Abstract
Above-water radiometry depends on estimates of the reflectance factor ρ of the sea surface to compute the in situ water-leaving radiance. The Monte Carlo code for ocean color simulations MOX is used in this study to analyze the effect of different environmental components on ρ values. A first aspect is examining the reflectance factor without and by accounting for the sky-radiance polarization. The influence of the sea-surface statistics at discrete grid points is then considered by presenting a new scheme to define the variance of the waves slope. Results at different sun elevations and sensor orientations indicate that the light polarization effect on ρ simulations reduces from ∼17 to ∼10% when the wind speed increases from 0 to 14m s-1. An opposite tendency characterizes the modeling of the sea-surface slope variance, with ρ differences up to ∼12% at a wind speed of 10m s-1. The joint effect of polarization and the the sea-surface statistics displays a less systematic dependence on the wind speed, with differences in the range ∼13 to ∼18%. The ρ changes due to the light polarization and the variance of the waves slope become more relevant at sky-viewing geometries respectively lower and higher than 40° with respect to the zenith. An overall compensation of positive and negative offsets due to light polarization is finally documented when considering different sun elevations. These results address additional investigations which, by combining the modeling and experimental components of marine optics, better evaluate specific measurement protocols for collecting above-water radiometric data in the field.
Highlights
Reflectance factor ρ = Lr/Li quantifies the fraction of reflected over incident radiance at the seaair interface (Lr and Li, respectively)
The proof-ofconcept GPS algorithm presented in this study complements the HWS scheme demonstrating the possibility to reach a statistical convergence between the slope variance computed at the sea-surface grid points and the target value
Ρ above-water systems [2] as a trade-off between uncertainties affecting ρ simulations due to a limited capability to account for light polarization and detail the sea-surface statistics
Summary
Above-water radiometric measurements require accurate ρ estimates to compute in situ water-leaving radiance Lw = Lt−ρ ·Li, where the total radiance Lt is collected in the sea-viewing mode (Fig. 1). In situ above-water systems have been used for the validation of radiometric data produced by space-born sensors like the Sea-viewing Wide Field-of-view Sensor SeaWiFS, the Moderate Resolution Imaging Spectroradiometer MODIS, the Medium Resolution Imaging Spectrometer MERIS and the Visible Infrared Imagery Radiometer Suite VIIRS [1]. Recent extensions account for the polarization of the incident light [10,11,12,13]. The accuracy target for in situ radiometric measurements underpins additional analyses to comprehensively account for specific environmental conditions and extensively assess the performance of measurement protocols
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