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Conversion of sub-surface reflectances to above-surface MERIS reflectance

Factors for converting sub-surface reflectances to above-surface MERIS reflectances have been determined both as analytic functions and average numbers for solar zenith angles in the range 30°–75°, wind speeds up to 10 m s−1, and the spectral domain 400–700 nm. The conversion factors have been obtained by numerical and statistical computations based on field observations of spectral radiance and irradiance, above and below the surface of the sea. The estimated maximum errors of the different algorithms range from ≤0.1% up to 10%, depending on the chosen method and the types of optical quantities that are available. The errors are smallest for solar zenith angles between 30° and 60° and increase when the solar zenith angle approaches 75°. The influence of the wind on the conversion factors is practically negligible. The algorithms, which have been derived for conditions representative of the Skagerrak and the adjacent seas, are assumed to be valid for both Case 1 and 2 waters.