Quality control tests for automated above-water hyperspectral measurements: Radiative Transfer assessment

Abstract

Automated above-water hyperspectral observations are often subject to inaccuracies caused by instrument malfunction and environmental conditions. This study evaluates the influence of atmospheric and water surface conditions on above-water hyperspectral measurements through statistical methods and Radiative Transfer (RT) modelling. Initially, we developed a general quality control method based on statistical assessment to detect the suspicious spectra. Subsequently, Radiative Transfer (RT) models were used to assess low light conditions, distortions in the spectral shape of above-water solar downwelling irradiance (E_S(λ), mW m⁻² nm⁻¹) particularly those caused by intense atmospheric scattering and/or reddish hue of dusk or dawn radiation, the effect of atmospheric humidity and precipitation on the intensity and shape of spectra, and the influence of sun glint and surface perturbations on sky (L_S(λ), mW m⁻² nm⁻¹ sr⁻¹) and water surface (L_T(λ), mW m⁻² nm⁻¹ sr⁻¹) radiances. The proposed methods were applied to the entire archive of automated above-water hyperspectral measurements collected every ten minutes from 2020 to 2022 at the Royal Netherland Institute for Sea Research (NIOZ) at Jetty Station (NJS) located in the Marsdiep tidal inlet of the Duch Wadden Sea, the Netherlands. The findings demonstrate that low light conditions are characterized by E_S(λ)_max ≤ 25 mW m⁻² nm⁻¹. Red-shifted or distorted spectra are indicated by a ratio of E_S(4 8 0)/E_S(6 8 0) ≤ 1.0 and E_S(λ_max)/ E_S(8 6 5) ≤ 1.25. High humidity/precipitation conditions are identified by the ratio of E_S(9 4 0)/E_S(8 6 5), which varies with the Solar Zenith Angle (SZA). Furthermore, significant sun glint and surface perturbations, such as whitecaps and foam, are indicated when the minimum ratio of L_T(800 nm-950 nm)/E_S(800 nm-950 nm) > 0.025 sr⁻¹, and the ratio of L_T(850 nm)/E_S(850 nm) ≥ 0.025 sr⁻¹.