Absorption correction and phase function shape effects on the closure of apparent optical properties

We present a closure experiment between new inherent optical properties (IOPs: absorption a, scattering b, back-scattering b(b)) and apparent optical properties (AOPs: remote-sensing reflectance R-rs, irradiance reflectance R, and anisotropic factor at nadir Q(n)) data of Ionian and Adriatic seawaters, from very clear to turbid waters, ranging across one order of magnitude in R-rs. The internal consistency of the IOP-AOP matchups was investigated though radiative transfer closure. Using the in situ IOPs, we predicted the AOPs with the commercial radiative transfer solver Hydrolight. Closure was limited by two unresolved issues, one regarding processing of in situ data and the other related to radiative transfer modeling. First, different correction methods of the absorption data measured by the Wetlabs ac-s produced high variations in simulated reflectances, reaching 40% for the highest reflectances in our dataset. Second, the lack of detailed volume scattering function measurements forces us to adopt analytical functions that are consistent with a given particle backscattering ratio. The analytical phase functions named Fournier-Forand and two-term Kopelevich presented reasonable angular shapes with respect to measurements at a few backward angles. Between these phase functions, induced changes were within 4% for R-rs, within 11% for R, and within 10% for Q(n). Additionally, closure of Q(n) was generally not successful considering radiometric uncertainties. Simulated Q(n) overestimated low values and underestimated high values, especially at 665 nm, where Hydrolight was unable to predict measured Q(n) values greater than 6 sr. The physical nature of Q(n) makes this mismatch almost independent of the measured IOPs, thus precluding Q(n) tuning by varying the former. The non-closure of Q(n) might be caused by an inaccurate phase function and, to a lesser extent, by the modeling of the incoming radiance. For the future, this remains the task of accurate absorption and phase function measurements, especially at red wavelengths. (C) 2016 Optical Society of America