An Evaluation of Models for the Satellite-Estimation of Phytoplankton Absorption Coefficients in Coastal/ Oceanic Waters

TLDR: Though the TS model is mathematically simple, it overcomes such limitations and yields excellent results in terms of reproducing measured aph spectra that are highly desired by the ocean color community for inputting in various bio-optical models and studying the spatial structure of the different phytoplankton communities from satellite remote sensing observations in global waters.

Abstract: Several semi-analytical models for the satellite-based estimation of absorption coefficients of phytoplankton aph (λ) have been used to routinely produce aph (λ) product from satellite ocean color data However, these models are generally applicable for clear ocean waters where they produce aph (λ) values only at a few wavelengths in the blue-green domain; this causes the main difficulty in making these models more usable with any suite of wavelengths Further, recent studies have shown the performance of these models to be highly questionable in optically complex waters This emphasized the need for developing a more accurate model for the satellite-based estimation of aph (λ) in a wide range of oceanic waters In our previous study, we developed an empirical model (hereafter referred as Tiwari-Shanmugam model - “TS model”) based on the relationship of the in situ remote sensing reflectance ratio Rrs (670)/Rrs (490) and in situ aph(λ) which is best fit to a third order polynomial In the present study, we rigorously test this model along with three global inversion models (eg, Constrained Linear Matrix (LM) model, Quasi-analytical algorithm (QAA), and GSM semi-analytical model which are often used by the ocean color community) using three independent in situ data sets from clear to turbid coastal waters and satellite match-ups data from global waters When applied to these data sets, the TS model produces more accurate aph values across the entire visible wavelengths (400-700 nm) in all these waters, whereas LM, QAA and GSM models yield significant errors in addition to being restricted to produce aph values only in the blue-green wavelengths (LM and GSM) Though the TS model is mathematically simple, it overcomes such limitations and yields excellent results in terms of reproducing measured aph spectra that are highly desired by the ocean color community for inputting in various bio-optical models and studying the spatial structure of the different phytoplankton communities from satellite remote sensing observations in global waters