Influence of Sun Elevation under clear skies Influence of Cloud cover Near-surface radiometric measurements are perturbated by the effect of capillary and gravity waves. This is not compatible with the plane parallel assumption, which assumes a horizontally homogeneous ocean (no horizontal gradient of radiance; see, e.g., Zaneveld et al., 2001) Moored at a fixed position and depth, the BOUSSOLE structure is designed to maximize the stability of the sensors. Time-integration makes it possible to filter out the interface fluctuations. Using the median value reduces the noise due to surface waves. In consequence, reliable determination of the diffuse attenuation coefficients just beneath the surface (at z = 4m and z = 9m) can be performed. VARIABILITY OF THE SURFACE LAYER DIFFUSE ATTENUATION COEFFICIENTS WITH CONSIDERATION OF SOLAR ELEVATION AND ENVIRONNEMENTAL CONDITIONS Pierre Gernez 1,2 and David Antoine 2, 1: ACRI-st, Sophia Antipolis, France, 2: LOV, Quai de la Darse, Villefranche-sur-Mer, France K d seasonal variability Ship measurements of the upward and downward diffuse attenuation coefficients are often performed under a relatively limited range of solar elevations and sea roughness. K d s are usually determined as average values determined over the first 20 or 30 meters, especially in clear waters. Upper-layer (< ~10 meters) radiometric data collected in the Mediterranean Sea on a new type of optics mooring (BOUSSOLE project) are used here to investigate the variability of the diffuse attenuation coefficients in the near surface. The design of the mooring and the measurement protocol indeed allow the perturbation due to the air-sea interface to be filtered out, in order to get near-surface measurements of the downward irradiance. In situ time-series measurements of K d, K u and K Lu are investigated in the surface layer under various illumination and environmental conditions. Under a clear sky, the shape of the circadian variations of these coefficient is strongly correlated with the solar zenith angle, as expected and confirmed by theoretical computations. Variations of the diffuse K-coefficients under overcast skies and at different trophic states (as depicted by the upper-layer chlorophyll concentration) are also discussed in comparison with theoretical computations. Context In Case I waters, e.g. in waters where the IOPs are indexed on the chlorophyll concentration, K d has been statistically linked to the chlorophyll concentration, e.g., Morel and Maritorena, 2001 : with K w is the hypothetical attenuation coefficient for downward irradiance in pure water. In a first order approximation, This K d to Chl relationship explains the seasonal variability in K d. Nevertheless, when considering the K d variability at a finer scale, the relative contribution of phytoplankton and other marine particle is to be taken into account. The variability of external conditions is also important and has to be considered. During the bloom and until mid April, the increase in K d is essentially correlated with the fluorescence pattern (1). Later, the decrease in fluorescence does not result in a corresponding decrease of K d, which is somewhat correlated to the variations in backscattering (2). This probably indicates the presence of detritus. The absence of a Chl-correlated decrease of K d is also probably due to CDOM accumulation. To take into account the influence of sky diffusion and sun elevation, [Gordon, 1989] proposed, for K d computed just beneath the surface, the following relationship: (Corresponding plots of measured K d vs modeled IOPs). Data and method Example of data collected during a 1-min acquisition period for E s and E d (4 and 9 meters) at 412 nm. Dashed lines are the medians of the 360 measurements. Influence of external conditions Conclusions, perspectives High-frequency, near-surface in situ measurements make it possible to investigate the variability in the diffuse attenuation coefficients from the daily to the seasonal scale. The variations of the diffuse attenuation coefficients have been investigated under clear and overcast skies and are coherent with theoretical models. Further analyses are envisaged: Influence of wind and sea roughness on the K d Investigation of the K d to IOPs relationships when using measured IOPs instead of modeled ones. K d variations in the near surface Ocean Optics XVIII, Montreal, October 9-13, 2006 (1)(2) High temporal resolution measurements allows a detailed (unprecedented) description of the influence of the sun elevation. K d from [Morel and Maritorena, 2001] (sun-independent) are represented as dashed lines. Plain curves, from [Morel and Gentili, 2004] (RT computations), are in good agreement with the field determinations. (Chl is about 1 mg.m -3 ) In the opposite of sunny days, the measurements are nearly constant when the sky is totally overcast. References: Gordon, H.R., 1989, Can the Lambert-Beer law be applied to the diffuse coefficient of ocean water? Limnol. Oceanogr., 34, Morel A. and B. Gentili, 2004, Radiation transport within oceanic (case 1) waters. J. Geophys. Res., 109, C06008, / 2003JC Morel A. and S. Maritorena, 2001, Bio-optical properties of oceanic waters: A reappraisal. J. Geophys. Res, 106, Zaneveld, J.R.V., E. Boss and A.H. Barnard, 2001, The influence of surface waves on measured and modeled irradiance profiles. Applied Optics. 40(9): EsEs 4-m E d 9-m E d