Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica
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...
| Main Authors: | , , , , , , |
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| Format: | Text |
| Language: | English |
| Published: |
2020
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/tc-2020-97 https://tc.copernicus.org/preprints/tc-2020-97/ |
| Summary: | 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 km 2 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. |
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