A chlorophyll‐dependent semianalytical reflectance model derived from field measurements of absorption and backscattering coefficients within the Southern Ocean

Abstract

A semianalytical model was developed for the prediction of spectral remote sensing reflectance (Rrs) as a function of fluorometric chlorophyll a concentration (Chl) for two regions within the Southern Ocean: the Ross Sea and the Antarctic Polar Front Zone (APFZ). The model is based upon Chl‐dependent parameterizations of the spectral absorption, a(λ), and backscattering, bb(λ), coefficients of seawater which were derived from field measurements. The relationships between a(λ) and Chl were similar in both regions, but for comparable Chl the particulate backscattering was on average 4 times greater in the APFZ. Measurements of particle size distributions suggest that particle assemblages in the APFZ were characterized by a greater predominance of smaller particles, consistent with the observed regional differences in backscattering properties. The model is used to examine the separate influences of absorption and backscattering on the blue to green ratio of reflectance, Rrs(490)/Rrs(555). Variability in the spectral absorption ratio, resulting principally from changes in the relative contribution of water to total absorption in each band, contributes >75% to changes in the Rrs(490)/Rrs(555) ratio as a function of Chl. However, variability in the spectral backscattering ratio appears to be the primary cause for the observed differentiation in the Rrs versus Chl relationships between the two regions.