A synthetic optical database generated by radiative transfer simulations in support of studies in ocean optics and optical remote sensing of the global ocean
Authors: Loisel Hubert , Jorge Daniel Schaffer Ferreira , Reynolds Rick A. , Stramski Dariusz 
Published: August 2023

Abstract. Radiative transfer (RT) simulations have long been used
to study the relationships between the inherent optical properties (IOPs) of
seawater and light fields within and leaving the ocean, from which ocean
apparent optical properties (AOPs) can be calculated. For example, inverse
models used to estimate IOPs from ocean color radiometric measurements have been
developed and validated using the results of RT simulations. Here we describe the
development of a new synthetic optical database based on hyperspectral RT
simulations across the spectral range of near-ultraviolet to
near-infrared performed with the HydroLight radiative transfer code. The key
component of this development is the generation of a synthetic dataset of
seawater IOPs that serves as input to RT simulations. Compared to similar
developments of optical databases in the past, the present dataset of IOPs
is characterized by the probability distributions of IOPs that are consistent
with global distributions representative of vast areas of open-ocean pelagic
environments and coastal regions, covering a broad range of optical water
types. The generation of synthetic data of IOPs associated with
particulate and dissolved constituents of seawater was driven largely by an
extensive set of field measurements of the phytoplankton absorption
coefficient collected in diverse oceanic environments. Overall, the
synthetic IOP dataset consists of 3320 combinations of IOPs. Additionally,
the pure seawater IOPs were assumed following recent recommendations. The RT
simulations were performed using 3320 combinations of input IOPs, assuming
vertical homogeneity within an infinitely deep ocean. These input IOPs were
used in three simulation scenarios associated with assumptions about
inelastic radiative processes in the water column (not considered in
previous synthetically generated optical databases) and three simulation
scenarios associated with the sun zenith angle. Specifically, the simulations
were made assuming no inelastic processes, the presence of Raman scattering
by water molecules, and the presence of both Raman scattering and
fluorescence of chlorophyll a pigment. Fluorescence of colored dissolved
organic matter was omitted from all simulations. For each of these three
simulation scenarios, the simulations were made for three sun zenith angles
of 0, 30, and 60∘ assuming clear skies, standard
atmosphere, and a wind speed of 5 m s−1. Thus, overall 29 880 RT
simulations were performed. The output results of these simulations include
radiance distributions, plane and scalar irradiances, and a whole set
of AOPs, including remote-sensing reflectance, vertical diffuse
attenuation coefficients, and mean cosines, where all optical variables are
reported in the spectral range of 350 to 750 nm at 5 nm intervals for
different depths between the sea surface and 50 m. The consistency of this
new synthetic database has been assessed through comparisons with in situ
data and previously developed empirical relationships involving IOPs and
AOPs. The database is available at the Dryad open-access repository of research
data (https://doi.org/10.6076/D1630T, Loisel et al., 2023).