Grant B. Deane

Scale dependence of bubble creation mechanisms in breaking waves

Breaking ocean waves entrain air bubbles that enhance air–sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants. The size distribution of the entrained bubbles is the most important factor in controlling these processes, but little is known about bubble properties and formation mechanisms inside whitecaps. We have measured bubble size distributions inside breaking waves in the laboratory and in the open ocean, and provide a quantitative description of bubble formation mechanisms in the laboratory. We find two distinct mechanisms controlling the size distribution, depending on bubble size. For bubbles larger than about 1 mm, turbulent fragmentation determines bubble size distribution, resulting in a bubble density proportional to the bubble radius to the power of -10/3. Smaller bubbles are created by jet and drop impact on the wave face, with a -3/2 power-law scaling. The length scale separating these processes is the scale where turbulent fragmentation ceases, also known as the Hinze scale. Our results will have important implications for the study of air–sea gas transfer.

Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60–180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Surface wave focusing and acoustic communications in the surf zone

The forward scattering of acoustic signals off of shoaling surface gravity waves in the surf zone results in a time-varying channel impulse response that is characterized by intense, rapidly fluctuating arrivals. In some cases, the acoustic focusing by the curvature of the wave crest results in the formation of caustics at or near a receiver location. This focusing and the resulting caustics present challenges to the reliable operation of phase coherent underwater acoustic communications systems that must implicitly or explicitly track the fluctuations in the impulse response. The propagation physics leading to focusing are studied with both experimental data and a propagation model using surface wave profiles measured during the collection of the experimental data. The deterministic experimental and modeled data show good agreement and demonstrate the stages of the focusing event and the impact of the high intensity arrivals and rapid fluctuations on the ability of an algorithm to accurately estimate the impulse response. The statistical characterization of experimental data shows that the focusing by surface gravity waves results in focused surface reflected arrivals whose intensity often exceeds that of the direct arrival and the focusing and caustic formation adversely impacts the performance of an impulse response estimation algorithm.

Sea spray aerosol as a unique source of ice nucleating particles

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Air Entrainment Processes and Bubble Size Distributions in the Surf Zone

Abstract A new optical instrument was deployed in the surf zone in a trial experiment to measure bubble size distributions and visualize air entrainment and bubble formation mechanisms within breaking surf. Images of bubbles and the evolving air–water mixture inside and beneath breaking wave crests are presented. The images resolve features of the air–water mixture to length scales of hundreds of microns across a 3.7-cm field of view. Two qualitatively different large-scale air entrainment processes are observed. First, intrusions of air and water, thought to be created by jets penetrating the water’s surface, fragment into plumes of bubbles. Second, an air cavity trapped by the overturning wave crest is observed to disintegrate into bubbles. The timescale for the evolution from a compacted air–water mass to individual bubbles was on the order of 90 ms or less for both of these processes. In addition, small-scale air filaments hundreds of microns wide and millimeters long have been discovered beneath wave...

Spatial and Temporal Variability of Internal Wave Forcing on a Coral Reef

The deployment of a dense spatial array of temperature sensors on a coral reef in the Florida Keys provided a unique view of the interaction of cool water incursions generated by internal waves with the three-dimensional reef bathymetry. Water temperature on the reef surface was sampled every 5 s at 100 points on a 100 m by 150 m grid with concomitant measurements of water column velocity and temperature from mid-May through mid-August 2003. Episodic incursions of cool, subsurface water were driven by periods of strong semidiurnal internal tide and higher-frequency internal wave activity. For every time step in the data record the mean profile of temperature as a function of depth is calculated with a 3-m vertical averaging length scale. Subtracting this mean profile from the raw record yields a within depth, horizontal temperature anomaly. Visualization through time of the anomaly mapped onto the measured reef bathymetry reveals episodic variability of thermal patchiness across the reef as well as persistent features associated with reef bathymetry. Variation in the nature of the cooling and resulting thermal heterogeneity among events and seasons suggests multiple modes of cool water incursion ranging from unbroken, tidal period internal waves to packets of higher-frequency, energetic, broken internal bores.

Imaging in the ocean with ambient noise: the ORB experiments

Acoustic daylight imaging is a new technique that has been proposed for creating pictorial images of objects in the ocean from the ensonification provided by the incident ambient noise field. To investigate the feasibility of the technique, a series of experiments was performed from the research platform ORB, moored in San Diego Bay, Southern California. Central to these experiments was an acoustic receiver known as ADONIS (acoustic daylight ocean noise imaging system), which consists of a spherical reflector, 3 m in diameter, with an elliptical array of 130 hydrophones at the focal surface. This system, which is broadband, operating between 8 and 80 kHz, forms a total of 126 receive-only beams spanning the vertical and horizontal. The ambient noise power in each beam is mapped into a pixel on a VDU. Various types of targets were used in the experiments, including planar panels and cylindrical, polyethylene drums containing wet sand, seawater or syntactic foam (essentially air), and most of the experiment...

An analysis of the three‐dimensional sound field in a penetrable wedge with a stratified fluid or elastic basement

A new analytical solution for the three‐dimensional sound field due to an harmonic, point source in a broad class of penetrable wedges is presented. The only restriction on the wedge basement is that it be horizontally stratified. Thus realistic models of the ocean floor, such as a fluid or elastic sediment layer overlying a hard elastic substrate, can be handled. The wave‐theoretic solution is based on the method of source images in conjunction with an extension of Brekhovskikh and Lysanov’s analysis of the reflection of a spherical wave field from a planar interface. The new solution is accurate throughout the wedge, including the normally difficult modal cutoff regions, where energy propagates through the wedge at grazing angles close to the critical grazing angle; and it is also valid through the hyperbolic caustics that border the region ensonified by each mode.

Estimating the compressional and shear wave speeds of a shallow water seabed from the vertical coherence of ambient noise in the water column

Due to the multiple bottom reflections encountered in shallow water environments, the spatial structure of the ambient noise field depends strongly on the geoacoustic properties of the seabed, which are invariant over time scales associated with most measurements. The vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed, rather than temporal variations in the surface source distribution. In this paper, estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds. Using a model of wind-generated noise over an elastic seabed, it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed. An inversion procedure is developed based on a matched field of the complex, broadband coherence from a single hydrophone pair. Using ambient noise data from two shear supporting sites, compressional and shear wave estimates are o...

Two Regimes of Laboratory Whitecap Foam Decay: Bubble-Plume Controlled and Surfactant Stabilized

A laboratory experiment to quantify whitecap foam decay time in the presence or absence of surface active material is presented. The investigation was carried out in the glass seawater channel at the Hydraulics Facility of Scripps Institution of Oceanography. Whitecaps were generated with focused, breaking wave packets infilteredseawaterpumpedfromLaJollaShoresBeachwithandwithouttheadditionofthesurfactantTriton X-100. Concentrations of Triton X-100 (204 m gL 21 ) were chosen to correspond to ocean conditions of mediumproductivity.Whitecapfoamandsubsurfacebubble-plumedecaytimesweredeterminedfromdigital images for a range of wave scales and wave slopes. The experiment showed that foam lifetime is variable and controlledbysubsurfacebubble-plume-degassingtimes,whichareafunctionofwavescaleandbreakingwave slope. This is true whether or not surfactants are present. However, in the presence of surfactants, whitecap foam is stabilizedand persists for roughly a factorof 3 times its clean seawater value. The range of foam decay times observed in the laboratory study lie within the range of values observed in an oceanic dataset obtained off Martha’s Vineyard in 2008.