README: A synthetic database of hyperspectral ocean optical properties

Hubert Loisel and Daniel Schaffer Ferreira Jorge
Laboratoire d'Océanologie et de Géosciences, Université du Littoral-Côte-d'Opale, Université Lille, CNRS, IRD, UMR 8187, LOG, 32 avenue Foch, Wimereux, France

Rick A. Reynolds and Dariusz Stramski
Marine Physical Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0238, USA

Correspondence: Hubert Loisel (hubert.loisel@univ-littoral.fr)

This database is associated with the article:
Loisel, H., D. S. F. Jorge, R. A. Reynolds, and D. Stramski. 2023. A synthetic optical database generated by radiative transfer simulations in support of studies in ocean optics and optical remote sensing of the global ocean. Earth Syst. Sci. Data, 15, 3711–3731. doi: 10.5194/essd-15-3711-2023.

This dataset represents a new synthetic optical database based on hyperspectral RT simulations across the spectral range from the near-ultraviolet to near-infrared performed with the HydroLight radiative transfer code. The key component of this database was the generation of the synthetic dataset of seawater IOPs which served as input to RT simulations. Compared to similar developments of optical databases in the past, the present dataset of IOPs is characterized by 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 the 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 and three simulation scenarios associated with 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 wind speed of 5 m/s. Thus, overall 29880 RT simulations were performed. The output results of these simulations include the radiance distributions, plane and scalar irradiances, and the whole set of AOPs including the remote-sensing reflectance, vertical diffuse attenuation coefficients, and mean cosines where all optical variables are reported in the spectral range from 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 the IOPs and AOPs.

Users are referred to the above article for an in-depth description and discussion of the database.
Please use the latest version (currently October 2023)!

Version history:
2023-10-18: All nine *_profile.nc files were updated. An error was discovered in the naming and content of these files with regards to the labeling of simulation scenarios for the inclusion of inelastic radiative transfer processes. The nine files with the IOP scenarios (HydrolightXYY.nc) are unchanged from the previous version.
2023-08-18: Readme and metadata files updated to reflect final article citation
2023-07-19: Updated database netcdf files to repair some corrupted data values
2023-05-31: Revised database submitted following changes made in response to article peer review
2023-03-03: Initial database submission to Dryad during article peer review

Description of the data and file structure

Data File Organization

The synthetic database of seawater IOPs used as input for performing RT simulations and the resulting output radiometric data from the HydroLight model runs are provided in 18 individual files. The file format is netCDF-4. Data for the variables contained within these files are in single-precision floating-point format. Missing values are identified with -999, for example situations where z1_lambda exceeded the maximum depth of our simulations.

The 18 data files are organized into 9 pairs of files representing different simulation scenarios. The file naming convention for these pairs is:

HydrolightXYY.nc
HydrolightXYY_profile.nc

The first file contains the spectral absorption and scattering coefficients of seawater constituents (assumed to be vertically homogenous) used in a given simulation scenario as well as selected sea-surface radiometric quantities of interest to ocean color radiometry. The second file contains both surface and depth-dependent values of calculated spectral radiometric quantities and selected AOPs along with other ancillary information.

The value of X in the filename refers to a specific simulation scenario with regards to the assumption of inelastic radiative processes occurring in the water column:
X = 1: No inelastic processes included.
X = 2: Raman scattering by water molecules included.
X = 4: Raman scattering by water molecules and fluorescence of chlorophyll-a included.
Note that fluorescence by colored dissolved organic matter was omitted from all simulations.

The value of YY in the filename refers to the solar zenith angle used in the simulation, and represents a value of 00, 30, or 60 degrees.

All simulations assume clear skies, standard atmosphere, a wind speed of 5 m/s, and an infinitely deep ocean.

Data File Descriptions

1. HydrolightXYY.nc These files contain the spectral absorption and scattering coefficients of seawater constituents used as input in the simulation scenario. In addition, selected sea-surface radiometric quantities output from the Hydrolight model run are included. Most variables are stored as 81 × 3320 numerical arrays, representing 81 light wavelengths (350–750 nm at 5 nm increments) and 3320 scenarios of different IOP combinations.
2. HydrolightXYY_profile.nc These files contain the surface and depth-dependent values of selected spectral radiometric quantities and AOPs along with other ancillary information. Surface variables are 81 × 3320 as described above, but depth-dependent variables include an additional dimension representing values at 61 discrete depths in the water column from the sea surface to 50 m (variable name includes “_z”). A depth of –1 indicates a quantity above the sea surface (z = 0+), a depth of 0 indicates a quantity immediately beneath the sea surface (z = 0-), and the remaining depths are positive in the downward direction.

Further details on variable names, units, and descriptions can be found in the file Database_README.pdf