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We revisit the classical but as yet unresolved problem of predicting the breaking onset of 2D and 3D irrotational gravity water waves. Based on a fully nonlinear 3D boundary element model, our numerical simulations investigate geometric, kinematic and energetic differences between maximally tall non-breaking waves and marginally breaking waves in focusing wave groups. Our study focuses initially on unidirectional domains with flat bottom topography and conditions ranging from deep to intermediate depth (depth to wavelength ratio from 1 to 0.2). Maximally tall non-breaking (maximally recurrent) waves are clearly separated from marginally breaking waves by their normalised energy fluxes localised near the crest tip region. The initial breaking instability occurs within a very compact region centred on the wave crest. On the surface, this reduces to the local ratio of the energy flux velocity (here the fluid velocity) to the crest point velocity for the tallest wave in the evolving group. This provides a robust threshold parameter for breaking onset for 2D wave packets propagating in uniform water depths from deep to intermediate. Further targeted study of representative cases of the most severe laterally focused 3D wave packets in deep and intermediate depth water shows that the threshold remains robust. These numerical findings for 2D and 3D cases are closely supported by our companion observational results. Warning of imminent breaking onset is detectable up to a fifth of a carrier wave period prior to a breaking event.

The breaking probability is investigated for the dominant surface waves observed in three geographically diverse natural bodies of water: Lake Washington, the Black Sea and the Southern Ocean. The breaking probability is taken as the average number of breaking waves passing a fixed point per wave period.

In this paper we report on a laboratory study, the Spray Production and Dynamics Experiment (SPANDEX), conducted at the University of New South Wales Water Research Laboratory in Australia. The goals of SPANDEX were to illuminate physical aspects of spume droplet production and dispersion; verify theoretical simplifications used to estimate the source function from ambient droplet concentration measurements; and examine the relationship between the implied source strength and forcing parameters such as wind speed, surface turbulent stress, and wave properties. Observations of droplet profiles give reasonable confirmation of the basic power law profile relationship that is commonly used to relate droplet concentrations to the surface source strength. This essentially confirms that, even in a wind tunnel, there is a near balance between droplet production and removal by gravitational settling. The observations also indicate considerable droplet mass may be present for sizes larger than 1.5 mm diameter. Phase Doppler Anemometry observations revealed significant mean horizontal and vertical slip velocities that were larger closer to the surface. The magnitude seems too large to be an acceleration time scale effect. Scaling of the droplet production surface source strength proved to be difficult. The wind speed forcing varied only 23% and the stress increased a factor of 2.2. Yet, the source strength increased by about a factor of 7. We related this to an estimate of surface wave energy flux through calculations of the standard deviation of small‐scale water surface disturbance, a wave‐stress parameterization, and numerical wave model simulations. This energy index only increased by a factor of 2.3 with the wind forcing. Nonetheless, a graph of spray mass surface flux versus surface disturbance energy is quasi‐linear with a substantial threshold.

During the Southern Ocean Waves Experiment (SOWEX), registered ocean wave topography and backscattered power data at Ka band (36 GHz) were collected with the NASA Scanning Radar Altimeter (SRA) off the coast of Tasmania under a wide range of wind and sea conditions, from quiescent to gale-force winds with 9-m significant wave height. Collection altitude varied from 35 m to over 1 km, allowing determination of the sea surface mean square slope (mss), the directional wave spectrum, and the detailed variation of backscattered power with incidence angle, which deviated from a simple Gaussian scattering model. The non-Gaussian characteristics of the backscatter increased systematically with the mss, suggesting that a global model to characterize Ka-band radar backscatter from the sea surface within 25° of nadir might be possible.

Optical variability occurs in the near‐surface and upper ocean on very short time and space scales (e.g., milliseconds and millimeters and less) as well as greater scales. This variability is caused by solar, meteorological, and other physical forcing as well as biological and chemical processes that affect optical properties and their distributions, which in turn control the propagation of light across the air‐sea interface and within the upper ocean. Recent developments in several technologies and modeling capabilities have enabled the investigation of a variety of fundamental and applied problems related to upper ocean physics, chemistry, and light propagation and utilization in the dynamic near‐surface ocean. The purpose here is to provide background for and an introduction to a collection of papers devoted to new technologies and observational results as well as model simulations, which are facilitating new insights into optical variability and light propagation in the ocean as they are affected by changing atmospheric and oceanic conditions. Key Points Optical variability ocean on very short time and space scales Factors affecting optical properties and propagation of light in the ocean New research and increased understanding of light propagation at small scale

Particle image velocimetry (PIV) is a state-of-the-art non-intrusive technique for velocity and fluid flow measurements. Due to ongoing improvements in image hardware and processing techniques, the diversity of applications of the PIV method continues to increase. This study presents an accurate thermal image velocimetry (TIV) technique using a CO2 laser source to measure the surface wave particle velocity using infrared imagery. Experiments were carried out in a 2-D wind wave flume with glass side walls for deep-water monochromatic and group waves. It was shown that the TIV technique is robust for both unforced and wind-forced group wave studies. Surface wave particles attain their highest velocity at the group crest maximum and slow down thereafter. As previously observed, each wave crest slows down as it approaches its crest maximum but this study demonstrates that the minimum crest speed coincides with maximum water velocity at the wave crest. Present results indicate that breaking is initiated once the water surface particle velocity at the wave crest exceeds a set proportion of the velocity of the slowing crest as it passes through the maximum of a wave group.

The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air-sea interaction experiment in which a specially instrumented meterological research aircraft simultaneously gathered atmospheric turbulence data in the marine boundary layer and sea surface topography data over the Southern Ocean for a wide range of wind speeds.

An experimental study of wind energy and momentum input into finite-depth wind waves was undertaken at Lake George, New South Wales, Australia. To measure microscale oscillations of induced pressure above surface waves, a high-precision wave-follower system was developed at the University of Miami, Florida. The principal sensing hardware included Elliott pressure probes, hot-film anemometers, and Pitot tubes. The wave-follower recordings were supplemented by a complete set of relevant measurements in the atmospheric boundary layer, on the surface, and in the water body. This paper is dedicated to technical aspects of the measurement procedure and data analysis. The precision of the feedback wave-following mechanism did not impose any restrictions on the measurement accuracy in the range of wave heights and frequencies relevant to the problem. Thorough calibrations of the pressure transducers and moving Elliott probes were conducted. It is shown that the response of the air column in the connecting tubes provides a frequency-dependent phase shift, which must be accounted for to recover the low-level induced pressure signal. In the finite-depth environment of Lake George, breaking waves play an important role in the momentum exchange between wind and waves, as will be shown in a subsequent paper.

Microscale breaking wind waves cover much of the surface of open waters exposed to moderate wind forcing. Recent studies indicate that understanding the nature and key features of the surface skin flows associated with these small waves is fundamental to explaining the dramatic enhancement of constituent exchange that occurs in their presence. We describe a laboratory study in which velocity measurements were made within a few hundred micrometres of the surface of microscale breaking wind waves without bubble entrainment, using flow visualization and particle image velocimetry (PIV) techniques for a range of wind speed and fetch conditions. Our measurements show that for each experiment, the mean surface drift directly induced by the wind on the upwind faces and crests of these waves is ($0.23\\,{\\pm}\\,0.02$)${u}^a_\\ast$ in the trough increasing to ($0.33\\,{\\pm}\\,0.07$)${u}^a_\\ast$ at the crest, where ${u}^a_\\ast$ is the wind friction velocity. About these mean values, there is substantial variability in the instantaneous surface velocity up to approximately ${\\pm}\\,0.17{u}^a_\\ast$ in the trough and ${\\pm}\\,0.37{u}^a_\\ast$ at the crest. This variability can be attributed primarily to the modulation of the wave field, with additional contributions arising from fluctuations in wind forcing and near-surface turbulence generated by shear in the drift layer or by the influence of transient microscale breaking.

A field experiment to study the spectral balance of the source terms for wind-generated waves in finite water depth was carried out in Lake George, Australia. The measurements were made from a shore-connected platform at varying water depths from 1.2 m down to 20 cm. Wind conditions and the geometry of the lake were such that fetch-limited conditions with fetches ranging from approximately 10 km down to 1 km prevailed. The resulting waves were intermediate-depth wind waves with inverse wave ages in the range 1 < U10/Cp < 8. The atmospheric input, bottom friction, and whitecap dissipation were measured directly and synchronously by an integrated measurement system, described in the paper. In addition, simultaneous data defining the directional wave spectrum, atmospheric boundary layer profile, and atmospheric turbulence were available. The contribution to the spectral evolution due to nonlinear interactions of various orders is investigated by a combination of bispectral analysis of the data and numerical modeling. The relatively small scale of the lake enabled experimental conditions such as the wind field and bathymetry to be well defined. The observations were conducted over a 3-yr period, from September 1997 to August 2000, with a designated intensive measurement period [the Australian Shallow Water Experiment (AUSWEX)] carried out in August–September 1999. High data return was achieved.