OCEANcode: The complete set of algorithms and models for the level_2 processing of European CZCS historical data

Abstract

A complete set of algorithms and models for the level_2 processing of the European CZCS historical data was integrated in the OCEANcode software package. The OCEANcode allows the calibration of the sensor-recorded signal taking into account the instrument sensitivity loss; the correction of the calibrated signal for atmospheric contamination and derive sub-surface reflectances; and then the estimation of the concentration of water constituents. The atmospheric correction is performed on the basis of a reflectance-model-based algorithm. The Rayleigh correction is applied consistently for all water pixels, using a multiple scattering approach, and introducing atmospheric pressure and Ozone concentration data in the computation. The marine aerosol correction uses a pixel-by-pixel iterative procedure, allowing successive estimates of both the marine reflectance in the red spectral region (670nm) and the Angstrom exponent, which links simple wavelengths ratios to reflectance ratios. For case 1 waters, the optical properties of which are essentially dominated by planktonic pigments, the interrelations between marine reflectances and reflectance ratios at various wavelengths are derived from modelled calculations. For identified case 2 waters, where water constituents other than planktonic pigments (i.e. dissolved organics and suspended sediments) dominate the water optical properties, the evaluation of marine reflectances is approximated by means of interpolated Angstrom exponent values computed over case 1 water pixels and of empirical relationships derived from in situ measurements. The computation of chlorophyll-like pigments is performed with algorithms based on blue/green (443-550nm) reflectance ratios, for lower pigment concentration, or on green/green (520-550nm) reflectance ratios, for higher pigment concentration. As for the case of atmospheric corrections, the inter-relations between pigment concentration and reflectance ratios are model-derived for case 1 waters, and empirically determined for case 2 waters.