Closure and uncertainty assessment for ocean color reflectance using measured volume scattering functions and reflective tube absorption coefficients with novel correction for scattering

Abstract

Optical closure is assessed between measured and simulated remote-sensing reflectance (Rrs) using Hydrolight radiative transfer code for five data sets that included a broad range of both Case I and Case II water types. Model-input inherent optical properties (IOPs) were the absorption coefficient determined with a WET Labs ac9 and the volume scattering function (VSF) determined with a custom in situ device called MASCOT. Optimal matchups were observed using measured phase functions and reflective tube absorption measurements corrected using a scattering error independently derived from VSF measurements. Absolute bias (δ) for simulations compared to measured Rrs was 20% for the entire data set, and 17% if a relatively shallow station with optical patchiness was removed from the analysis. Approximately half of this δ is estimated to come from uncertainty in radiometric measurements of Rrs, with the other half arising from combined uncertainties in IOPs, radiative transfer modeling, and related assumptions. For exercises where such δ can be tolerated, IOPs have the potential to aid in ocean color validation. Overall, δ was roughly consistent with the sum of uncertainties derived from associated measurements, although larger deviations were observed in several cases. Applying Fournier–Forand phase functions derived from particulate backscattering ratios according to Mobley et al. [Appl. Opt.41, 1035 (2002)APOPAI0003-693510.1364/AO.41.001035] resulted in overall δ that was almost as good (23%) as simulations using measured phase functions. Possibilities for improving closure assessments in future studies are discussed.