Reconstruction of vertical profiles of chlorophyll concentration

Abstract

This work presents the multispectral reconstruction of the vertical profile of Chlorophyll concentration (Chl-a) in Case 1 waters. In such ocean waters, optical properties are driven by the presence of phytoplankton, allowing the use of bio-optical models, where the absorption and scattering coefficients are related with Chl-a. The vertical profile of Chl-a is reconstructed from experimental measurements of water-leaving radiances (nLw) at 10 wavelengths. These radiances are considered for a discrete number of upward polar directions. The inverse problem is formulated as an optimization problem, and iteratively solved by the ACO using the radiative transfer equation as direct model. The objective function is given by the square difference between computed and experimental radiances. For each iteration a population of candidate solutions is generated, pre-selected and evaluated by means of the objective function. Each candidate solution corresponds to a discrete Chl-a profile. The radiative transfer equation is solved for each candidate solution yielding the radiances that are used in the objective function to evaluate it. Since this equation requires the absorption and scattering coefficients, these are calculated using bio-optical models. The radiative transfer equation is solved using the Laplace transform discrete ordinate LTS_<i>N</i> method. A parallel implementation of the ACO is employed and executed in a distributed memory machine. The multispectral approach allows estimating the vertical profile using only nLw, instead of in situ measurements on several depths.