ANALYSIS OF THE VARIATIONS OF THE WHITECAP FRACTION AS MEASURED IN A COASTAL ZONE C. LAFON 1 , J. PIAZZOLA 1 , P. FORGET 2 , O. LE CALVE 1 and S. DESPIAU 1 1 LEPI, University of Toulon-Var, France; 2 LSEET, University of Toulon-Var, France (Received in final form 1 July 2003) Abstract. 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 condi- tions 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. Keywords: Air-sea interactions, Fetch, Waves, Whitecap coverage. 1. Introduction When the wind blows over an ocean surface, waves are generated and for certain conditions of wind speed and duration, they break, resulting in the occurrence of whitecaps. The knowledge of the breaking rate is important, in particular, to estim- ate aerosol and gas fluxes at the air-sea interface (Liss and Merlivat, 1986), which, in turn, influence the radiative budget, and hence the global climate (Charlson et al., 1992). A source function for the aerosol generation during breaking wave episodes requires an accurate model for the whitecap fraction (Monahan and O’Muircheartaigh, 1986; Andreas, 1992; Piazzola et al., 2002). The whitecap frac- tion, commonly denoted W , is the ratio between the sea surface area covered by breaking waves to the total sea surface area. In coastal zones, the whitecap fraction can be different from open ocean conditions (Monahan and Woolf, 1989) due to the occurrence of specific coastal processes, such as wave-current interactions, bottom influence, the geometry of the coastline and variations of the wind speed and direction. These strongly influence variations in the whitecap ratio (Van Eijk Boundary-Layer Meteorology 111: 339–360, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.