Proceedings Article | 11 December 2012
KEYWORDS: Polymers, Reflectivity, Clouds, Magnetoencephalography, Atmospheric modeling, Oceanography, Aerosols, Scattering, Image processing, Water
Atmospheric correction of ocean-color imagery in the Arctic brings some specific challenges that the standard
atmospheric correction algorithm does not address, namely low solar elevation, high cloud frequency, multi-layered
polar clouds, presence of ice in the field-of-view, and adjacency effects from highly reflecting surfaces covered by
snow and ice and from clouds. The challenges may be addressed using a flexible atmospheric correction algorithm,
referred to as POLYMER (Steinmetz and al., 2011). This algorithm does not use a specific aerosol model, but fits
the atmospheric reflectance by a polynomial with a non spectral term that accounts for any non spectral scattering
(clouds, coarse aerosol mode) or reflection (glitter, whitecaps, small ice surfaces within the instrument field of
view), a spectral term with a law in wavelength to the power -1 (fine aerosol mode), and a spectral term with a law
in wavelength to the power -4 (molecular scattering, adjacency effects from clouds and white surfaces). Tests are
performed on selected MERIS imagery acquired over Arctic Seas. The derived ocean properties, i.e., marine
reflectance and chlorophyll concentration, are compared with those obtained with the standard MEGS algorithm.
The POLYMER estimates are more realistic in regions affected by the ice environment, e.g., chlorophyll
concentration is higher near the ice edge, and spatial coverage is substantially increased. Good retrievals are
obtained in the presence of thin clouds, with ocean-color features exhibiting spatial continuity from clear to cloudy
regions. The POLYMER estimates of marine reflectance agree better with in situ measurements than the MEGS
estimates. Biases are 0.001 or less in magnitude, except at 412 and 443 nm, where they reach 0.005 and 0.002,
respectively, and root-mean-squared difference decreases from 0.006 at 412 nm to less than 0.001 at 620 and 665
nm. A first application to MODIS imagery is presented, revealing that the POLYMER algorithm is robust when
pixels are contaminated by sea ice.
