Validating remotely-sensed ocean colour data using a moored databuoy

Abstract

SeaWiFS, and the other new generation of ocean colour sensors, must be backed by comprehensive calibration and validation programmes if they are to achieve their aims of accurate, synoptic, global measurement of oceanic phytoplankton biomass and production. In situ measurements of normalised water-leaving radiance {L^) made simultaneously with satellite measurements can complement on-orbit methods of tracking changes in the calibration of the satellite radiometer and allow end-to-end vicarious validation of the whole remote sensing method, including new atmospheric correction methods. A moored, optical databuoy was developed to measure L^ in the western English Channel for extended periods. Tests indicate that the buoy is capable of measuring spectral incident irradiance with less than 10% error and water-leaving radiance with less than 20% error; the errors are reduced by averaging the data. There were 24 match-ups with good quality SeaWiFS data at the buoy site during the 10 months of deployment between May 1997 and September 1998. The RMS differences between the buoy and SeaWiFS measurements ofLw varied from 49.5% at 555 nm to 101.5% at 412 nm. This leads to the diffuse attenuation at 490 nm [^490)] being systematically overestimated by the SeaWiFS algorithm by between 50 and 70%, with an RMS difference of 68.6%. Near surface chlorophyll-a concentrations (Ca) differ from those based on the buoy optical measurements by between -52% (SeaWiFS estimate lower than buoy estimate) to +123%, with an RMS difference of 57%. The RMS differences in measurements of Lwn could be reduced to less than 13% by increasing the calibration coefficients of SeaWiFS by between 3.7% and 0.25% in the visible bands and by tuning the extrapolation of aerosol radiances from the near infra red to the visible wavelengths. This study also found that remotely sensed measurements of A^490)an^ Ca may be biased because of undersampling.

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