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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-97
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-97
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 May 2020

26 May 2020

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A revised version of this preprint is currently under review for the journal TC.

Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica

Tim Carlsen1,a, Gerit Birnbaum2, André Ehrlich1, Veit Helm2, Evelyn Jäkel1, Michael Schäfer1, and Manfred Wendisch1 Tim Carlsen et al.
  • 1Leipzig Institute for Meteorology, Leipzig University, Leipzig, Germany
  • 2Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • anow at: Department of Geosciences, University of Oslo, Oslo, Norway

Abstract. The angular reflection of solar radiation by snow surfaces comprises an important boundary condition for radiative transfer simulations. In polar regions, the surface reflection is particularly anisotropic due to low sun elevations and the highly anisotropic scattering phase function of ice crystals. This anisotropy needs to be considered in the angular modeling of the surface reflection, which is essential for satellite remote sensing techniques. To quantify the snow reflection properties, the hemispherical-directional reflectance function (HDRF) of snow surfaces was derived from airborne measurements using a digital 180° fish-eye camera (green channel, 490–585 nm wavelength band) in Antarctica during austral summer in 2013/14. This function was measured for different surface roughnesses, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 × 1000 km2 in the vicinity of Kohnen station (75°0′ S, 0°4′ E) at the outer part of the East Antarctic Plateau. The observations over Dronning Maud Land include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera was calibrated in terms of spectral radiances and installed in a downward-looking configuration on a research aircraft. It provides upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of HDRF data derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical, kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized with respect to solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations generally underestimated the anisotropy of the surface reflection. Some of these deviations between airborne and MODIS satellite retrievals are likely linked to short-term changes in snow properties.

Tim Carlsen et al.

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Tim Carlsen et al.

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Short summary
The angular reflection of solar radiation by snow surfaces is particularly anisotropic and highly variable. We measured the angular reflection from an aircraft using a digital camera in Antarctica in 2013/14 and studied its variability: the anisotropy increases with a lower sun, but decreases for rougher surfaces and larger snow grains. The applied methodology allows for a direct comparison with satellite observations, which generally underestimated the anisotropy measured within this study.
The angular reflection of solar radiation by snow surfaces is particularly anisotropic and...
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