Validation of ocean color remote sensing reflectance data: Analysis of results at European coastal sites

From its initial measurements 20 years ago, the Ocean Color component of the Aerosol Robotic Network (AERONET-OC) has produced large validation data sets to assess the ocean color satellite data records. This study, applied to the standard atmospheric correction algorithm l2gen of the National Aeronautics and Space Administration, analyses the populations of residuals (differences between matching satellite and field data) of remote sensing reflectance R-RS and, secondarily, aerosol optical thickness za, and their validation statistics associated with data collected at seven AERONET-OC sites located in European coastal regions for six satellite missions. Validation statistics do not appear particularly sensitive to observation conditions, represented by viewing and solar geometry, aerosol optical thickness tau(a), angstrom ngstrom exponent or water properties indexed by R-RS, the most obvious exceptions being a systematic increase of the residuals for high solar zenith angles for all missions and sites. Residuals of R-RS and tau(a) tend to be inversely correlated as deviations in aerosol radiance and water-leaving radiance compensate each other. The spectra of a R-RS uncertainty estimate sigma decrease in a monotonous manner from 0.7 to 1.5 10(-3) sr(-1) for wavelengths around 412 nm to 0.1-0.35 10(-3) sr(-1) around 667 nm, with sigma usually relatively high for waters associated with high amplitudes of R-RS. At 412 nm, sigma for tau(a) is mostly in the interval 0.03-0.04. For R-RS, variations across missions of a or of the relative differences between satellite and field data are overall smaller than variations across sites, which suggests that the products derived from various missions are rather consistent at a given location when processed with the considered algorithm. For a given mission, R-RS residuals are well correlated between bands (inter-band correlation coefficient mostly larger than 0.5), which has implications on the propagation of uncertainties through bio-optical algorithms. For a given wavelength, residuals are correlated between missions to various degrees (most often with inter-mission correlation coefficients larger than 0.5). Besides implications as far as uncertainties of multi-mission merged products are concerned, this inter-mission correlation among residuals suggest that the residuals are not random and could be reduced.

Automated summary

The Ocean Color component of AERONET-OC has provided validation data sets for ocean color satellite data records. This study analyzed the residuals of remote sensing reflectance and aerosol optical thickness using the standard atmospheric correction algorithm l2gen, and validated the algorithm using data from seven AERONET-OC sites in European coastal regions. The results showed that the validation statistics were not sensitive to observation conditions, and the residuals were correlated between bands and missions.