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This study investigates the sensitivity of the Madden–Julian oscillation (MJO) to changes to the bulk flux parameterization and the role of ocean surface waves in air–sea coupling using a fully coupled ocean–atmosphere–wave model. The atmospheric and ocean model components of the Energy Exascale Earth System Model (E3SM) are coupled to a spectral wave model, WAVEWATCH III (WW3). Two experiments with wind speed–dependent bulk algorithms (NCAR and COARE3.0a) and one experiment with wave-state-dependent flux (COR3.0a-WAV) were conducted. We modify COARE3.0a to include surface roughness calculated within WW3 and also account for the buffering effect of waves on the relative difference between air-side and ocean-side momentum flux. Differences in surface fluxes, primarily caused by discrepancies in drag coefficients, result in significant differences in MJO's properties. While COARE3.0a has better convection–circulation coupling than NCAR, it exhibits anomalous MJO convection east of the date line. The wave-state-dependent flux (COR3.0-WAV) improves the MJO representation over the default COARE3.0 algorithm. Strong easterlies over the Pacific Ocean in COARE3.0a enhance the latent heat flux (LHFLX). This is responsible for the anomalous MJO propagation after the date line. In COR3.0a-WAV, waves reduce the anomalous easterlies, leading to a decrease in LHFLX and MJO dissipation after the date line. These findings highlight the role of surface fluxes in MJO simulation fidelity. Most importantly, we show that the proper treatment of wave-induced effects in bulk flux parameterization improves the simulation of coupled climate variability. [ABSTRACT FROM AUTHOR]

Air bubbles in the upper ocean are generated mainly by waves breaking at the air–sea interface. As such, after the waves break, entrained air bubbles evolve in the form of plumes in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the link between bubble plume penetration depth, mechanical and thermal forcings, and the air–sea transfer velocity of CO2 , a field experiment was conducted from an oceanographic research tower in the north Adriatic Sea. Air bubble plumes were observed using high-resolution echosounder data from an upward-looking 1000 kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organized into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17 m depth under strong forcing. We verify that bubble plumes adapt rapidly to wind and wave conditions and have depths that scale approximately linearly with wind speed. Scaling with the wind–wave Reynolds number is also proposed to account for the sea-state severity in the plume depth prediction. Results show a correlation between measured bubble depths and theoretical air–sea CO2 transfer velocity parametrized with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air–sea interaction studies and pave the way for progress in CO2 gas exchange formulations. [ABSTRACT FROM AUTHOR]

We conducted experiments in a laboratory to study turbulent flow over wind generated water waves. The experiments were performed in a wind-wave-current flume with three free stream wind speeds of Uref  = 6.0, 8.0 and 10.0 m s−1, corresponding to 10 m equivalent wind speed of U 10 = 10.2, 12.2 and 14.1 m s−1 and the root-mean-square wave height of 0.7, 1.1 and 1.7 cm, respectively, at a fetch of 6.2 m. The instantaneous velocity fields above the waves were obtained by using a particle image velocimetry (PIV) technique. The velocity fields were decomposed into the mean, wave-induced and turbulent velocity components. The tested wind waves were primarily dissipated by capillaries and microscale breaking waves. The Bond number and the shear velocity-fetch based Reynolds number were found to correlate with the wind wave regimes well. The turbulent dissipation rates above the water surface were determined based on resolved spatial gradient of instantaneous velocities, where the time-averaged dissipation rate values were calibrated using those estimated from the one-dimensional velocity spectrum in the temporal space. Subsequently, the turbulent kinetic energy (TKE) budget including its production, dissipation, advection and turbulent transport was presented. In addition, conditional averaging analysis of the TKE budgets over leeward, windward sides and all phases was performed. The results showed a strong dependency with the wave phase in the TKE budget terms except for the dissipation. The production-dissipation ratio increased significantly as the wind speed increased, likely attributed to the increased roughness over the substantial coverage of micro-breaking waves. [ABSTRACT FROM AUTHOR]

While numerous modeling studies have focused on the interaction of ocean surface waves with the atmospheric boundary layer, most employ idealized waves that are either monochromatic or synthetically generated from a theoretical wave spectrum, and the atmospheric solvers are typically incompressible. To study wind–wave coupling in real-world scenarios, a model that can simulate both realistic meteorological and wave conditions is necessary. In this paper we describe the implementation of a moving bottom boundary condition into the Weather Research and Forecasting Model for large-eddy simulation applications. We first describe the moving bottom boundary conditions within WRF's pressure-based vertical coordinate system. We then validate our code with idealized test cases that have analytical solutions, including flow over a monochromatic wave with and without viscosity. Finally, we present results from turbulent flows over a moving monochromatic wave with different wave ages, and demonstrate satisfactory agreement of the wave growth rate with results from the literature. We also compare atmospheric stress and wind parameters from two physically equivalent cases. The first specifies a wind moving in the same direction as a propagating wave, while the second involves a stationary wave with the wind adjusted such that the wind relative to the wave is the same as in the first case. Results indicate that the velocity and Reynolds stress profiles for the two cases match, further validating the moving bottom implementation. [ABSTRACT FROM AUTHOR]

The results of measurements of atmospheric turbulence characteristics were used to obtain parameterizations for calculating the dynamic roughness parameter and the roughness parameters for temperature and humidity for a shallow closed water body. At medium wind speeds, the results of calculations by Charnock formula are in agreement with observation data; in this case, the c parameter is three times as large as that in the case of an open ocean, and the passage from the viscous to wave mechanism occurs at high wind speeds, while the dynamic roughness parameter at the same wind speeds is greater. The roughness parameters for temperature and humidity at wind speed from 0.5 to 3 m/s are not equal. The empirical coefficients in the equations describing the ratio of the dynamic roughness to the roughness parameter for temperature (humidity) on Reynolds number are close to those obtained before for other closed water bodies, thus suggesting a common formation mechanism of transport processes in a viscous sublayer. The obtained parameterizations can be used in Earth system models and lake models for calculating turbulent flows over continental water bodies. [ABSTRACT FROM AUTHOR]

Due to the long-standing lack of understanding the role of wind waves on wind stress at moderate to high wind speeds, a high-frequency turbulence observation system is used in this study to obtain air-sea momentum flux data under pure wind wave conditions based on the tower-based marine meteorological observation platform in the southern Bohai Sea. Moreover, the modulation of wind waves on wind stress under wind speeds greater than 10 m s−1 is analyzed. The results indicated that the wind wave states caused by winds from the northwest and northeast are different under the influence of cold air, resulting in different wind stresses and drag coefficients. The wind stress increases with an increasing wind speed, reaching its maximum value when the northwest wind is nearly 20 m s−1, while the extreme value of the drag coefficient is basically the same when the northwest wind speed is the maximum and the northeast wind wave significant wave height is the maximum. The drag coefficient increases with an increasing wind speed within the range of 10–15 m s−1, reaching saturation at 15 m s−1. The critical wind speed is smaller than other observed results. Further analysis showed that wind-induced turbulent stress deviates from the observed values, and the degree of deviation depends on the wind speed and wave state, with a greater deviation caused by strong winds and waves. The wave-induced stress can correct the negative deviation between wind-induced turbulent stress and the observed value, and the drag coefficient calculated based on the modified wind stress tends to be close to the observed value overall. [ABSTRACT FROM AUTHOR]

Studying the dynamic characteristics of the platform under unsteady loads is of great significance to the development of floating offshore wind turbines (FOWTs). In this work, a 15 MW semi-submersible FOWT is extensively simulated using OPENFAST software, and the influence of turbulent wind and irregular waves on the platform's motion is analyzed in detail. Results show that the surge motion of the platform of a complete FOWT system is mainly affected by wind speed, and the influence of turbulence intensity (TI) is slightly greater than the wave height. The heave motion is most significantly affected by the wave height. When the wave height is large, the influence of wind speed on the platform gradually decreases, and the effect of TI becomes more evident. The pitch and yaw motions are greatly affected by the wind speed and TI. The research results can serve as a reference for the platform design of large FOWTs in the future. [ABSTRACT FROM AUTHOR]

The seasonality of the vertical mixing at coastal sites is not well characterized yet. Here, a time series of the dissipation rate of turbulent kinetic energy (ε) was obtained from weekly morning microstructure observations covering the destratification period (July 2015, February 2016) at a coastal site in the western Mediterranean Sea, influenced by freshwater runoffs. Estimated with bulk parameters from the public re-analyzed dataset ERA5, the Ekman layer, and the convective penetration depth scale with the mixed layer depth (MLD) with a good agreement. Below the MLD, peaks of ε are observed in the baroclinic layers that progressively overlap with the bottom layer, where repeated near-bottom turbidity peaks provide evidence of sediment resuspension, suggesting energetic processes within the bottom boundary layer. In the subsurface, moderate values ( 10 − 9 to 10 − 8   W   kg − 1 ) are observed, following a Burr type XII distribution. Significant correlation with ε at MLD is obtained with a model combining the effects of wind, wind–wave, and convection, highlighting a calm sea bias in our data, plus a sunrise bias when morning buoyancy fluxes are stabilizing. Another correlation, obtained from a pure-wind estimation 18 h before, suggests the role of wind in generating internal waves in the stratified layers, thus, impacting mixing intensity. [ABSTRACT FROM AUTHOR]

Many researchers and engineers have shown great interest in mass transfer processes produced by wind-induced waves. Such waves contribute significantly to the transfer of environmental materials, such as sediment and marine debris, and the turbulence occurring beneath the waves further complicates wave-induced mass transport. The phase cycle of wave motion is generally considered to be a key determinant of mean flow and turbulence. In aqueous environmental engineering, this relationship is a crucial one to investigate, since turbulence is closely related to mass transport. To address this question, we measured the time-series of instantaneous velocity vectors by means of particle image velocimetry in a laboratory flume to reveal the turbulence structure induced by wave motion. By using a wavelet analysis free of specific assumptions, we were able to decompose the instantaneous velocity data into mean current, wave motion, and turbulence components. This analysis allowed for the objective evaluation of the shear stresses related to wave energy and turbulence energy production. Furthermore, we found significant phase characteristics of energy transfer among mean velocity, wave, and turbulence components. In order to examine the diffusion and convection properties induced by wind waves, we also conducted tracking analysis of imaginary sediment markers. Our results support the conclusion that mass transfer induced by wind waves impacts the entire range of water depths, at least in relatively shallow aqueous environments. [ABSTRACT FROM AUTHOR]

Monin–Obukhov similarity theory (MOST) is a well-tested approach for specifying the fluxes when the roughness surfaces are homogeneous. For flow over waves (inhomogeneous surfaces), phase-averaged roughness length scales are often prescribed through models based on the wave characteristics and the wind speed. However, such approaches lack generalizability over different wave ages and steepnesses due to the reliance on model coefficients tuned to specific datasets. In this paper, a sea surface–based hydrodynamic drag model applicable to moving surfaces is developed to model the pressure-based surface drag felt by the wind due to the waves. The model is based on the surface gradient approach of Anderson and Meneveau applicable to stationary obstacles and extended here to the wind–wave problem. The wave drag model proposed specifies the hydrodynamic force based on the incoming momentum flux, wave phase speed, and the surface frontal area. The drag coefficient associated with the wind–wave momentum exchange is determined based on the wave steepness. The wave drag model is used to simulate turbulent airflow above a monochromatic wave train with different wave ages and wave steepnesses. The mean velocity profiles and model form stresses are validated with available laboratory-scale experimental data and show good agreement across a wide range of wave steepnesses and wave ages. The drag force is correlated with the wave surface gradient and out-of-phase with the wave height distribution by a factor of π/2 for the sinusoidal wave train considered. These results demonstrate that the current approach is sufficiently general over a wide parameter space compared to wave phase-averaged models with a minimal increase in computational cost. Significance Statement: Understanding the physics of wind waves plays an important role in the context of numerous geophysical and engineering applications. A drag-based model is developed that characterizes the effect of the sea surface waves on the wind above. The model is validated with existing experimental datasets and is shown to be effective in predicting the average wind velocity and stress over waves with varied steepnesses and phase speeds. The ease of implementation and low computational cost of the model make it useful for studying turbulent atmospheric-scale flows over the sea surface important in offshore wind energy research as well as for modeling air–sea fluxes of momentum, heat, and mass. [ABSTRACT FROM AUTHOR]

Estimates of turbulence kinetic energy (TKE) dissipation rate (ε) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates of ε with concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates of ε at a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scale (LM) $({L}_{M})$ and the newer Langmuir stability length scale (LL) $({L}_{L})$, suggesting that ocean‐side friction velocity u∗ $\left({u}_{\ast }\right)$ implicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes drift us $\left({u}_{s}\right)$ and u∗ ${u}_{\ast }$ that is likely facilitated by the steady Southeast trade winds regime. In certain regimes, u∗3κz $\frac{{u}_{\ast }^{3}}{\kappa z}$, where κ $\kappa $ is the von Kármán constant and z $z$ is instrument depth, and surface buoyancy flux capture our estimates of ε $\varepsilon $ well, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based on us ${u}_{s}$ and u∗ ${u}_{\ast }$, scales ε well at times but is overall less consistent than u∗3κz $\frac{{u}_{\ast }^{3}}{\kappa z}$. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes. Plain Language Summary: Surface ocean turbulence is key to the transfer of heat, gas, and other climate‐relevant properties between the ocean and atmosphere. Because turbulence is difficult to measure in the field, it is often parameterized using more easily obtained variables such as wind speed, wave measurements, and surface heat flux. Here, we test such parameterizations against an extensive time series of turbulence measurements collected on a mooring line attached to a surface buoy in the Southeast Pacific Ocean. This region is known to support important South American fisheries as well play a significant role in the global radiation budget, yet is poorly represented in climate models. We find the parameterizations to describe our measurements well, and we explore how conditions at the study site influence their performance. Key Points: Moored instrumentation allows for prolonged time series of turbulence estimates with concurrent in‐situ meteorological and wave measurementsLaw of the Wall and Monin‐Obukhov Similarity theory are able to predict measurements of ε in certain turbulence regimesIn the context of turbulence scaling, it may be unnecessary to distinguish between a wind‐driven and Langmuir‐driven turbulence regime [ABSTRACT FROM AUTHOR]

Wind speeds measured by the Mars 2020 Perseverance rover in Jezero crater were fitted as a Weibull distribution. InSight wind data acquired in Elysium Planitia were also used to contextualize observations. Jezero winds were found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observed. However, a great influence of turbulence and wave activity was observed in the wind speed variations, thus driving the probability of reaching the highest wind speeds at Jezero, instead of sustained winds driven by local, regional, or large‐scale circulation. The power spectral density of wind speed fluctuations follows a power‐law, whose slope deviates depending on the time of day from that predicted considering homogeneous and isotropic turbulence. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. The signature of convection cells was also found during dust storm conditions, when prevailing winds were consistent with a tidal drive. Nighttime fluctuations were also intense, suggesting strong mechanical turbulence. Convective vortices were usually involved in rapid wind fluctuations and extreme winds, with variations peaking at 9.2 times the background winds. Transient high wind events by vortex‐passages, turbulence, and wave activity could be driving aeolian activity at Jezero. We report the detection of a strong dust cloud of 0.75–1.5 km in length passing over the rover. The observed aeolian activity had major implications for instrumentation, with the wind sensor suffering damage throughout the mission, probably due to flying debris advected by winds. Plain Language Summary: Jezero winds as measured in the crater floor by Perseverance were found to be much calmer on average than in previous landing sites. Turbulence and wave activity provoked rapid fluctuations that changed wind speed from calm conditions to more than 10–15 ms−1 in the timescale of seconds to minutes. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. These convection cells are produced under strong thermal gradients typically present during daytime. Pressure drops, associated with convective vortices, were usually involved in rapid wind fluctuations and, in some cases, in extreme winds as measured by Perseverance. An intense aeolian activity was observed at Jezero crater produced by transient high wind events. This aeolian activity had major implications for instrumentation, with the Perseverance wind sensor suffering damage probably due to flying debris advected by winds. Also, we report the detection of a strong dust cloud of 0.75–1.5 km passing over the rover. This paper has a companion paper (part 1) in the same issue, which is focused on wind patterns and analyzed the mechanisms driving atmospheric circulation at Jezero. Key Points: Jezero winds are found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observedTurbulence, wave activity, and convective vortices drive the peak wind speeds observed at JezeroWe report the detection of a dust cloud of 0.75–1.5 km in length passing over the rover [ABSTRACT FROM AUTHOR]

Wind gustiness in the marine atmospheric boundary layer affects significantly the dynamics of air–sea interaction. To understand the impacts of wind gust events, we perform large-eddy simulation of wind turbulence over a travelling wave to investigate the response of the wind field to an impulsive wind speed increase or decrease. It is found that the turbulence fluctuations and the terms in the turbulent kinetic energy budget equation have a delayed response to the change in the mean flow, while the response of the wave-coherent motions is quasi-stationary. The wave-coherent motions are investigated quantitatively through comparison with a viscous curvilinear model developed by Cao et al. (J. Fluid Mech., vol. 901, 2020, A27) and Cao & Shen (J. Fluid Mech., vol. 919, 2021, A38). We observe an asymmetric hysteresis between the growing wind and the decaying wind in the evolution of the form drag and the viscous drag. We find further that the variation of the wave growth rate during the wind gust is related closely to the contribution from the out-of-phase component of the vertical velocity. Our discoveries provide evidence for the necessity of improving non-equilibrium turbulence and wind input modelling to account for the wind gustiness effect in future studies. [ABSTRACT FROM AUTHOR]

Bubbles directly link sea surface structure to the dissipation rate of turbulence in the ocean surface layer through wave breaking, and they are an important vehicle for air–sea transfer of heat and gases and important for understanding both hurricanes and global climate. Adequate parameterization of bubble dynamics, especially in high winds, requires simultaneous measurements of surface waves and breaking-induced turbulence; collection of such data would be hazardous in the field, and they are largely absent from laboratory studies to date. We therefore present data from a series of laboratory wind-wave tank experiments designed to observe bubble size distributions in natural seawater beneath hurricane conditions and connect them to surface wave statistics and subsurface turbulence. A shadowgraph imager was used to observe bubbles in three different water temperature conditions. We used these controlled conditions to examine the role of stability, surface tension, and water temperature on bubble distributions. Turbulent kinetic energy dissipation rates were determined from subsurface ADCP data using a robust inertial-subrange identification algorithm and related to wind input via wave-dependent scaling. Bubble distributions shift from narrow to broadbanded and toward smaller radius with increased wind input and wave steepness. TKE dissipation rate and shear were shown to increase with wave steepness; this behavior is associated with a larger number of small bubbles in the distributions, suggesting shear is dominant in forcing bubbles in hurricane wind-wave conditions. These results have important implications for bubble-facilitated air–sea exchanges, near-surface ocean mixing, and the distribution of turbulence beneath the air–sea interface in hurricanes. Significance Statement: Bubbles are a vehicle for the flux of heat, momentum, and gases between the atmosphere and ocean. These fluxes contribute to the energy budgets of hurricanes, climate, and upper-ocean biology. Few to no simultaneous measurements of surface waves, bubbles, and turbulence have been made in hurricane conditions. To improve numerical model representation of bubbles, we performed laboratory experiments to parameterize bubble size distributions using physical variables including wind and waves. Bubble distributions were found to become broadbanded and shift toward smaller radius with increased wind stress and wave steepness. Turbulence dissipation rate and shear were shown to increase with wave steepness. Our results give the first physically based bubble distribution parameterization from naturally breaking waves in hurricane-force conditions. [ABSTRACT FROM AUTHOR]

Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the "inertial chimney" effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs' downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement: We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models. [ABSTRACT FROM AUTHOR]

Recent studies in hydrodynamic flows and nonlinear plasma waves have demonstrated the turbulent transitions from ordered laminar flows and ordered plane waves, respectively, with the formation of a large percolating turbulent cluster, after the sporadic emergence and decay of turbulent puffs in the spatiotemporal space. These transitions follow the similar order–disorder transition scenario in nonequilibrium extended systems, governed by percolation theory. Here, we experimentally investigate the unexplored issue of whether a similar transition scenario can be extended to wind-driven water waves, especially for the transition from weak to strong turbulent states. Localized sites in the y–t (y is normal to the wind direction) space are binarized into hot turbulent sites (HTSs) and cold turbulent sites depending on the instantaneous energy of the local wave height fluctuations. It is found that increasing the fetch (the distance x from the wind entrance) as increasing the effective drive leads to the transition from the weak to the strong turbulent state with a smooth rapid rise of the area fraction occupied by HTSs, and the formation of a large HTS cluster percolating through the y–t space after the sporadic emergence of HTS clusters. This generic transition behavior and the scaling exponents of the HTS fraction around the critical (percolating) fetch, and of the quiescent time and the quiescent distance between adjacent HTS clusters at the critical fetch, are akin to those around and at the critical point, respectively, for the 1 + 1D (dimensional) nonequilibrium system governed by the directed percolation theory. [ABSTRACT FROM AUTHOR]

To achieve more accurate weather and climate forecasting, and propose efficient engineering solutions for exploiting offshore renewable energies, it is imperative to accurately describe the atmospheric turbulent flow in the offshore environment. The ocean's dynamics raise specific challenges for the aforementioned applications, as they significantly alter the atmospheric flow through complex wind–wave interactions. These interactions are important in fairly common situations and notably in old-sea conditions, where ocean waves travel fast, under comparatively slow wind velocities. In the present study, a scanning LiDAR (sLiDAR) was deployed on the shore to study micro-scale wind–wave interactions by performing horizontal scans 18 m above the ocean, and as far as 2 km from the coast. In the proposed configuration, and in the test cases presented in old seas, the sLiDAR captures wave-induced disturbances propagating into the lower part of the marine atmospheric boundary layer. Based on measurements of high-resolution space–time maps of the Radial Wind Speed, an original two-dimensional spectral analysis of the space–time auto-correlation functions was performed. Unlike more conventional data-processing techniques, and as long as the waves travel sufficiently (∼twofold) faster than the mean wind at the measurement height, the upward transfer of motions from the waves to the wind can be clearly distinguished from the atmospheric turbulence in the wave-number–angular-frequency (k–w) turbulent spectra. These are the first space–time auto-correlation functions of the wind velocity fluctuations obtained at micro-scales above the ocean. The analyses demonstrate sLiDAR systems' applicability in measuring k–w-dependent turbulent spectra in the coastal environment. The findings present new perspectives for the study of micro-scale wind–wave interactions. [ABSTRACT FROM AUTHOR]

Wind-wave interaction affects the wind-wave fields and the combined loading on structures. This study aims to characterize turbulent airflow over non-breaking and breaking waves using a high-fidelity two-phase model in OpenFOAM. The volume of fluid method is utilized to model the complex air–water interface. Wind turbulence is considered via prescribing it at the inlet boundary. The model is validated against experimental data, and a numerical case study is conducted to simulate extreme wind and wave-plunging conditions. Results reveal that non-breaking waves induce wind turbulence and affect averaged wind velocity. For wave steepness exceeding 0.35, extreme wind forces amplify the turbulence by over 100% at wave crests. The amplified turbulence region extends to about 0. 6 λ in height. When waves plunge, an overturning jet propels the wind forward, generates a counterclockwise vortex, and enhances wind turbulence. Averaged wind speeds increase by over 20% above wave crests, with the enhanced region extending to a height of around 0. 2 λ for old waves. Maximum turbulence kinetic energy transiently increases by around 0. 1 C p 2 and maximum kinematic energy of wave-coherent velocity increases and subsequently decreases during wave plunging. • An LES-based two-phase model is developed to simulate coupled turbulent wind-wave flows. • Simulated wind-wave flow characteristics agree well with the experimental data. • Over non-breaking waves, the wave generated wind turbulence depends on wave ages. • Over plunging waves, a counterclockwise vortex forms, enhancing wind turbulence. • Wave breaking has a significant impact on the pattern of phase-averaged wind speed. [ABSTRACT FROM AUTHOR]

Air–sea momentum and scalar fluxes are strongly influenced by the coupling dynamics between turbulent winds and a spectrum of waves. Because direct field observations are difficult, particularly in high winds, many modeling and laboratory studies have aimed to elucidate the impacts of the sea state and other surface wave features on momentum and energy fluxes between wind and waves as well as on the mean wind profile and drag coefficient. Opposing wind is common under transient winds, for example, under tropical cyclones, but few studies have examined its impacts on air–sea fluxes. In this study, we employ a large-eddy simulation for wind blowing over steep sinusoidal waves of varying phase speeds, both following and opposing wind, to investigate impacts on the mean wind profile, drag coefficient, and wave growth/decay rates. The airflow dynamics and impacts rapidly change as the wave age increases for waves following wind. However, there is a rather smooth transition from the slowest waves following wind to the fastest waves opposing wind, with gradual enhancement of a flow perturbation identified by a strong vorticity layer detached from the crest despite the absence of apparent airflow separation. The vorticity layer appears to increase the effective surface roughness and wave form drag (wave attenuation rate) substantially for faster waves opposing wind. Significance Statement: Surface waves increase friction at the sea surface and modify how wind forces upper-ocean currents and turbulence. Therefore, it is important to include effects of different wave conditions in weather and climate forecasts. We aim to inform more accurate forecasts by investigating wind blowing over waves propagating in the opposite direction using large-eddy simulation. We find that when waves oppose wind, they decay as expected, but also increase the surface friction much more drastically than when waves follow wind. This finding has important implications for how waves opposing wind are represented as a source of surface friction in forecast models. [ABSTRACT FROM AUTHOR]

The coupled dynamics of turbulent airflow and a spectrum of waves are known to modify air–sea momentum and scalar fluxes. Waves traveling at oblique angles to the wind are common in the open ocean, and their effects may be especially relevant when constraining fluxes in storm and tropical cyclone conditions. In this study, we employ large-eddy simulation for airflow over steep, strongly forced waves following and opposing oblique wind to elucidate its impacts on the wind speed magnitude and direction, drag coefficient, and wave growth/decay rate. We find that oblique wind maintains a signature of airflow separation while introducing a cross-wave component strongly modified by the waves. The directions of mean wind speed and mean wind shear vary significantly with height and are misaligned from the wind stress direction, particularly toward the surface. As the oblique angle increases, the wave form drag remains positive, but the wave impact on the equivalent surface roughness (drag coefficient) rapidly decreases and becomes negative at large angles. Our findings have significant implications for how the sea-state-dependent drag coefficient is parameterized in forecast models. Our results also suggest that wind speed and wind stress measurements performed on a wave-following platform can be strongly contaminated by the platform motion if the instrument is inside the wave boundary layer of dominant waves. Significance Statement: Surface waves increase friction at the sea surface and modify how wind forces upper-ocean currents and turbulence. Therefore, it is important to include effects of different wave conditions in weather and climate forecasts. We aim to inform more accurate forecasts by investigating wind blowing over waves propagating in oblique directions using large-eddy simulation. We find that waves traveling at a 45° angle or larger to the wind grow as expected, but do not increase or even decrease the surface friction felt by the wind—a surprising result that has significant implications for how oblique wind-waves are represented as a source of surface friction in forecast models. [ABSTRACT FROM AUTHOR]

In shallow lakes, wind wave turbulence alters underwater spectral composition, but the influence of this phenomenon on phytoplankton community structure is poorly understood. We used 100L mesocosms to investigate the influence of light quality on a natural phytoplankton community collected from Taihu Lake in China. The communities in mesocosms were exposed to sunlight filtered for white, blue, green, and red light, while wave-making pumps simulated wind wave turbulence similar to Taihu Lake. Over the course of experiment, each filtered light reduced the total phytoplankton abundance compared to white light. The mean abundance of phytoplankton in controls was 1.72, 1.78, and 7.89 times of that in the red, blue, and green light treatments. Red, blue, and green light significantly promoted the growth of cyanobacteria, green algae, and diatoms, respectively, and induced successional change of the phytoplankton species under the tested conditions. The proportion of Microcystis to total phytoplankton abundance in controls and red light shifted from 87.09% at the beginning to 37.95% and 56.30% at the end of the experiment, respectively, and maintained its dominance, whereas Microcystis lost its dominance and was replaced by Scenedesmus (53.78%) and Synedra (53.18%) in the blue and green light, respectively. Given the process of how these phytoplankton compete in designated spectrum, exploring these influences could help provide new insights into the dominance formation of toxic cyanobacteria. [ABSTRACT FROM AUTHOR]

Winds, wind waves, and turbulence are essential variables and playing critical role in regulating a series of physical and biogeochemical processes in large shallow lakes. However, parameterizing winds, waves, currents and turbulence and simulating the interaction between them in large shallow lakes haven't been evaluated strictly because of a lack of field observations of lake hydrodynamics process. To address this problem, two process-based field observations were conducted to record the development of summer and winter wind-driven currents in Lake Taihu, a large shallow lake in China. Based on these observations and numerical experiments, a wave-current coupled model (WCCM) is developed by rebuilding expression of wind drag coefficient, introducing wave-induced radiation stress, and adopting a simple turbulence scheme, and then used to simulate wind-driven currents in Lake Taihu. The results show that, the WCCM can accurately simulate the upwelling process resulting from the wind-driven currents during the field observations. Comparing with other model, there is a 42.9 % increase of WCCM-simulated current speed which is mainly attributed to the new expression of wind drag coefficient. Meanwhile WCCM-simulated current direction and field are also improved due to the introduction of wave-induced radiation stress. Furthermore, the use of the simple turbulent scheme in the WCCM makes the simulation of the upwelling processes more efficient. The WCCM provides a sound basis for simulating shallow lake ecosystems. [ABSTRACT FROM AUTHOR]

Three velocity components ui (i = x, y, z) have been measured in a wind-wave tank at three levels in water in the presence of wind waves. The degree of anisotropy of a wind wave–induced turbulence and the rate of its dissipation ε are estimated versus system parameters. For this, standard deviations σi and the frequency spectra Si(f) are calculated for the components of the flows measured, as well as σiF and SiF(f) for the turbulent components of the flows with filtered wave motions. The following is ascertained: (a) the turbulence is not isotropic; (b) σx> σy> σz and σxF> σyF> σzF; and (c) more than 70% of the kinetic energy is contained in the turbulent components of flow fluctuations. The spectra of the horizontal velocity components are similar in shape and intensity. In the frequency range below wave spectrum peak frequency fp, spectra Si(f) of all the flow components decay by the power law with the exponent equal to –1.7 ± 0.1. In the frequency range f > 2fp, spectra Sx(f) and Sy(f) are close in intensity and their decay obeys the same law, but the intensities of the spectra of the vertical component Sz(f) are almost an order of magnitude weaker and spectra Sz(f), in the presence of a power-law tail, decay by the law –2.0 ± 0.1. These segments of the spectra are interpreted as analogs of the Kolmogorov spectra. The intensity of spectra Sz(f) was used to estimate the turbulence dissipation rate ε. Semiempirical parameterizations of the dependences of σiF and ε on wave parameters and the measuring depth are constructed; their differences from the known parameterizations and possible causes of features of the spectra Si(f) are discussed. [ABSTRACT FROM AUTHOR]

Turbulent diffusivity D is estimated in laboratory experiments in a wind-wave tank. The estimates make an initial verification of the available theoretical results on the dependence of D on different parameters possible. To estimate D by the Einstein formula, we use measurements of the spreading rate of an inkblot in a water layer under waves. A comparison between the estimates and a set of model parameterizations of D has shown the statistical advantage of parameterization in the presence of wind waves and parameterization in the presence of mechanical waves ( is the friction velocity, is the wave amplitude on the surface, is the wave-spectrum peak frequency, is wave steepness, and z is the measuring depth). The measurement procedure and the technique for comparison between the theory and experiment are discussed. [ABSTRACT FROM AUTHOR]

Coherent vortices in the aqueous surface layer beneath wind waves manifest themselves by inducing elongated high‐speed streaks on the interface. Analyses of thermal images taken in the wind‐wave flume reveal that the mean streak spacing scaled by the viscous length, λ+¯, depends strongly on the wind‐wave condition. This contradicts the wall‐bounded turbulent flow in which the scaled mean streak spacing approaches a canonical value, λ+¯≈100. Comparative numerical simulations of shear flow bounded by flat and wavy surfaces are conducted to explain the variation. In the low‐wind range with insignificant surface waves, λ+¯<100; the reduction of λ+¯ is attributed to the insufficient shear rate to form the elongated streaks. For the moderate‐wind range in which surface waves become pronounced but remain to be unbroken, λ+¯ is still less than 100. Analysis of the vorticity transport in the simulated wavy flows reveals that the presence of non‐breaking waves enhances the formation of quasi‐streamwise vortices and the surface streaks. For the high‐wind range, surface waves break and the breakers wipe out the streaks. The streaks quickly reestablish in the wake of the breaker with mean spacing λ+¯≈100 but are destructed again by the following breaker. Plain Language Summary: Coherent vortices within the aqueous shear layer beneath wind waves contribute to the interfacial scalar transfer and induce elongated high‐speed streaks on the interface. Analyses of thermal images from wind‐wave flume experiments reveal that the scaling between the mean streak spacing and the surface friction velocity is different from that of wall‐bounded flow. Comparative numerical simulations of shear flow bounded by flat and wavy surfaces are conducted to explain such a deviation. In the low‐wind range with insignificant surface waves, the difference is attributed to the insufficient boundary shear to maintain the turbulence. For the range of moderate wind speeds in which surface waves become pronounced but remain to be unbroken, surface waves enhance the formation of coherent vortices and reduce the streak spacings. For the high‐wind range in which the surface waves break, the mean streak spacing approximately follows the wall‐flow scaling. Key Points: The mean streak spacings on non‐breaking wind waves exhibit different scaling with surface friction velocity from that of wall‐bounded flowNon‐breaking waves enhance the formation of quasi‐streamwise vortices and reduce the streak spacingThe mean streak spacings on the breaking wind waves approximately follow the wall‐flow scaling [ABSTRACT FROM AUTHOR]

Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Using a large-eddy simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik–Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments in which winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases as a result of reduced TKE production. Stage 2 is characterized by TKE increasing, with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values because of inertial resonance and nonequilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts as a result of slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian shear TKE production exceeding Stokes drift shear TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier–Stokes equation models indicate that Stokes drift shear production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics. [ABSTRACT FROM AUTHOR]

Dissipation within the turbulent boundary layer under sea ice is one of many processes contributing to wave energy attenuation in ice-covered seas. Although recent observations suggest that the contribution of that process to the total energy dissipation is significant, its parameterizations used in spectral wave models are based on fairly crude, heuristic approximations. In this paper, an improved source term for the under-ice turbulent dissipation is proposed, taking into account the spectral nature of that process (as opposed to parameterizations that are based on the so-called representative wave), as well as effects related to sea ice concentration and floe-size distribution, formulated on the basis of the earlier results of discrete-element modeling. The core of the new source term is based on an analogous model for dissipation due to bottom friction derived by Weber in 1991 (https://doi.org/10.1017/S0022112091003634). The shape of the wave energy attenuation curves and the frequency dependence of the attenuation coefficients are analyzed in detail for compact sea ice. The role of floe size in modifying the attenuation intensity and spectral distribution is illustrated by calibrating the model to observational data from a sudden sea ice breakup event in the marginal ice zone. [ABSTRACT FROM AUTHOR]

We present research on the excitation of ocean surface wind waves in non-homogeneous situations, for the case of a deep water strait in the presence of a constant wind, blowing perpendicular to the coast line. The statistical wave model used is based on the Hasselmann equation with high-wavenumbers wave-breaking dissipation, exact non-linear four-wave interaction, and ZRP (Zakharov-Resio-Pushkarev (Zakharov et al. Nonlin Process Geophys 24:581–597 2017) wind input term. At the first stage, the waves propagate in the wind direction in a step-like moving front manner, which is the combination of self-similar fetch-limited and duration-limited solutions of the Hasselmann equation. The second stage begins after intermediate self-similar linear asymptotics for wave energy is built along the fetch. Beginning with that time, the wave groups, propagating across and against the wind due to nonlinear interaction, are observed. Despite the absence of long-wave dissipation, the system asymptotically evolves into a complex quasi-stationary state, comprised of the self-similar "wind sea" in the wind direction, and quasi-monochromatic waves, radiating close to orthogonally with respect to the wind, while slightly tilting from perfectly orthogonal to the wind direction, with the angle slant increasing toward the wave turbulence origination shore line, and reaching 15∘ close to it. The total wave energy in the asymptotic state exceeds the wave sea energy propagating along the wind by a factor of two due to the presence of quasi-orthogonal and counter the wind wave fields. Very similar turbulence structure was previously observed experimentally; this paper presents a theoretical explanation of these results. [ABSTRACT FROM AUTHOR]

The nonlinear dependence of gas transfer velocity on wind speed typically results in the best fit of observational data; however, gas transfer velocity is dimensionally inconsistent with the nonlinear wind speeds. The objective of the current study was to show that, in the case of wind waves, gas transfer velocity with a consistent dimension should be determined by turbulent viscosity instead of by the viscosity of water when parameterised using the renewal model. Turbulent kinetic energy (TKE) near the air–sea interface is significantly intensified by breaking wind waves. By analyzing various models, we found that the TKE dissipation rate was explicitly linearly related to wind speed and dependent on wave age, with powers ranging from –2.36 to 4.0. Various models show that wave energy dissipation (WED) due to wind wave breaking explicitly increases with the cube of the wind speed and weakly depends on wave age. Assuming a balance between WED and total TKE dissipation in a constant dissipation layer, the depth of this layer was shown to be comparable to the wave height. Using the traditional renewal model with wind wave turbulent viscosity and TKE dissipation rate at the sea surface, we found that the gas transfer velocity was explicitly linearly related to wind speed in a dimensionally consistent manner, and depended simultaneously on the wave age and drag coefficient. These results are consistent with observational data obtained using the eddy correlation method. We emphasise that the linear dependence on wind speed is only valid when the wave age and drag coefficient are fixed; thus, this finding cannot be directly confirmed by currently available observational data due to a lack of wave state information.

A combined directional Stokes drift profile for swell and wind sea is presented. The profile can be used to calculate the shear under crossing seas and as such is relevant for Langmuir turbulence and Stokes-Coriolis forcing, but also for material advection. The swell is represented as either a monochromatic wave or as a Phillips spectrum, while the wind sea is represented as a Phillips spectrum. The profile is found to compare well against the full directional Stokes drift calculated from the 2D spectrum of ERA-Interim in an openocean location in the North Atlantic. The error compared to a Phillips-type unidirectional Stokes drift profile is markedly lower for a combined profile with a monochromatic swell Stokes profile. However, representing the swell as a Phillips-type Stokes drift profile yields even better results. The combined profile relies on integrated wave parameters readily available from wave models and can be calculated at low cost. The global Stokes drift climate is investigated using ERA-Interim reanalysis data with the intention of identifying regions dominated by crossing Stokes drift. We find that the eastern equatorial Pacific Ocean probably experiences the greatest degree of crossing Stokes drift, and the entire subtropical band 208-308S/N exhibits a significant degree of crossing Stokes drift and swell dominance over the Stokes drift. [ABSTRACT FROM AUTHOR]

It is important to investigate the effects of air-side freestream turbulence on the development of air–liquid surface waves. Two turbulent-grid air flows are generated by regular and fractal grids in a wind-wave tank. Wind velocity and water-level fluctuations are measured with and without grids. The results show that regular and fractal grids can generate turbulent-grid air flows with different wind velocity fluctuations. The spectra of water-level fluctuations in the regular-type and fractal-type grid cases are consistent with those of pure wind-driven waves without a grid. Furthermore, as wind-wave properties, including the fetch law, dispersion relation, and Toba's 3/2 power law, all hold in cases with and without grids, it is confirmed that the waves develop through the turbulence only near the free water surface, and not through the air-side freestream turbulence. This suggests that the effects of air-side freestream turbulence on wave development are negligible. [ABSTRACT FROM AUTHOR]

Vertical profiles of the mean horizontal currents U(z), the vertical momentum fluxes τ(z), and the turbulence kinetic-energy dissipation rates (TKE dissipation rate) ε(z) in the upper water layer (UWL) are considered and their joint analysis is carried out. For this purpose, data from laboratory measurements performed in the wind-wave channel of the Institute of Applied Physics, Russian Academy of Sciences (RAS) [1, 2], are used. They correspond to conditions of strong wind and breaking wind waves. The profiles of the currents and momentum fluxes are estimated for x and z components of the velocity at five horizons in the UWL at four various wind values. The empirical estimates of ε(z) obtained from the same data in the previous work [3] are used in the joint analysis. It is established that (a) velocity of currents U(z) increases noticeably when compared to the values of U(z) in the absence of waves, (b) the momentum flux in the water τw(z) decreases noticeably when compared to that in the air τa(z), and (c) τw(z) significantly attenuates with depth according to ratio τw(z) ~ 1/z2. The mentioned anomalies of profiles U(z) and τ(z) in the UWL are analyzed together with the previously determined pattern of TKE dissipation-rate falloff with depth according to ratio ε(z) ~ 1/z2 in order to search for an interpretation of the results. [ABSTRACT FROM AUTHOR]

We propose a new phenomenological model to represent the impact of wind-waves on the dissipation of turbulence kinetic energy near the sea surface. In this model, the momentum flux at a given height results from the averaged contribution of eddies attached to the sea surface whose sizes are related to the surface geometry. This yields a coupling between long wind-waves and turbulence at heights of about 10 m. This new wind-and-waves coupling is thus not exclusively confined to the short wave range and heights below 5 m, where most of the momentum transfer to the waves is known to occur. The proposed framework clarifies the impact of wind-waves on Monin–Obukhov similarity theory, and the role of long wind-waves on the observed wind-wave variability of momentum fluxes. This work reveals which state variables related to the wind-wave coupling require more accurate measurements to further improve wind-over-waves models and parametrizations. [ABSTRACT FROM AUTHOR]

A robust spectral dissipation term for wind waves has long been a goal of detailed-balance spectral modeling and is represented by many different approximations in spectral models of random wave fields. A Monte Carlo approach is employed here to create a random-phase sea surface that is used to simulate the distribution of horizontal surface velocities at the sea surface and to relate these velocities to deep-water wind wave breaking. Results are consistent with many recent studies that show a kinematic-based breaking criterion can provide a consistent depiction of the onset of wave breaking. This criterion is combined with the calculated nonlinear flux rates to estimate a transition point within a spectrum at which a spectrum changes from an f−4 equilibrium-range form to an f−5 region dominated by dissipation, potentially an important factor within several air–sea interaction mechanisms, turbulence at the sea surface, and remote sensing applications. It also has the potential to improve operational modeling capabilities. [ABSTRACT FROM AUTHOR]

This paper presents a detailed investigation on unsteady effects in bottom boundary layer beneath a tsunami. In numerical simulation of tsunami, bottom shear stress has commonly been evaluated by using steady flow friction laws such as Manning equation, simply assuming that long-period wave motion satisfies quasi-steady flow conditions. However, the present study found that the behavior of tsunami-induced bottom boundary layer has an unsteady characteristics and acts similar to that induced by wind-generated waves even under long-period wave motion. As a result, bottom shear stress under tsunami is much larger than the estimation using steady friction coefficient due to a steep velocity gradient in the bottom boundary layer. Surprisingly, the steady flow friction law is not valid in almost the entire computational domain, from the source area to shallow region. This result highly coincides with the field measurement data obtained by Lacy group during the 2010 Chilean Tsunami at the Monterey Bay mouth in U.S. A correction coefficient is proposed to take into account unsteady effects in the conventional estimation method using a steady friction factor; this approach is validated by comparing with numerical simulation results using k-ω turbulence model. [ABSTRACT FROM AUTHOR]

The aim of this work is to obtain estimates and parameterization of the dissipation rate of the turbulence kinetic energy (TKE dissipation), ε, induced in the upper water layer by the presence of wind waves at the surface. For this purpose, data from the laboratory measurements of the wind waves and three components of currents at six horizons in the upper water layer and four different winds are used. The measurements are performed in the wind-wave channel of the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS) [1, 2]. It is established that, for most horizons, Kolmogorov-type ranges with the property are clearly seen in the frequency spectra for the vertical velocity component Uz of the flow induced by wind and waves. Using the algorithms described in [3, 4], this fact allows us to obtain estimates of the TKE dissipation at the corresponding horizons and then establish the dependence of ε on the friction velocity , the height of waves at the surface a0, the peak frequency of the spectrum ωp, and the depth of the horizon z. An analysis of the results (according to the available data) makes it possible to propose a parameterization of the form ε ≈ 0.00025 for which a physical interpretation is proposed. [ABSTRACT FROM AUTHOR]

The impact of sea state on air–sea momentum flux (or wind stress) is a poorly understood component of wind–wave interactions, particularly in high wind conditions. The wind stress and mean wind profile over the ocean are influenced by the characteristics of boundary layer turbulence over surface waves, which are strongly modulated by transient airflow separation events; however, the features controlling their occurrence and intensity are not well known. A large-eddy simulation (LES) for wind over a sinusoidal wave train is employed to reproduce laboratory observations of phase-averaged airflow over waves in strongly forced conditions. The LES and observation both use a wave-following coordinate system with a decomposition of wind velocity into mean, wave-coherent, and turbulent fluctuation components. The LES results of the mean wind profile and structure of wave-induced and turbulent stress components agree reasonably well with observations. Both LES and observation show enhanced turbulent stress and mean wind shear at the height of the wave crest, signifying the impact of intermittent airflow separation events. Disparities exist particularly near the crest, suggesting that airflow separation and sheltering are affected by the nonlinearity and unsteadiness of laboratory waves. Our results also suggest that the intensity of airflow separation is most sensitive to wave steepness and the surface roughness parameterization near the crest. These results clarify how the characteristics of finite-amplitude waves can control the airflow dynamics, which may substantially influence the mean wind profile, equivalent surface roughness, and drag coefficient. [ABSTRACT FROM AUTHOR]

There are few observational techniques for measuring the distribution of kinetic energy within the mesosphere with a wide range of spatial and temporal scales. This study describes a method for estimating the three‐dimensional mesospheric wind field correlation function from specular meteor trail echoes. Each radar echo provides a measurement of a one‐dimensional projection of the wind velocity vector at a randomly sampled point in space and time. The method relies on using pairs of such measurements to estimate the correlation function of the wind with different spatial and temporal lags. The method is demonstrated using a multistatic meteor radar data set that includes ≈105 meteor echoes observed during a 24‐hr time period. The new method is found to be in good agreement with the well‐established technique for estimating horizontal mean winds. High‐resolution correlation functions with temporal, horizontal, and vertical lags are also estimated from the data. The temporal correlation function is used to retrieve the kinetic energy spectrum, which includes the semidiurnal mode and a 3‐hr period wave. The horizontal and vertical correlation functions of the wind are then used to derive second‐order structure functions, which are found to be compatible with the Kolmogorov prediction for spectral distribution of kinetic energy in the turbulent inertial range. The presented method can be used to extend the capabilities of specular meteor radars. It is relatively flexible and has a multitude of applications beyond what has been shown in this study. Key Points: A novel method for estimating the mesospheric wind field correlation and structure function from specular meteor trail echoesSpatial and temporal correlations of mesospheric wind fields are obtained with unprecedented coverage and resolutionStructure functions of horizontal wind fluctuations in the horizontal and vertical directions show a −5/3 power spectral index [ABSTRACT FROM AUTHOR]

• Under high wind conditions, relationship between overwater downwind turbulence intensity (TI) and wind speed is obtained from measurements. • Based on the proposed TI parameterization, overwater friction velocity (U *) can also be obtained. • The formula is verified by state-of-the-art results from the eddy-covariance, the wind gust and the wind-current interaction methods. • In addition, the proposed formula is shown to be valid up to U 10 = 56 m s−1 as compared to recent analysis on Super Typhoon Megi (2010). Analyses of simultaneous measurements of wind speed at 10 m, U 10 , significant wave height, H s and peak wave period, T p from 3 National Data Buoy Center (NDBC) buoys during the passage of 6 hurricanes over the wind seas when U 10 ≥ 9 m s −1 indicate that overwater downwind turbulence intensity (TI) can be formulized as T I = - 6.06 × 10 - 5 U 10 2 + 4.93 × 10 - 3 U 10 + 3.95 × 10 - 2 , here U 10 is the wind speed at 10 m height over sea surface. It is found that TI = 0.08 when U 10 = 9 m s−1, and it peaks up to approximately 0.14 when U 10 ≈ 40 m s −1. Thereafter, TI saturates as U 10 continuously increases. From this TI formulation, overwater friction velocity (U *) is inferred as U * = 0.4 0 U 10 (- 6.06 × 10 - 5 U 10 2 + 4.93 × 10 - 3 U 10 + 3.95 × 10 - 2). This friction velocity parameterization is further verified by 3 independent methods: the direct eddy covariance, the wind gust and the wind-current interaction. It shows that this proposed U * formula is valid up to U 10 = 56 m s−1 during Super Typhoon Megi (2010) as validated by measurements based on the wind-current interaction method. [ABSTRACT FROM AUTHOR]

• Applicability of EL point-particle approach to gyrotactic swimmer dynamics in wind-sheared free-surface turbulence. • Simulations with turbulence and swimmer dynamics based on LPT in DNS flow fields. • Phenomenology of gyrotaxis-biased surfacing of swimmers in free-surface turbulence. We examine the effect of wind-induced shear on the orientation and distribution of motile micro-swimmers in free-surface turbulence. Winds blowing above the air–water interface can influence the distribution and productivity of motile organisms via the shear generated just below the surface. Swimmer dynamics depend not only on the advection of the fluid but also on external stimuli like nutrient concentration, light, gravity, which are in turn coupled to and influenced by the distribution of the swimmers. Here we focus on gyrotaxis, resulting from the gravitational torque generated by an asymmetric mass distribution within the organism. The combination of such torque with the viscous torque due to shear can reorient swimmers, reducing their vertical migration and causing entrapment in horizontal fluid layers. Through DNS-based Euler–Lagrangian simulations we investigate the effect of wind-induced shear on the motion of gyrotactic swimmers in turbulent open channel flow. We consider different wind forcing and swimmers with different reorientation time (reflecting the ability to react to turbulent fluctuations). We show that only stable (high-gyrotaxis) swimmers may reach the surface and form densely concentrated filaments, the topology of which depends on the wind direction. Otherwise swimmers exhibit weaker vertical fluxes and loose segregation at the surface. [ABSTRACT FROM AUTHOR]

Vertically 0.042‐m‐spaced moored high‐resolution temperature sensors are used for detailed internal wave‐turbulence monitoring near Texel North Sea and Wadden Sea beaches on calm summer days. In the maximum 2‐m deep waters, irregular internal waves are observed supported by the density stratification during day‐times' warming in early summer, outside the breaking zone of < 0.2 m surface wind waves. Internal‐wave–induced convective overturning near the surface and shear‐driven turbulence near the bottom are observed in addition to near‐bottom convective overturning due to heating from below. Small turbulent overturns have durations of 5–20 s, close to the surface wave period and about one third to one tenth of the shortest internal wave period. The largest turbulence dissipation rates are estimated to be of the same order of magnitude as found above deep‐ocean seamounts, whereas overturning scales are observed 100 times smaller. The turbulence inertial subrange is observed to link between the internal and surface wave spectral bands. [ABSTRACT FROM AUTHOR]

Interactions between a patchy degraded Zostera noltei seagrass meadow and waves, currents, and sedimentary processes were analyzed from data obtained from a strongly wind-influenced micro-tidal brackish water lagoon in southeastern France. Measurements were conducted on offshore and foreshore morphology (topography, bathymetry), on hydrodynamics (waves, water levels, and currents) under different wind conditions within and outside the meadow, and on meadow biometry (shoot density, leaf length). The main impact of this patchy meadow on wind-wave transformations seems to be attenuation of waves further offshore than in the absence of vegetation. This attenuation is particularly notable above the meadow front edge, and is related to wave heights, water levels, and wave periods that are, in turn, dependent on wind intensity and fetch length. The data show that the patchy meadow does not attenuate small and short waves, especially when water levels are high, but is capable, like salt marshes and artificial seagrass, of attenuating relatively high and long waves. Notwithstanding its patchy and degraded character, the meadow also strongly influences the vertical distribution of currents. Whereas currents are strong and significantly influenced by wind and wind waves above the meadow, both waves and currents are dissipated in a transitional canopy-water layer. These wave and current modifications are reflected in the evolution of the seabed. Erosion and sedimentation are mainly controlled by the hydrodynamics but the seasonal state of the meadow plays a role by modulating the hydrodynamics. These substrate changes are, important, in turn, in influencing protection of the shoreline. [ABSTRACT FROM AUTHOR]

Designing reliable and cost-effective floating bridges for wide and deep fjords is very challenging. The floating bridge is subjected to various environmental loads, such as wind, wave, and current loads. All these loads and associated load effects should be properly evaluated for ultimate limit state design check. In this study, the wind-, wave-, and current-induced load effects are comprehensively investigated for an end-anchored curved floating bridge, which was an early concept for crossing the Bjørnafjorden. The considered floating bridge is about 4600 m long and consists of a cable-stayed high bridge part and a pontoon-supported low bridge part. It also has a large number of eigen-modes, which might be excited by the environmental loads. Modeling of wind loads on the bridge girder is first studied, indicating that apart from aerodynamic drag force, aerodynamic lift and moment on the bridge girder should also be considered due to their significant contribution to axial force. Turbulent wind spectrum and spatial coherence play an important role and should also be properly determined. The sway motion, axial force, and strong axis bending moment of the bridge girder are mainly induced by wind loads, while the heave motion, weak axis bending moment, and torsional moment are mainly induced by wave loads. Turbulent wind can cause significant larger low-frequency eigen-mode resonant responses than the second-order difference frequency wave loads. Current loads mainly contribute damping and reduce the variations of sway motion, axial force, and strong axis bending moment. [ABSTRACT FROM AUTHOR]

We analyze surface wave data taken in Currituck Sound, North Carolina, during a storm on 4 February 2002. Our focus is on the application of nonlinear Fourier analysis (NLFA) methods (Osborne 2010) to analyze the data set: The approach spectrally decomposes a nonlinear wave field into sine waves, Stokes waves, and phase-locked Stokes waves otherwise known as breather trains. Breathers are nonlinear beats, or packets which "breathe" up and down smoothly over cycle times of minutes to hours. The maximum amplitudes of the packets during the cycle have a largest central wave whose properties are often associated with the study of "rogue waves." The mathematical physics of the nonlinear Schrödinger (NLS) equation is assumed and the methods of algebraic geometry are applied to give the nonlinear spectral representation. The distinguishing characteristic of the NLFA method is its ability to spectrally decompose a time series into its nonlinear coherent structures (Stokes waves and breathers) rather than just sine waves. This is done by the implementation of multidimensional, quasi-periodic Fourier series, rather than ordinary Fourier series. To determine preliminary estimates of nonlinearity, we use the significant wave height Hs, the peak period Tp, and the length of the time series T. The time series analyzed here have 8192 points and T =1677.72 s = 27.96 min. Near the peak of the storm, we find Hs ≈ 0.55 m, Tp ≈ 2.4 s so that for the wave steepness of a near Gaussian process, S=π5/2/gHs/Tp2, we find S ≈ 0.17, quite high for ocean waves. Likewise, we estimate the Benjamin-Feir (BF) parameter for a near Gaussian process, IBF=π5/2/gHsT/Tp3, and we find IBF ≈ 119. Since the BF parameter describes the nonlinear behavior of the modulational instability, leading to the formation of breather packets in a measured wave train, we find the IBF for these storm waves to be a surprisingly high number. This is because IBF, as derived here, roughly estimates the number of breather trains in a near Gaussian time series. The BF parameter suggests that there are roughly 119 breather trains in a time series of length 28 min near the peak of the storm, meaning that we would have average breather packets of about 14 s each with about 5-6 waves in each packet. Can these surprising results, estimated from simple parameters, be true from the point of view of the complex nonlinear wave dynamics of the BF instability and the NLS equation? We analyze the data set with the NLFA to verify, from a nonlinear spectral point of view, the presence of large numbers of breather trains and we determine many of their properties, including the rise time for the breathers to grow to their maximum amplitudes from a quiescent initial state. Energetically, about 95% of the NLFA components are found to consist of breather trains; the remaining small amplitude components are sine and Stokes waves. The presence of a large number of densely packed breather trains suggests an interpretation of the data in terms of breather turbulence, highly nonlinear integrable turbulence theoretically predicted for the NLS equation, providing an interesting paradigm for the nonlinear wave motion, in contrast to the random phase Gaussian approximation often considered in the analysis of data. [ABSTRACT FROM AUTHOR]

The nonlinear dependence of gas transfer velocity on wind speed typically results in the best fit of observational data; however, gas transfer velocity is dimensionally inconsistent with the nonlinear wind speeds. The objective of the current study was to show that, in the case of wind waves, gas transfer velocity with a consistent dimension should be determined by turbulent viscosity instead of by the viscosity of water when parameterised using the renewal model. Turbulent kinetic energy (TKE) near the air-sea interface is significantly intensified by breaking wind waves. By analyzing various models, we found that the TKE dissipation rate was explicitly linearly related to wind speed and dependent on wave age, with powers ranging from -2.36 to 4.0. Various models show that wave energy dissipation (WED) due to wind wave breaking explicitly increases with the cube of the wind speed and weakly depends on wave age. Assuming a balance between WED and total TKE dissipation in a constant dissipation layer, the depth of this layer was shown to be comparable to the wave height. Using the traditional renewal model with wind wave turbulent viscosity and TKE dissipation rate at the sea surface, we found that the gas transfer velocity was explicitly linearly related to wind speed in a dimensionally consistent manner, and depended simultaneously on the wave age and drag coefficient. These results are consistent with observational data obtained using the eddy correlation method. We emphasise that the linear dependence on wind speed is only valid when the wave age and drag coefficient are fixed; thus, this finding cannot be directly confirmed by currently available observational data due to a lack of wave state information. [ABSTRACT FROM AUTHOR]

An analysis of coherent measurements of winds and waves from data collected during the ONR Southern California 2013 (SoCal2013) program from R/P FLIP off the coast of Southern California in November 2013 is presented. An array of ultrasonic anemometers mounted on a telescopic mast was deployed to resolve the vertical profile of the modulation of the marine atmospheric boundary layer by the waves. Spectral analysis of the data provides the wave-induced components of the wind velocity for various wind-wave conditions. Results show that the wave-induced fluctuations depend both on the spectral wave age and the normalized height , where c is the linear phase speed of the waves with wavenumber k and is the mean wind speed measured at the height z. The dependence on the spectral wave age expresses the sensitivity of the wave-induced airflow to the critical layer where . Across the critical layer, there is a significant change of both the amplitude and phase of the wave-induced fluctuations. Below the critical layer, the phase remains constant while the amplitude decays exponentially depending on the normalized height. Accounting for this double dependency, the nondimensionalization of the amplitude of the wave-induced fluctuations by the surface orbital velocity collapses all the data measured by the array of sonic anemometers, where a is the amplitude of the waves. [ABSTRACT FROM AUTHOR]

• Strip waves into surge and wind-wave, with wind-wave being more relevant to TKE. • In addition to wind speed, wind direction also has a significant effect on TKE. • "wind value": coupling wind direction into speed using trigonometric functions. • Sediment resuspension is not necessarily proportional to flow velocity, but to TKE. • High-density plants can change the flow pattern to slow down sediment suspension. Wind direction and velocity determines the hydrodynamic structure in the shallow lakes. Notably, submerged vegetation also affect the hydrodynamic structure of the thermocline region of the lake which in turn affects the resuspension of benthic sediments. Therefore, based on meteorological and high-frequency hydrodynamic data, this study analyses the effect of wind field on the vertical shear of the flow field. Besides, study used Pierson–Moskowitz (PM) spectra to strip wind and surge waves to elucidate the response mechanism of turbulence in Meiliang Bay of Taihu Lake. Based on indoor experiments, we simulate the "wind value" (joint effect of wind speed and direction) on hydrodynamics and sediment release under different hydrophyte density cover. The results show that both wind and wave fields affect the hydrodynamic structure. In summer, the vertical shear layer of the flow field will shift downwards due to the wind field. The wave field consists of wind waves and surge waves, the wind waves having a greater impact on turbulence energy. A characteristic quantity 'wind value' is constructed using trigonometric functions which can characterize wind speed and wind direction of arbitrary magnitude. The correlation between 'wind value' and near-bottom turbulence is significantly better than that of wind speed alone, indicating that the effect of wind direction on turbulence cannot be ignored. Although, within a certain range, sediment re-suspension is not necessarily proportional to flow velocity but to turbulent kinetic energy due to the effect of wind direction. High-density submerged vegetation can form a wall boundary protection, which changes the flow pattern of the lake from mixed laminar flow to boundary laminar flow, reducing the intensity of turbulence and impeding sediment re-suspension. Findings of this study can provide a reference for endogenous control and aquatic ecological restoration in shallow lakes. [ABSTRACT FROM AUTHOR]

The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for the development of theories and methods on ocean remote sensing, in particular for oil spill remote sensing. Recently it has been shown that microwave radar return is determined by both Bragg and non-Bragg (non-polarized) scattering mechanisms and some evidence has been given that the latter is associated with wave breaking, in particular, with strong breaking such as spilling or plunging. However, our understanding of mechanisms of the action of strong wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. In this paper an effect of suppression of radar backscattering after strong wave breaking has been revealed experimentally and has been attributed to the wind ripple suppression due to turbulence generated by strong wave breaking. The experiments were carried out in a wind wave tank where a frequency modulated wave train of intense meter-decimeter-scale surface waves was generated by a mechanical wave maker. The wave train was compressed according to the gravity wave dispersion relation (“dispersive focusing”) into a short-wave packet at a given distance from the wave maker. Strong wave breaking with wave crest overturning (spilling) occurred for one or two highest waves in the packet. Short decimeter-centimeter-scale wind waves were generated at gentle winds, simultaneously with the long breaking waves. A Ka-band scatterometer was used to study microwave backscattering from the surface waves in the tank. The scatterometer looking at the area of wave breaking was mounted over the tank at a height of about 1 m above the mean water level, the incidence angle of the microwave radiation was about 50 degrees. It has been obtained that the radar return in the presence of short wind waves is characterized by the radar Doppler spectrum with a peak roughly centered in the vicinity of Bragg wave frequencies. The radar return was strongly enhanced in a wide frequency range of the radar Doppler spectrum when a packet of long breaking waves arrived at the area irradiated by the radar. After the passage of breaking waves, the radar return strongly dropped and then slowly recovered to the initial level. Measurements of velocities in the upper water layer have confirmed that the attenuation of radar backscattering after wave breaking is due to suppression of short wind waves by turbulence generated in the breaking zone. A physical analysis of the effect has been presented.