Abstract
The atmospheric correction algorithm employed by the NASA Ocean Biology Processing Group requires an assumption of negligible water-leaving reflectance in the near-infrared region of the spectrum. For waters where this assumption is not valid, an optical model is used to estimate near-infrared water-leaving reflectance. We describe this optical model as implemented for the sixth reprocessing of the SeaWiFS mission-long time-series (September 2009). Application of the optical model resulted in significant reductions in the number of negative water-leaving reflectance retrievals in turbid and optically complex waters, and improved agreement with in situ chlorophyll-a observations. The incidence of negative water-leaving reflectance retrievals at 412 nm was reduced by 40%, while negative reflectance at 490 nm was nearly eliminated.
Highlights
The retrieval of accurate geophysical data products (e.g., spectral remote sensing reflectance Rrs( ), and concentrations of the phytoplankton pigment chlorophyll-a, Ca) from satelliteborne radiometers requires the use of an ‘atmospheric correction’ algorithm [1]
For waters where this assumption is not valid, an optical model is used to estimate near-infrared water-leaving reflectance. We describe this optical model as implemented for the sixth reprocessing of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) missionlong time-series (September 2009)
The algorithm implemented by the NASA Ocean Biology Processing Group (OBPG) [1] requires an assumption of negligible water-leaving reflectance in the near infrared (NIR) region of the spectrum (i.e., Rrs(NIR) = 0 sr 1)
Summary
The retrieval of accurate geophysical data products (e.g., spectral remote sensing reflectance Rrs( ), and concentrations of the phytoplankton pigment chlorophyll-a, Ca) from satelliteborne radiometers requires the use of an ‘atmospheric correction’ algorithm [1]. The NIR bands of the NASA Sea-viewing Wide Field-of-view Sensor (SeaWiFS) are centered on 765 and 865 nm With this ‘black-pixel’ assumption, the measured top-of-atmosphere (TOA) reflectance in two NIR bands can be used to estimate both the magnitude and spectral dependence of a(NIR). This preliminary Ca was used to estimate spectral particulate backscattering, bbp(NIR), which was used in turn to reconstruct Rrs(NIR) With this modeled Rrs(NIR) removed from the TOA signal, the atmospheric correction process was repeated, Ca was recalculated, the process was iterated upon until convergence in Ca was achieved [2,4]. The OBPG switched from [2] to [5] after observing that [2] depressed ratios of a(NIR) in optically complex waters This often resulted in the selection of spectrally flat aerosol models within the atmospheric correction process, which artificially depressed the final Ca retrievals.
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