Optical properties of Forel-Ule water types deduced from 15 years of global satellite ocean color observations

Abstract

The Forel-Ule (FU) color comparator scale is the oldest set of optical water types (OWTs). This scale was originally developed for visual comparison and generated an immense amount of data, with hundreds of thousands of observations being gathered from the last 130 years. Since recently, the FU scale is also applicable to remote sensing data. This has been possible thanks to an optical characterization of the 21 FU colors in terms of the (x,y) CIE standards and new algorithms that convert remote-sensing reflectances (Rrs) from satellite-borne ocean color sensors to FU. Rrs-derived hue angle and FU have been recently applied with success in the assessment of color variability of lakes and specific shelf areas, but an evaluation over global oceanic waters is still missing. By clustering global climatological ESA-OC-CCI v2.0 Rrs with the derived FU, we obtain a set of Rrs to be used as optical water types (OWTs). Diffuse attenuation coefficient, Secchi disk depth and chlorophyll concentration are also associated to the FU classes. The angular distances of a given Rrs to the two nearest FU classes are proposed as simple and robust membership indexes, adding up to one. We also evaluate the advantages and limitations of FU and the hue angle as monitoring tools over the full marine range, from the most oligotrophic areas to the turbid and productive coastal zones. The first 7 FU indexes cover 99% of global surface waters. Unlike the hue angle, that resolves all spatio-temporal color variations, the FU scale is coarse as a monitoring tool for oligotrophic waters as all the subtropical gyres saturate to FU = 1, while the color of other seas varies across 2, 3 or even 4 FU classes. We illustrate the introduction of a new “zero” FU class that increases monitoring resolution at the blue end of the color range. Finally, we show how optical diversity varies across the color range and compare several sets of OWTs from a color perspective. Overall, we provide a valuable and self-consistent dataset that enhances the usefulness of the FU scale by converting it to useful information for the oceanographic community. This OWT scheme keeps the advantages of other datasets, like being useful to study ocean color product quality and characterize the uncertainties, but also allows to continue to monitor long-term change in optical diversity over the global ocean color. Integration into the optical modules of ecosystem models can help verify past simulations that predate the satellite age, through comparisons with in-situ FU data collected at the time.