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 providing 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.