Bubbles generated from wind-steepened breaking waves: 2. Bubble plumes, bubbles, and wave characteristics

Measurements of breaking-wave-generated bubble plumes were made in fresh (but not clean) water in a large wind-wave tunnel. To preserve diversity, a classification scheme was developed on the basis of plume dimensions and "optical density," or the plume's ability to obscure the background. Optically dense plumes were due to the presence of a peak at large radius in the plume bubble size distribution. For each class, the plume formation rate, P, was measured at different fetches. The relationship between wavebreaking characteristics and the bubble plume evolution is examined in detail for these experiments. The wave-breaking rate and intensity were strongly fetch-dependent as the mechanically steepened wind waves rapidly evolved with fetch because of wind, dissipation, and nonlinear wave-wave interactions. P followed the trend in wave breaking, reaching a maximum at the fetch of maximum wave breaking. The ratio of dense to diffuse plumes was more sensitive to the wave-breaking intensity. Using P and the bubble population size distributions for each class, the global bubble plume injection size distribution, Ψi(r), where r is radius, was calculated. Ψi decreased as Ψi ∼ r-1.2 for r < 1700 μm and Ψi ∼ r-3.9 for larger r. Total volume injection was 640 cm3 S-1, divided approximately equally between bubbles smaller and larger than 1700-μm radius. Using plume volumes at maximum penetration for each class, a concentration distribution was calculated and showed plume concentrations greater than the background population by one to several orders of magnitude, depending upon r. Copyright 2006 by the American Geophysical Union

HE and RN gas exchange experiments in the large wind-wave facility of IMST

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LUMINY: an overview

Experiments were undertaken in the Large Air Sea Interaction Simulation Tunnel of IRPHE-IOA, Laboratoire de LUMINY, in Marseille, France, aimed at improving our understanding of the effects of breaking waves on gas transfer, and providing parameterisations for the transfer velocities. Detailed studies were made of breaking wave phenomena, bubbles and turbulence in water and air, and exchange rates of gases with a variety of physical properties (CO2, CH4, N2O, DMS, CH3Br, 4He and SF6). A simple scaling of air-water transfer velocities with friction velocity and Schmidt number breaks down at high wind speeds. A solubility-dependent enhancement of transfer velocity by bubbles can explain only part of the behaviour. An "interfacial resistance" model can explain much of the outstanding behaviour at high wind speeds. Bubble-mediated transfer, surface disruption by turbulence and surfacing bubbles, and interfacial resistance, are all identified as significant to air-sea gas exchange at high wind and sea states

Modelling of bubble-mediated gas transfer : fundamental principles and a laboratory test

gas transfer; wave breaking, air-sea interaction; sea surfaceInternational audienceThe air–water exchange of gases can be substantially enhanced by wave breaking and specifically by bubble-mediated transfer. A feature of bubble-mediated transfer is the additional pressure on bubbles resulting from the hydrostatic forces on a submerged bubble and from surface tension and curvature. This peculiarity results in asymmetry of bubble-mediated gas transfer and equilibrium supersaturations of dissolved gases in a bubbly ocean. A second peculiarity is the finite capacity of bubbles, so that the composition of a bubble may change during the exchange. The result is that gas transfer mediated by bubbles is characterized by an altered dependence on the molecular properties of the dissolved gas compared to direct transfer across the main air–water interface. A related phenomenon for bubble plumes with a high void fraction (air volume to total volume ratio) is that the composition of the dissolved gas within the interstitial water of a plume may alter during the exchange process and only mix into the full water reservoir later. Three asymptotes are identified for gas exchange mediated by high-void-fraction bubble plumes and a semi-empirical parameterization of bubble-mediated gas transfer is devised on the basis of these asymptotes, which describes the dependence of the overall transfer velocity on plume properties and molecular properties of the gas. These models are confronted with data from laboratory experiments. The experiments use artificial aeration with the gas source switched during each run. Measurements of the bubble distribution enable calculation of the theoretical transfer of the gases. A parameterization fits the theoretical transfer satisfactorily. Gas measurements are used to test if the actual transfer of gases is similar to the theoretical transfer. The experimental method enables separation of bubble-mediated transfer from transfer directly across the main air–water interface. The agreement between gas and bubble-derived values of transfer velocity is sufficient to generally validate the theory, but is imprecise. The results suggest that the interstitial water plays a significant role in limiting gas transfer–in particular, limiting transfer of helium–despite the fact that typical void fractions were low (< 0.1%). It should be possible to predict gas transfer velocities in the field by simulating oceanic bubble plumes sufficient to constrain that part of the transfer, but targets of 10% or 20% may be beyond reach especially for the most poorly soluble gases (for which the bubble-mediated mechanism is particularly important). These simulations require accurate bubble distributions, void fractions and a good description of the entire plume dynamics. Such simulations are particularly important for interpreting dual tracer and nitrogen/oxygen experiments in stormy conditions, where the relative transfer of different gases is a non-trivial problem

Wind and wave characteristics observed during the LUMINY gas transfer experiments

The parameterization of the greenhouse gas fluxes between the atmosphere and oceans as function of wind and sea state parameters remains a challenging problem, of key importance for climate modelling. It is well-known that exchange across the air-water interface of gases of poor solubility as carbon dioxide, methane, is governed by the mixing phenomena which affect the very upper water boundary layer in so far as this layer concentrates most of the resistance to transfer. However, these phenomena, dependent on various processes as momentum transfer from wind to waves and currents, turbulence generation in water, wave interaction with shear, wave breaking, thermal stratification or water surface contamination by surfactants, are complex and consequently, have been poorly described up to now. Therefore, most attempts to parameterize gas transfer have consisted essentially in measuring gas transfer rates over a large range of wind and wave conditions both in laboratory and field experiments and then, searching for empirical relations describing the gas flux evolution with wind speed (Liss et Merlivat, 1986; Wanninkhof, 1992). However, the available experimental data exhibit large discrepancies, in particular at high wind speeds, making these first attempts far from being completely satisfactory. The experiments planned within the framework of the LUMINIY project aimed at providìng a better description of the dynamics of the air-water interface observed at high wind speeds when wave breaking is dominant, in order to identify more precisely the wind and wave parameters which control gas transfer in such conditions (for a more detailed presentation of the project, see De leeuw et al (1998)). This paper is devoted to the observations performed during the gas transfer experiments in order to describe the "sea state" and the wind stress at the water surface. The approach adopted to characterize wave breaking is presented in detail and the first results obtained at high wind speeds are discussed briefly