Influence of Sun Elevation under clear skies

1. Influence of Cloud cover Influence of Sun Elevation under clear skies Context Data and method Ship measurements of the upward and downward diffuse attenuation coefficients are often performed under a relatively limited range of solar elevations and sea roughness. Kds 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 Kd, Ku and KLu 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. 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 CONDITIONSPierre Gernez1,2 (pierre.gernez@obs-vlfr.fr) and David Antoine2, 1: ACRI-st, Sophia Antipolis, France, 2: LOV, Quai de la Darse, 06238 Villefranche-sur-Mer, France Es 4-m Ed 9-m Ed Example of data collected during a 1-min acquisition period for Es and Ed (4 and 9 meters) at 412 nm. Dashed lines are the medians of the 360 measurements. Kd seasonal variability -1 In Case I waters, e.g. in waters where the IOPs are indexed on the chlorophyll concentration, Kd has been statistically linked to the chlorophyll concentration, e.g., Morel and Maritorena, 2001 : with Kw is the hypothetical attenuation coefficient for downward irradiance in pure water. In a first order approximation, This Kd to Chl relationship explains the seasonal variability in Kd. Nevertheless, when considering the Kd 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. (1) (2) During the bloom and until mid April, the increase in Kd is essentially correlated with the fluorescence pattern (1). Later, the decrease in fluorescence does not result in a corresponding decrease of Kd , 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 Kd is also probably due to CDOM accumulation. Influence of external conditions To take into account the influence of sky diffusion and sun elevation, [Gordon, 1989] proposed, for Kd computed just beneath the surface, the following relationship: (Corresponding plots of measured Kdvs modeled IOPs). • 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 Kd • Investigation of the Kd to IOPs relationships when using measured IOPs instead of modeled ones. • Kd variations in the near surface (Chl is about 1 mg.m-3) (Chl is about 1 mg.m-3) References: Gordon, H.R., 1989, Can the Lambert-Beer law be applied to the diffuse coefficient of ocean water? Limnol. Oceanogr., 34, 1389. Morel A. and B. Gentili, 2004, Radiation transport within oceanic (case 1) waters. J. Geophys. Res., 109, C06008, 10.1029 / 2003JC002259 Morel A. and S. Maritorena, 2001, Bio-optical properties of oceanic waters: A reappraisal. J. Geophys. Res, 106, 7763-7780 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): 1442-1449 High temporal resolution measurements allows a detailed (unprecedented) description of the influence of the sun elevation. Kd 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. In the opposite of sunny days, the measurements are nearly constant when the sky is totally overcast. Ocean Optics XVIII, Montreal, October 9-13, 2006