Analysis of the Variations of the Whitecap Fraction as Measured in a Coastal Zone

The whitecap coverage generated by breaking waves plays a major role in the transfer of heat, momentum, water vapour and particles at the air-sea interface. The sea surface covered by whitecaps strongly depends on both the wind and the wave-field characteristics. In particular, in coastal zones, the variations of the whitecap fraction, commonly noted W, differ from open ocean conditions, due to fetch effects, wave-current interactions, bottom influence and irregular coastlines affecting both the wind properties and the wave development. We present an analysis of the whitecap-fraction variations using experimental data acquired during FETCH (Flux, Etat de mer, et Télédétection en Condition de fetcH variable), an experimental campaign that took place in the Gulf of Lion off the French Mediterranean coast in 1998. The data include various conditions of wave development associated with fetch and unsteady effects. The whitecap fraction W was measured using an original image processing technique applied to sea surface photography. Specific relationships between the whitecap fraction and both atmospheric and oceanic parameters are investigated, which confirm the suitability of the friction velocity for modelling W. However, W has also been parameterised using wave-parameter dependent relationships, which is of interest for very short fetches.

[1]  Gerrit de Leeuw,et al.  Modeling aerosol particle size distributions over the North Sea , 1992 .

[2]  M. Donelan,et al.  Radar scattering and equilibrium ranges in wind‐generated waves with application to scatterometry , 1987 .

[3]  Directional wave measurements from the ASIS (Air-Sea Interaction Spar) buoy , 1998, IEEE Oceanic Engineering Society. OCEANS'98. Conference Proceedings (Cat. No.98CH36259).

[4]  Philippe Forget,et al.  A sea spray generation function for fetch-limited conditions , 2002 .

[5]  E. L. Andreas.  Sea spray and the turbulent air‐sea heat fluxes , 1992 .

[6]  T. E. Nordeng.  On the wave age dependent drag coefficient and roughness length at sea , 1991 .

[7]  D.J.T. Carter,et al.  Prediction of wave height and period for a constant wind velocity using the JONSWAP results , 1982 .

[8]  Edward C. Monahan,et al.  Optimal Power-Law Description of Oceanic Whitecap Coverage Dependence on Wind Speed , 1980 .

[9]  R. M. Kennedy,et al.  On the Formation of Whitecaps by a Threshold Mechanism. Part I:. Basic Formalism , 1983 .

[10]  A. Townsend.  Flow in a deep turbulent boundary layer over a surface distorted by water waves , 1972, Journal of Fluid Mechanics.

[11]  V. Kudryavtsev,et al.  Impact Of Dominant Waves On Sea Drag , 2002 .

[12]  S. Hsu,et al.  A Mechanism for the Increase of Wind Stress (Drag) Coefficient with Wind Speed over Water Surfaces: A Parametric Model , 1986 .

[13]  Jin Wu.  Variations of whitecap coverage with wind stress and water temperature , 1988 .

[14]  J. F. Meirink.  The role of wind waves and sea spray in air-sea interaction , 2002 .

[15]  P. Mulhearn,et al.  Photographic measurements of bubble populations from breaking wind waves at sea , 1987 .

[16]  Richard J. Seymour,et al.  Detection of Breaking in a Deep Water Wave Record , 2002 .

[17]  W. Kendall Melville,et al.  Distribution of breaking waves at the ocean surface , 2002, Nature.

[18]  D. Xu,et al.  Probability of wave breaking and whitecap coverage in a fetch-limited sea , 2000 .

[19]  M. Donelan,et al.  Directional spectra of wind-generated ocean waves , 1985, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[20]  Frédéric Bouchara,et al.  Development of the Mediterranean extinction code (MEDEX) , 2003 .

[21]  W. Oost,et al.  Wave Energy Dissipation by Whitecaps , 1996 .

[22]  Kimmo K. Kahma,et al.  A Study of the Growth of the Wave Spectrum with Fetch , 1981 .

[23]  D. E. Spiel,et al.  A Model of Marine Aerosol Generation Via Whitecaps and Wave Disruption , 1986 .

[24]  Leo H. Holthuijsen,et al.  Statistics of Breaking Waves Observed as Whitecaps in the Open Sea , 1986 .

[25]  Edward C. Monahan,et al.  Whitecaps and the passive remote sensing of the ocean surface , 1986 .

[26]  David K. Woolf,et al.  Comments on “Variations of Whitecap Coverage with Wind stress and Water Temperature , 1989 .

[27]  Edward C. Monahan,et al.  The Ocean as a Source for Atmospheric Particles , 1986 .

[28]  Fukuzo Tasai,et al.  Observation of the Power Spectrum of Ocean Waves Using a Cloverleaf Buoy , 1980 .

[29]  M. Donelan,et al.  On the Dependence of Sea Surface Roughness on Wave Development , 1993 .

[30]  Yoshiaki Toba,et al.  A Parameter Describing Overall Conditions of Wave Breaking, Whitecapping, Sea-Spray Production and Wind Stress , 1986 .

[31]  Jin Wu.  Stability Parameters and Wind-Stress Coefficients under Various Atmospheric Conditions , 1986 .

[32]  L. Merlivat,et al.  Air-Sea Gas Exchange Rates: Introduction and Synthesis , 1986 .

[33]  J. Coakley,et al.  Climate Forcing by Anthropogenic Aerosols , 1992, Science.

[34]  C. Pounder.  Sodium Chloride and Water Temperature Effects on Bubbles , 1986 .

[35]  S. Petrinec,et al.  Cusp observations of high- and low-latitude reconnection for northward interplanetary magnetic field , 2000 .