Assessment of Radiometric Data from a Buoy in the St. Lawrence Estuary

Abstract

AbstractFisheries and Oceans Canada maintains a network of scientific buoys in the St. Lawrence estuary and gulf. Among a suite of environmental parameters documented, the in-water upwelling radiance is measured using an ocean color radiometer located underneath the center of the buoy. The shadow effect from the 1.05-m-radius buoy on the measured upwelling radiance is estimated and empirical models to correct for it are proposed. On average, the shading error (i.e., the percent of missing radiance) was and for the 555- and 412-nm channels, respectively. Two analytical models were tested to predict the shading error using measured inherent optical properties, the sun zenith angle, and the fraction of diffuse sky irradiance. Neglecting light scattering led to overestimates of the shading error. In contrast, the bias was removed when the scattering coefficient was accounted for, but the overall error was only barely improved (root-mean-square error ). Empirical relationships based on the uncorrected reflectance ratio measured by the buoy were used to predict both the shading error and the diffuse attenuation of the upwelling radiance, two quantities needed to calculate remote sensing reflectance . Overall, was retrieved with an averaged absolute percent difference ranging from 12% to 20%, which appears adequate for the validation of ocean color data such as the Moderate Resolution Imaging Spectroradiometer (MODIS-Aqua) and Visible Infrared Imaging Radiometer Suite (VIIRS) products in the optically complex waters of the St. Lawrence estuary.