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- Title
Inherent Optical Properties‐Reflectance Relationships Revisited.
- Authors
Lo Prejato, Marilisa; McKee, David; Mitchell, Catherine
- Abstract
Understanding the relationship between remote sensing reflectance, Rrs(λ) and the inherent optical properties (IOPs) of natural waters is potentially a key to improving our ability to determine biogeochemical constituents from radiometric measurements. These relationships are usually described as a function of absorption, a(λ), and backscattering, bb(λ), coefficients, with the literature providing various forms of equation operating on either bb(λ)/a(λ) or bb(λ)/[a(λ)+bb(λ)] to represent the impact of variations in light field geometries and changes in sea‐water composition. The performance of several IOP‐reflectance relationships is assessed using HydroLight radiative transfer simulations covering a broad range of Case 1 and Case 2 water conditions. While early versions of IOP‐reflectance relationships assigned variability to associated proportionality factors (e.g., f/Q) or low‐order polynomial functions, recent studies have demonstrated relationships between Rrs(λ) and bb(λ)/[a(λ)+bb(λ)] are well‐characterized by nonlinear (high‐order polynomial), monotonic functions. This study demonstrates that this approach is also valid for relationships operating on bb(λ)/a(λ) and that there is no intrinsic benefit to functions operating on bb(λ)/[a(λ)+bb(λ)] compared to bb(λ)/a(λ) for Case 2 waters, contrary to recent suggestions in the literature. In all cases it is necessary to carefully consider the performance of best fit relationships across the full range of variability of IOPs and Rrs(λ), with higher order polynomials required to enable equivalent performance across the range of natural variability. The analysis further demonstrates insignificant wavelength sensitivity across the visible region, limited sensitivity to changes in solar zenith angle and extends to relationships for below surface remote sensing reflectance, rrs(λ). Plain Language Summary: Ocean color satellites measure the remote sensing reflectance of oceans. The magnitude and spectral distribution of this signal (the ocean color) is determined by the inherent optical properties (IOPs) of the ocean, primarily the absorption, a(λ), and backscattering, bb(λ), coefficients. These IOPs are, in turn, determined by the biogeochemical composition of material dissolved and suspended in seawater, usually considered to be a mixture of phytoplankton, colored dissolved organic material and sediments. In this study relationships between remote sensing reflectance and IOPs are re‐examined taking into account differences between forward and inverse directions and also considering above and below surface reflectances. It is shown that IOP reflectance relationships operating on two previously suggested IOP expressions, bb(λ)/a(λ) and bb(λ)/[a(λ)+bb(λ)], are essentially equivalent across a wide range of water types. It is also shown that higher order polynomial forms of relationship are needed to ensure accurate return of derived parameters across the full range of material concentrations. Existing lower order relationships are shown to perform well, sometimes better, over subranges of material concentrations. Key Points: The mathematical form and performance of relationships between above and below surface remote sensing reflectance and inherent optical properties (IOPs) of natural waters are examined for both forward and inverse applicationsTwo forms of IOPs relationship that have previously been presented in the literature are shown to be equivalent in performance across a wide range of optically complex water conditionsPerformance of new and previously published relationships varies across the range of natural optical variability and higher order polynomial functions are needed for accuracy in some cases
- Subjects
OPTICAL properties; REMOTE sensing; BIOGEOCHEMICAL cycles; OCEAN dynamics; MARINE geophysics
- Publication
Journal of Geophysical Research. Oceans, 2020, Vol 125, Issue 11, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016661