An algorithm is proposed to perform atmospheric correction of ocean-color imagery in the presence of semi-transparent
clouds. The atmospheric “path” reflectance, due to scattering by molecules, aerosols, and droplets, absorption by
aerosols, and reflection by the surface, including coupling terms, is modeled by a polynomial with three terms, i.e., three
unknown coefficients. The marine reflectance is modeled as a function of chlorophyll concentration and a backscattering
coefficient that accounts for scattering by non-algal particles (or deviation from the backscattering coefficient specified
for typical phytoplankton), i.e., two additional unknown variables. The cloud transmittance, assumed constant spectrally,
is estimated separately from top-of-atmosphere reflectance in the near infrared. The five unknowns are retrieved by an
iterative, spectral matching scheme. The methodology, including the decomposition of the top-of-atmosphere signal and
the modeling of the path reflectance, is evaluated theoretically and applied to actual MODIS imagery acquired over
relatively thin clouds. Chlorophyll concentration is retrieved adequately under the clouds, and continuity is good
between the cloudy and adjacent clear regions. Values are similar to those obtained with the SeaDAS algorithm in clear
sky conditions, but cloud coverage is increased considerably. The algorithm is applicable operationally, but needs to be
further evaluated in varied cloudy situations.
A methodology is presented to estimate aerosol altitude from reflectance ratio measurements in the O2 absorption A-band. Previous studies have shown the impact of the vertical distribution of scatterers on the reflectance ratio. The reflectance ratio is defined as the ratio of the reflectance in a first spectral band, strongly attenuated by O2 absorption, to the reflectance in a second spectral band, minimally attenuated. First, a sensitivity study is performed to quantify the expected accuracy for various aerosol loadings and models. An accurate, high spectral resolution, radiative transfer model that fully accounts for interactions between scattering and absorption is used in the simulations. Due to their adequate spectral characteristics, POLDER and MERIS instruments are considered for simulations. For a moderately loaded atmosphere (i.e., aerosol optical thickness of 0.3 at 760 nm), the expected error on aerosol altitude is about 0.3 km for MERIS and 0.7 km for POLDER. More accurate estimates are obtained with MERIS, since the spectral reflectance ratio is more sensitive. Second, the methodology is applied to MERIS and POLDER imagery. Estimates of aerosol altitude are compared with lidar profiles of backscattering coefficient acquired during the AOPEX-2004 experiment. Retrievals are consistent with measurements and theory. These comparisons demonstrate the potential of the differential absorption methodology for obtaining information on aerosol vertical distribution.
The vertical distribution of absorbing aerosols affects significantly the reflectance of the ocean-atmosphere system. The effect, due to the coupling between molecule scattering and aerosol absorption, is important in the visible, especially in the blue, and becomes negligible in the near-infrared. Differences between top-of-atmosphere reflectance obtained with distinct vertical distributions increase with the sun, and view zenith angle, and the aerosol optical thickness, and with decreasing scattering albedo, but are practically independent of wind speed. In atmospheric correction algorithms, these differences are directly translated into errors on the retrieved water reflectance. They may reach large values even for small aerosol optical thickness, preventing accurate retrieval of chlorophyll concentration. A method has been developed to estimate aerosol altitude from data in the oxygen A-band of the MERIS, and POLDER sensors. The method is sufficiently sensitive to improve retrievals of water reflectance and chlorophyll concentration in the presence of absorbing aerosols.