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The small-scale physics within the first centimetres above the wavy air–sea interface are the gateway for transfers of momentum and scalars between the atmosphere and the ocean. We present an experimental investigation of the surface wind stress over laboratory wind-generated waves. Measurements were performed at the University of Delaware's large wind-wave-current facility using a recently developed state-of-the-art wind-wave imaging system. The system was deployed at a fetch of 22.7 m, with wind speeds from 2.19 to $16.63\\ \\textrm {m}\\ \\textrm {s}^{-1}$. Airflow velocity fields were acquired using particle image velocimetry above the wind waves down to $100\\ \\mathrm {\\mu }\\textrm {m}$ above the surface, and wave profiles were detected using laser-induced fluorescence. The airflow intermittently separates downwind of wave crests, starting at wind speeds as low as $2.19\\ \\textrm {m}\\ \\textrm {s}^{-1}$. Such events are accompanied by a dramatic drop in tangential viscous stress past the wave's crest, and a gradual regeneration of the viscous sublayer upon the following (downwind) crest. This contrasts with non-airflow separating waves, where the surface viscous stress drop is less significant. Airflow separation becomes increasingly dominant with increasing wind speed and wave slope $a k$ (where $a$ and $k$ are peak wave amplitude and wavenumber, respectively). At the highest wind speed ($16.63\\ \\textrm {m}\\ \\textrm {s}^{-1}$), airflow separation occurs over nearly 100 % of the wave crests. The total air–water momentum flux is partitioned between viscous stress and form drag at the interface. Viscous stress (respectively form drag) dominates at low (respectively high) wave slopes. Tangential viscous forcing makes a minor contribution (${\\sim }3\\,\\%$) to wave growth.

Highly resolved laboratory measurements of the airflow over wind-generated waves are examined using a novel wave growth diagnostic that quantifies the presence of Miles’ critical layer mechanism of wind-wave growth. The wave growth diagnostic is formulated based on a linear stability analysis, and results in growth rates that agree well with those found by a pressure reconstruction method as well as other, less direct, methods. This finding, combined with a close agreement between the airflow measurements and the predictions of linear stability (critical layer) theory, demonstrate that the Miles’ critical layer mechanism can cause significant wave growth in young (wave age $c/u_* = 6.3$, where $c$ is the wave phase speed, and $u_*$ the friction velocity) wind-forced waves.

We present results from experiments designed to measure near-surface turbulence generated by rainfall. Laboratory experiments were performed using artificial rain falling at near-terminal velocity in a wind–wave channel filled with synthetic seawater. In this first series of experiments, no wind was generated and the receiving seawater was initially at rest. Rainfall rates from 40 to $190~\\text{mm}~\\text{h}^{-1}$ were investigated. Subsurface turbulent velocities of the order of $O(10^{-2})~\\text{m}~\\text{s}^{-1}$ are generated near the interface below the depth of the cavities generated by the rain drop impacts. The turbulence appears independent of rainfall rates. At depth larger than the size of the cavities, the turbulent velocity fluctuations decay as $z^{-3/2}$ . Turbulent length scales also appear to scale with the size of the impact cavities. In these seawater experiments, a freshwater lens is established at the water surface due to the rain. At the highest rain rate studied, the resulting buoyancy flux appears to lead to a shallower subsurface mixed layer and a slight decrease of the turbulent kinetic energy dissipation. Finally, direct measurements and inertial estimates of the turbulent kinetic energy dissipation show that approximately 0.1–0.3 % of the kinetic energy flux from the rain is dissipated in the form of turbulence. This is consistent with existing freshwater measurements and suggests that high levels of dissipation occur at depths and scales smaller than those resolved here and/or that other phenomena dissipate a considerable amount of the total kinetic energy flux provided by rainfall.

Sea spray droplets are known to enhance the fluxes of momentum, heat, and mass at the air‐sea interface. Evaluating these fluxes depends in part on the so‐called “spray generation function”, the size distribution of droplets generated. At high wind speeds, spray is empirically observed to be plentiful near the ocean surface, however, the generation function has remained elusive both theoretically and experimentally. We report on a photographic laboratory experiment designed to directly quantify spume droplets observed at high wind speeds. The resulting sea spray concentration functions for spume droplets (diameter > 140μm) are reported for three high wind speed conditions (31.3, 41.2, and 47.1 ms−1). Our data suggest that large supra‐millimeter droplets are more prevalent than previously thought. We also observed a previously unreported spray generation mechanism whereby liquid sheets form at the crests of breaking waves and generate, upon breakup, a significant number of small spume drops. Key Points Presents new measurements of sea spray concentration in high wind speeds Illustrates a previously unobserved sea spray generation mechanism New knowledge for modeling of spray mediated air‐sea fluxes in high winds

Large-eddy simulation (LES) of a wind- and wave-forced water column based on the Craik–Leibovich (C–L) vortex force is used to understand the structure of small-scale Langmuir circulation (LC) and associated Langmuir turbulence. The LES also serves to understand the role of the turbulence in determining molecular diffusive scalar flux from a scalar-saturated air side to the water side and the turbulent vertical scalar flux in the water side. Previous laboratory experiments have revealed that small-scale LC beneath an initially quiescent air–water interface appears shortly after the initiation of wind-driven gravity–capillary waves and provides the laminar–turbulent transition in wind speeds between 3 and$6~\\text{m}~\\text{s}^{-1}$. The LES reveals Langmuir turbulence characterized by multiple scales ranging from small bursting eddies at the surface that coalesce to give rise to larger (centimetre-scale) LC over time. It is observed that the smaller scales account for the bulk of the near-surface turbulent vertical scalar flux. Although the contribution of the larger (centimetre-scale) LC to the near-surface turbulent flux increases over time as these scales emerge and become more coherent, the contribution of the smaller scales remains dominant. The growing LC scales lead to increased vertical scalar transport at depths below the interface and thus greater scalar transfer efficiency. Simulations were performed with a fixed wind stress corresponding to a$5~\\text{m}~\\text{s}^{-1}$wind speed but with different wave parameters (wavelength and amplitude) in the C–L vortex force. It is observed that longer wavelengths lead to more coherent, larger centimetre-scale LC providing greater contribution to the turbulent vertical scalar flux away from the surface. In all cases, the molecular diffusive scalar flux at the water surface relaxes to the same statistically steady value after transition to Langmuir turbulence occurs, despite the different wave parameters in the C–L vortex force across the simulations. This implies that the small-scale turbulence intensity and the molecular diffusive scalar flux at the surface scale with the wind shear and not with the wave parameters. Furthermore, it is seen that the Langmuir (wave) forcing (provided by the C–L vortex force) is necessary to trigger the turbulence that induces elevated molecular diffusive scalar flux at the water surface relative to wind-driven flow without wave forcing.

The combined effects of rain and wind on air‐water gas exchange were investigated with a series of experiments conducted at University of Delaware's Air‐Sea Interaction Laboratory (ASIL). During this study, the third ASIL Wind and Rain Experiment (WRX 3), a combination of three rain rates and eight wind speeds were executed using aqueous mass balances of SF6 to determine gas transfer velocities, k(600). In addition, measurements of wave properties, currents, and turbulence were obtained. Study results show that rain and wind effects combine nonlinearly to enhance air‐water gas exchange. Also, rainfall appears to contribute significantly to the total air‐water gas flux at low wind speeds, while at higher speeds rain effects appear to be negligible. We find that the range of conditions over which the rain effects are important is well defined by the ratio of rain kinetic energy flux to that of the wind. A nonlinear parameterization of k(600) for the combined effects of rain and wind is proposed. We extend this parameterization to field conditions and obtain the approximate rain rate and wind speed conditions where rain is expected to have a significant effect on air‐sea gas exchange. Low wind speed–high rain rate regions such as the tropics are regions where rain is expected to play a significant role. Key Points Rain and wind interact nonlinearly to enhance air‐sea gas exchange Rain effects are seen at lower wind speeds and disappear at higher speeds The limiting parameter is the ratio of the kinetic energy flux of rain to wind

We present the results of laboratory and field measurements on the stability of wind-driven water surfaces. The laboratory measurements show that when exposed to an increasing wind starting from rest, surface current and wave generation is accompanied by a variety of phenomena that occur over comparable space and time scales. Of particular interest is the generation of small-scale, streamwise vortices, or Langmuir circulations, the clear influence of the circulations on the structure of the growing wave field, and the subsequent transition to turbulence of the surface flow. Following recent work by Melville, Shear & Veron (1998) and Veron & Melville (1999b), we show that the waves that are initially generated by the wind are then strongly modulated by the Langmuir circulations that follow. Direct measurements of the modulated wave variables are qualitatively consistent with geometrical optics and wave action conservation, but quantitative comparison remains elusive. Within the range of parameters of the experiments, both the surface waves and the Langmuir circulations first appear at constant Reynolds numbers of 370 ± 10 and 530 ± 20, respectively, based on the surface velocity and the depth of the laminar shear layer. The onset of the Langmuir circulations leads to a significant increase in the heat transfer across the surface. The field measurements in a boat basin display the same phenomena that are observed in the laboratory. The implications of the measurements for air–sea fluxes, especially heat and gas transfer, and sea-surface temperature, are discussed.

Direct numerical simulations (DNS) of an initially quiescent coupled air-water interface driven by an air flow with free stream speed of 5 m s have been conducted and scalar transfer from the air side to the water side and subsequent vertical transport in the water column have been analysed. Two simulations are compared: one with a freely deforming interface, giving rise to gravity-capillary waves and aqueous Langmuir turbulence (LT) characterized by small-scale (centimeter-scale) Langmuir cells (LC), and the other with the interface intentionally held flat, i.e., without LC. It is concluded that LT serves to enhance vertical transport of the scalar in the water side and in the process increases scalar transfer efficiency from the air side to the water side relative to the shear-dominated turbulence in the flat interface case. Furthermore, transition to LT was observed to be accompanied by a spike in scalar flux characterized by an order of magnitude increase. These episodic flux increases, if linked to gusts and overall unsteadiness in the wind field, are expected to be an important contributor in determining the long-term average of the air-sea gas fluxes.

Air–sea fluxes of heat and momentum play a crucial role in weather, climate, and the coupled general circulation of the oceans and atmosphere. Much progress has been made to quantify momentum transfer from the atmosphere to the ocean for a wide range of wind and wave conditions. Yet, despite the fact that global heat budgets are now at the forefront of current research in atmospheric, oceanographic, and climate problems and despite the good research progress in recent years, much remains to be done to better understand and quantify air–sea heat transfer. It is well known that ocean-surface waves may support momentum transfer from the atmosphere to the ocean, but the role of the waves in heat transfer has been ambiguous and poorly understood. Here, evidence is presented that there are surface wave–coherent components of both the sensible and the latent heat fluxes. Presented here are data from three field experiments that show modulations of temperature and humidity at the surface and at 10–14 m above the surface, which are coherent with the surface wave field. The authors show that the phase relationship between temperature and surface displacement is a function of wind speed. At a 10–12-m elevation, a wave-coherent heat transfer of O(1) W m−2 is found, dominated by the latent heat transfer, as well as wave-coherent fractional contributions to the total heat flux (the sum of latent and sensible heat fluxes) of up to 7%. For the wind speeds and wave conditions of these experiments, which encompass the range of global averages, this wave contribution to total heat flux is comparable in magnitude to the atmospheric heat fluxes commonly attributed to the effects of greenhouse gases or aerosols. By analogy with momentum transfer, the authors expect the wave-coherent heat transfer to decay with height over scales on the order of k−1, where k is the characteristic surface wavenumber; therefore, it is also expected that measurements at elevations of O(10) m may underestimate the contribution of the wave-induced heat flux to the atmosphere.

Near the shore, cross-shore winds strongly affect the location of the break point and the breaking-wave height. From casual observation from the beach, wind direction (onshore or offshore) and speed also appear to affect wave shape (i.e., skewness and asymmetry), although as of yet this effect has not been quantified near the shore. The effect of wind on shoaling wave shape is investigated with laboratory experiments using monochromatic waves and onshore-directed wind. Wind increases the shoaling wave energy at discrete multiples of the primary frequency and has a significant effect on the wave shape at both a deeper and shallower shoaling locations. At the shallower location, the ratio of wave energy at 2 times the primary frequency to the primary frequency is also a function of wind speed, indicating interaction between the wind and the nonlinear wave shoaling process. Nearshore wave models do not account for these wind effects. Incorrect predictions of third-order velocity moments (wave shape), believed to control wave-driven sediment transport, would result in incorrect beach morphological evolution predictions. [PUBLICATION ABSTRACT]