Estimating pixel-level uncertainty in ocean color retrievals from MODIS
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Minwei Zhang, Amir Ibrahim, Bryan A. Franz, Ziauddin Ahmad, and Andrew M. Sayer, "Estimating pixel-level uncertainty in ocean color retrievals from MODIS," Opt. Express 30, 31415-31438 (2022). https://doi.org/10.1364/OE.460735
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
The spectral distribution of marine remote sensing reflectance, Rrs, is the fundamental
measurement of ocean color science, from which a host of bio-optical and biogeochemical
properties of the water column can be derived. Estimation of uncertainty in these derived
properties is thus dependent on knowledge of the uncertainty in satellite-retrieved Rrs (uc(Rrs)) at
each pixel. Uncertainty in Rrs, in turn, is dependent on the propagation of various uncertainty
sources through the Rrs retrieval process, namely the atmospheric correction (AC). A derivativebased method for uncertainty propagation is established here to calculate the pixel-level uncertainty
in Rrs, as retrieved using NASA’s multiple-scattering epsilon (MSEPS) AC algorithm and verified
using Monte Carlo (MC) analysis. The approach is then applied to measurements from the
Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite, with uncertainty
sources including instrument random noise, instrument systematic uncertainty, and forward model
uncertainty. The uc(Rrs) is verified by comparison with statistical analysis of coincident retrievals
from MODIS and in situ Rrs measurements, and our approach performs well in most cases. Based
on analysis of an example 8-day global products, we also show that relative uncertainty in Rrs at
blue bands has a similar spatial pattern to the derived concentration of the phytoplankton pigment
chlorophyll-a (chl-a), and around 7.3%, 17.0%, and 35.2% of all clear water pixels (chl-a ≤ 0.1
mg/m3
) with valid uc(Rrs) have a relative uncertainty ≤ 5% at bands 412 nm, 443 nm, and 488
nm respectively, which is a common goal of ocean color retrievals for clear waters. While the
analysis shows that uc(Rrs) calculated from our derivative-based method is reasonable, some
issues need further investigation, including improved knowledge of forward model uncertainty
and systematic uncertainty in instrument calibration.