Long Island Sound Coastal Observatory: assessment of above-water radiometric measurement uncertainties using collocated multi and hyper-spectral systems: reply to comment

摘要

Uncertainties associated with the derivation of the exact normalized water-leaving radiance (L_(WN)) from an above-water radiometric system were analyzed in Harmel et al. [Appl. Opt. 50, 5842 (2011)] based on collocated hyperspectral (HyperSAS) and multispectral (SeaPRISM) systems installed on the Long Island Sound Coastal Observational (LISCO) platform. Based on a 1.5 year time series of LISCO data, uncertainty contributors in the derivation of L_(WN) were quantified in units of unbiased relative percentage differences (URPD) by applying the different steps of the respective data processing incrementally. Results showed that discrepancy between L_(WN) data of two systems is significantly reduced when the average total sea radiance data of SeaPRISM is used in lieu of the standard one, which utilizes only the lowest total sea radiance measurements to remove the sky glint perturbations. The Zibordi comment [Appl. Opt. 51, 3888 (2012)] rejects the conclusion that attributes the sky glint removal step as the major uncertainty contributor in the SeaPRISM processing. It then states that the observed discrepancy might be due to an increased probability of sun-glint contamination in HyperSAS measurements because of its wider field of view and longer integration time. It was also underlined that observed dispersion between the atmospheric transmittance data derived from HyperSAS and SeaPRISM measurements can be attributed to probable contamination by stray light perturbation or issues with the noncosine response of the HyperSAS irradiance sensor. Finally, it was suggested to thoroughly investigate those instrumental perturbations. In this reply, impacts of non-perfect cosine response of the irradiance sensor are shown to be relatively low (<2percent on average) and therefore can only partially explain the bias in atmospheric transmittance. Additional discrepancies between the HyperSAS and SeaPRISM downwelling irradiance derivation are attributed to the presence of absorbing aerosols. Intercomparisons of the total sea radiance and nonnormalized water-leaving radiance, complementary to those discussed in the LISCO paper, are analyzed, and this analysis shows that discrepancies in normalized water-leaving radiance retrievals arise from data processing and not from instrumental uncertainty. In addition, limitations in the standard data processing to meaningfully derive normalized water-leaving radiance for various appropriate viewing configurations are discussed. It is finally advocated that the issue of sky glint perturbation correction requires further analysis based on radiative transfer computations, including refined modeling of wave slope distributions.