wave systems with the aim of identifying the interaction mechanisms transportable to prototype scale. In particular, a specific method is used to separate the two wave systems in the measured bimodal spectra. In fetch-limited conditions, pure wind-wave growth is in accordance with anterior experiments, but differs from the prototype scale in terms of energy and frequency variations with fetch. Monochromatic long waves are shown to reduce the energy of the wind-waves significantly, as it was observed in anterior laboratory experiments. The addition of JONSWAP-type long waves instead results in a downshift of the wind-wave peak frequency but no significant energy reduction. Overall, it is observed that the presence of long waves affects the wind-wave energy and frequency variations with fetch. Finally, in the presence of JONSWAP-type long waves, variations of wind-wave energy and peak frequency with fetch appear in close agreement with the wind-wave growth observed at prototype scale both in terms of variations and nondimensional magnitude.

Recent studies stress the importance of considering sea surface wave characteristics in sea spray generation functions (SSGFs). To this end, the effect of interacting winds and waves on sea spray generation was studied using data collected during the Marine Aerosol Tunnel Experiments (MATE2019) conducted at the OSU-Pytheas large wind-wave tunnel facility at Luminy, Marseille (France) (Study detailed in Bruch et al., in review). A total of 20 wind and wave combinations were tested, with wind speeds between 8 and 20 m s-1 combined with pure wind waves and waves generated by a wavemaker, allowing for a range of wave characteristics and wave ages. Similar wind speed profiles and whitecapping behavior between the laboratory and the field suggest that the laboratory is appropriate for the study of sea spray production. The sea spray generation flux was estimated from logarithmic vertical sea spray concentration profiles using a flux-profile method using Monin and Obukhov (1954) theory. Results show that the production of larger droplets at 20-35 µm radius is well correlated with the wave slope variance 2>, whilst the wind friction velocity cubed u*3 performs best over 7-20 µm. Two SSGFs are proposed. The original work presented here is an assessment of the validity of the two SSGFs in the field. The two laboratory-derived SSGFs are tested in two numerical models; the stationary Marine Aerosol Concentration Model (MACMod) (used in Laussac et al., 2018), and the non-hydrostatic mesocale atmospheric model Meso-NH (jointly developed by the LA - UMR 5560 - and the CNRM - UMR 3589). The 2> necessary required by both SSGFs is estimated using a wind-dependent formulation (Cox and Munk, 1956) and a spectral spectral model (Elfouhaily et al., 1997). Results show that the numerical simulations offer good results relative to sea spray measurements obtained in the North-West Mediterranean in fetch-limited conditions (Laussac et al., 2018), as well as other existing SSGFs in the literature. These results suggest that wind-wave tunnel facilities present an interesting alternative for determining the sea spray generation flux, especially in high wind speed conditions in which deployment in the field is difficult. References : Bruch, W., Piazzola, J., Branger, H., van Eijk, A. M. J., Luneau, C., Bourras, D., Tedeschi, G. (In review). Sea Spray Generation Dependence on Wind and Wave Combinations : A Laboratory Study. Submitted in : Boundary Layer Meteorology. Cox, C., & Munk, W. (1956). Slopes of the sea surface deduced from photographs of sun glitter. University of California Press. Vol. 6,9,401-488. Elfouhaily, T., Chapron, B., Katsaros, K., & Vandemark, D. (1997). A unified directional spectrum for long and short wind driven waves. Journal of Geophysical Research: Oceans, 102(C7),15781-15796. Monin, A. S., & Obukhov, A. M. (1954). Basic laws of turbulent mixing in the surface layer of the atmosphere. Contrib. Geophys. Inst. Acad. Sci. USSR,151(163),e187. Laussac, S., Piazzola, J., Tedeschi, G., Yohia, C., Canepa, E., Rizza, U., & Van Eijk, A. M. J. (2018). Development of a fetch dependent sea-spray source function using aerosol concentration measurements in the North-Western Mediterranean. Atmospheric Environment,193,177-189.