Session L26: Geophysical Fluid Dynamics: Air-Sea Interaction I

Direct numerical simulations of coupled turbulent wind-wave flow at high wind speed

The interaction between wind currents and ocean waves at wind speed above 25 m/s remains poorly understood. To address this research gap, we investigate a two-phase boundary layer at high wind speed fully resolving the evolving wave field using direct numerical simulations of the Navier-Stokes equations, combined with a geometric Volume-of-Fluid method, to capture the gas-liquid interface. The simulations utilize Basilisk's adaptive mesh refinement framework, ensuring sufficient resolution at the two-phase interface. In our study, we vary the ratio between wave speed and friction velocity c/u_s = [1,2] and the initial wave steepness a₀k=[0.2-0.3], while keeping the friction and wave Reynolds numbers and the Bond number equal to Re_τ=720, Re_w=25000 and Bo=200. Upon an initial transient, the water waves undergo a cycle of growth and breaking stages. For each case, we quantify the stress and energy budget partition between pressure and viscous stress contributions at the interface. We systematically evaluate how the velocity and pressure profiles change in both phases during pre- and post-breaking events. Finally, by increasing the Bond number to Bo=1000, we assess how a significant amount of sea spray affects the exchanged momentum and energy fluxes.   

Breaking induced turbulence generation and dissipation in simulated broadband wave fields

Wave breaking in steep ocean wave fields is the major dissipation mechanism for waves and is known to generate turbulence and mixing in the upper ocean. However, due to the complexity related to the multi-scale and nonlinear nature of wave breaking, both the generation of turbulence and dissipation are still not well understood and the parameterizations of both are largely empirical. In this follow-up study of breaking broadband wave fields using the multi-layer numerical model (Wu et al. JFM 2023), we characterize the generation of underwater turbulence and the breaking-related dissipation of the wave field. The simulations produce averaged mixing layer depth and shape similar to those previously observed, with additional dynamical features that can potentially help improve the wave-induced mixing parameterization. Dissipation is analyzed in the framework of the fifth moment of breaking front distribution, and uncertainties of the breaking strength parameter are discussed.   

Linking drops and associated bubbles in laboratory experiments on spray generation by collective bubble bursting

Bubbles entrained by breaking waves rise to the ocean surface, where they reside before bursting and releasing droplets into the atmosphere. The ejected aerosols affect the climate, motivating the study of spray generation by collective bubble bursting at the air-sea interface. To investigate the controlling parameters and mechanisms of this spray generation, experiments were conducted in a 50x50x60 cm³ bubbling tank filled with solutions of artificial seawater. Measurements of bulk bubbles, surface bubbles, drops, and dry aerosol particles were made for all cases, which included a variety of injection bubble sizes and salinity levels. We first varied the injection bubble size distribution, including both monodisperse and broad-banded cases, and we attempt to attribute drops of sizes 0.1-100 microns to bursting bubbles ranging from 0.05 to 5 millimeters. Experiments were then run using a lower salinity solution for each bubble size configuration to further study the effect of salinity on drop production. Using the results from numerous cases, we discuss the attribution of drops to the associated bursting bubbles and compare the collective bursting results to existing single bubble studies.   

Impact of Near-Surface Airflow Structure around Mechanical Breaking Waves on Inertial Marine Aerosol Dynamics

The contribution of inertial droplets to the overall air-sea fluxes induced by spray is not well understood. Despite significant progress in understanding the role of small passive particles through field and laboratory studies in the last two decades, there remains considerable uncertainty in estimating inertial droplet fluxes. This uncertainty arises from limited knowledge about the production and initial dynamics of inertial droplets.

This talk will present findings from two distinct experiments conducted at the University of Delaware's wind-wave flume. The first experiment measured the kinematics of inertial droplets generated from breaking waves with varying wave slopes and wave ages.  The second experiment mapped airflow velocities and structure in the vicinity of the breaking wave through particle imaging velocimetry. Utilizing results from both studies, inertial droplet dynamics will be discussed including estimated slip velocities and droplet trajectories for varying droplet sizes and wind/wave conditions.   

Laboratory measurements on both sides of the air-water interface during early wind-wave generation

Ocean waves play a key role in controlling air-sea exchanges of momentum, energy, and gasses. Several researchers studied theoretically, experimentally, and numerically the influence of surface waves on the wind stress and how wind contributes to wave generation and growth. From an experimental point of view, taking air- or water-side measurements close to the moving wavy surface, is very challenging. And, because of the difference in air and water densities (and viscosity), most existing experimental and numerical studies have not investigated both fluids simultaneously. Here, we present laboratory experiments using a combination of PIV-LIF (Particle Image Velocimetry – Laser Induced Fluorescence) techniques, where air and water velocity are measured simultaneously as close to the surface as O(100 um) in the early stage of wind-wave generation and until a steady wind-wave equilibrium is reached. Our aim is to observe coherence structures in both fluid phases and to link velocity and stresses at the interface. We use a triple decomposition of the velocity signal to partition the contributions of wave- and turbulent-related stresses, and to close momentum and energy balances in the observed control volume.   

The Dynamic Influence of Air-Sea Temperature Gradient on Near-Surface Turbulent Eddy Scales within the Marine Atmospheric Boundary Layer.

In the marine atmospheric boundary layer (MABL), the difference in temperature between the air just above the sea surface and the water at the sea surface, △T is a driving parameter for characterizing the vertical variability of the wind velocity field, and, indirectly, the atmospheric stability. For this study, wind velocity data collected at Galveston Island State Park, Texas, with a scanning wind LiDAR are coupled with wind/sea data collected from a buoy station deployed in the proximity of this coastal site at a distance of 32 miles from the shore to investigate the variability of the MABL velocity field with different atmospheric/wave conditions. Preliminary findings indicate the prevalence of old waves during onshore conditions and young waves during offshore conditions. Furthermore, onshore wind conditions are typically associated with a positive △T, namely the air temperature is larger than the water temperature. Conversely, the occurrence of negative △T is typically associated with offshore winds. The dimensionless velocity gradient derived from LiDAR measurements shows a correlation with △T, thus with the Bulk Richardson number. Finally, it is observed that positive △T, which can be associated with stable atmospheric conditions, leads to a significant increase in wind shear.   

Study of Turbulence Response to Wave Forcing using a Fast and Accurate Simulation Method

Surface waves play a crucial role in shaping Langmuir circulations and enhancing the turbulent mixing and transport in the upper-ocean boundary layer.  Conventionally, the wave effect on the turbulence is modeled using the wave-phase-averaged approach.  In this work, we introduce a wave-phase-resolved method for accurate and efficient simulations of turbulence under waves, directly coupling turbulence simulation with instantaneous wave orbital motions without the wave-phase averaging. We also introduce an approximate boundary condition, which allows us to formulate the turbulence simulation in a rigid-lid box domain while capturing the wave-phase effect. Using the proposed wave-phase-resolved method, we conduct simulations of a wind-shear-driven turbulence boundary layer interacting with a wave group. Our results show a significant increase in the turbulence intensity, particularly in the vertical fluctuations, after the wave group passes. This work elucidates how turbulence responds to the transient wave forcing in a wave group.   

Numerical Study of Droplet Generation by Breaking Wind Waves

Breaking waves have been extensively studied numerically, but still, there remains a notable gap in understanding turbulent wind’s impact on wave breaking and sea spray generation. Understanding wind’s role in sea-spray droplet generation is vital for the modeling of heat, mass, and momentum exchange between oceans and marine atmospheric boundary layer. In this study, we employ direct numerical simulation of two-fluid flows to explore the dynamic interplay between wind and breaking waves. To start the simulation, we use a fully developed turbulent wind flow over strongly forced steep waves with the initial wave elevation following a third-order Stokes wave of finite amplitude. The formation of droplets and bubbles from air-sea interactions is accurately captured using the coupled level-set and volume-of-fluid method. By analyzing the instantaneous distribution of spray droplets, we obtain the time history of droplet number and the droplet size spectrum. For a detailed analysis of spray droplets, we implement the optimal network (ON) algorithm for Lagrangian tracking of the droplets efficiently. The ON algorithm provides insights into droplet trajectories and residence times and enables the detection of droplet creation, extinction, fragmentation, and coalescence events.   

Evolution of ocean surface waves in the early stage of wind-wave generation

How the ocean surface waves respond to the turbulent wind in the early stage of the wind-wave generation process has been a fascinating research topic for several decades.  In this study, we conduct high-fidelity numerical simulations to reveal the multi-stage wave evolution.  We have discovered a nascent stage in wind-wave generation, which occurs immediately after a turbulent wind impacts a calm water surface.  This nascent stage precedes the initial stage described by Phillips' resonance theory and exhibits a new quartic law of growth over time.  Our study offers direct numerical evidence supporting the existence of the resonance mechanism in the initial stage of Phillips' theory.  The results shed light on the resonance mechanism's role in shaping the distribution of wave energy in the spectral space.  Based on numerical observations, we further investigate surface wave energy growth in the initial stage, for which a quantitative analysis was challenging.  We develop a novel complex analysis approach to quantify the temporal evolution of wave energy in Phillips' initial stage.  Our study presents a quantitative analysis of surface wave energy growth, revealing the significant influence of surface tension on both the resonance curve and wave energy amplification.

* This work was supported by Office of Naval Research.  Tianyi Li’s work after August 28, 2023 was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Laboratory study of the vertical concentration profile of water droplets near the ocean surface in a rain field

Secondary water droplets generated by the impact of raindrops on a deep-water pool are studied experimentally in an artificial rain facility. Artificial rain is produced by a rain generator that consists of a water-filled open-surface rectangular tank with an array of hypodermic needles attached to its bottom. Experiments are performed by mounting the rain generator above the water pool at a vertical distance of 2.2 m. With this distance, the impact velocities of the raindrops with a diameter of about 3.0 mm can reach 70% of their terminal velocity in natural conditions. Different rain rates are used. Droplets are measured at various heights above the pool's water surface by using a cinematic digital in-line holographic technique. Both diameters and motions of the secondary droplets and raindrops are extracted from the high-speed hologram movies. It is found that the diameters and concentrations of the secondary droplets in the rain field change drastically with the height above the pool's surface. The effects of rain rate and raindrop diameter on the vertical concentration profile of secondary droplets are investigated.   

Modeling microplastic selectivity and enhancement in small sea spray aerosols

Recent field studies have suggested that the ocean may be a source as well as a sink for micro- and nanoplastics through emissions in sea spray aerosols. Bursting bubbles are known to transport large quantities of particles, such as bacteria and sea salt, into the atmosphere. This effect is enhanced when hydrophobic particles are scavenged on the bubbles, producing highly enriched droplets. However, estimates of plastic transport via this pathway have large uncertainties due to limited size detection techniques in field studies and relatively few laboratory studies. Here we combine various physical models to provide some guiding principles. These new insights reveal the dominance of jet drops in particle transport via bursting bubbles and highlight the strong size-dependence of droplet enrichment, expanding our understanding of the complex fate and transport dynamics of micro and nanoparticulates in the ocean.