Guillemette Caulliez

Influence of energetic wind and waves on gas transfer in a large wind–wave tunnel facility

Air–water gas exchange experiments were carried out in a large wind wave tunnel in Marseille, France, to investigate gas transfer processes under energetic wind and wave fields, where macroscale breaking waves create bubble plumes (white caps) and turbulence on the water surface. We measured the gas transfer velocities of N2O, DMS, He, SF6, CH3Br, and total air. Their diffusivity and solubility span a large range, allowing us to investigate gas transfer mechanisms under a variety of physical conditions. We observed that the gas transfer velocities varied with friction velocity in a linear manner. Gas transfer in the presence of pure wind waves is generally consistent with the surface renewal model, as the gas transfer velocity has a strong dependence on diffusivity with an exponent of 0.53(±0.02). Contrary to expectations, the bubble plumes generated by breaking waves contributed relatively little in our pure wind wave experiments. Superposition of mechanically generated waves onto the wind waves in the high wind regime attenuated DMS gas transfer (as a function of friction velocity) across the air–water interface by ~20% compared with gas transfer under pure wind waves, implying suppression of gas transfer directly across the sheared water surface. Greater transfer of less soluble gases may result from bubble-mediated gas transfer.

Universality of sea wave growth and its physical roots

Modern day studies of wind-driven sea waves are usually focused on wind forcing rather than on the effect of resonant nonlinear wave interactions. The authors assume that these effects are dominating and propose a simple relationship between instant wave steepness and time or fetch of wave development expressed in wave periods or lengths. This law does not contain wind speed explicitly and relies upon this asymptotic theory. The validity of this law is illustrated by results of numerical simulations, in situ measurements of growing wind seas and wind wave tank experiments. The impact of the new vision of sea wave physics is discussed in the context of conventional approaches to wave modeling and forecasting.

Drag of the Water Surface at Very Short Fetches: Observations and Modeling

Abstract The specific properties of the turbulent wind stress and the related wind wave field are investigated in a dedicated laboratory experiment for a wide range of wind speeds and fetches, and the results are analyzed using the wind-over-waves coupling model. Compared to long-fetch ocean wave fields, wind wave fields observed at very short fetches are characterized by higher significant dominant wave steepness but a much smaller macroscale wave breaking rate. The surface drag dependence on fetch and wind then closely follows the dominant wave steepness dependence. It is found that the dimensionless roughness length z*0 varies not only with wind forcing (or inverse wave age) but also with fetch. At a fixed fetch, when gravity waves develop, z*0 decreases with wind forcing according to a −1/2 power law. Taking into account the peculiarities of laboratory wave fields, the WOWC model predicts the measured wind stress values rather well. The relative contributions to surface drag of the equilibrium-range w...

Ka‐band backscattering from water surface at small incidence: A wind‐wave tank study

We report on an experiment conducted at the large Pytheas wind-wave facility in Marseille to characterize the Ka-band radar return from water surfaces when observed at small incidence. Simultaneous measurements of capillary-gravity to gravity wave height and slopes and Normalized Radar Cross Section (NRCS) were carried out for various wind speeds and scattering angles. From this data set we construct an empirical two-dimensional wave number spectrum accounting for the surface current to describe water surface motions from decimeter to millimeter scales. Some consistency tests are proposed to validate the surface wave spectrum, which is then incorporated into simple analytical scattering models. The resulting directional NRCS is found in overall good agreement with the experimental values. Comparisons are performed with oceanic models as well as in situ measurements over different types of natural surfaces. The applicability of the present findings to oceanic as well as continental surfaces is discussed.