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Volume 11, issue 6 | Copyright

Special issue: Intercomparison of methods to characterise snow...

The Cryosphere, 11, 2727-2741, 2017
https://doi.org/10.5194/tc-11-2727-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Nov 2017

Research article | 27 Nov 2017

Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica

Tim Carlsen1, Gerit Birnbaum2, André Ehrlich1, Johannes Freitag2, Georg Heygster3, Larysa Istomina3, Sepp Kipfstuhl2, Anaïs Orsi4, Michael Schäfer1, and Manfred Wendisch1 Tim Carlsen et al.
  • 1Leipzig Institute for Meteorology, University of Leipzig, Leipzig, Germany
  • 2Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 3Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 4Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France

Abstract. The optical-equivalent snow grain size affects the reflectivity of snow surfaces and, thus, the local surface energy budget in particular in polar regions. Therefore, the specific surface area (SSA), from which the optical snow grain size is derived, was observed for a 2-month period in central Antarctica (Kohnen research station) during austral summer 2013/14. The data were retrieved on the basis of ground-based spectral surface albedo measurements collected by the COmpact RAdiation measurement System (CORAS) and airborne observations with the Spectral Modular Airborne Radiation measurement sysTem (SMART). The snow grain size and pollution amount (SGSP) algorithm, originally developed to analyze spaceborne reflectance measurements by the MODerate Resolution Imaging Spectroradiometer (MODIS), was modified in order to reduce the impact of the solar zenith angle on the retrieval results and to cover measurements in overcast conditions. Spectral ratios of surface albedo at 1280 and 1100nm wavelength were used to reduce the retrieval uncertainty. The retrieval was applied to the ground-based and airborne observations and validated against optical in situ observations of SSA utilizing an IceCube device. The SSA retrieved from CORAS observations varied between 27 and 89m2kg−1. Snowfall events caused distinct relative maxima of the SSA which were followed by a gradual decrease in SSA due to snow metamorphism and wind-induced transport of freshly fallen ice crystals. The ability of the modified algorithm to include measurements in overcast conditions improved the data coverage, in particular at times when precipitation events occurred and the SSA changed quickly. SSA retrieved from measurements with CORAS and MODIS agree with the in situ observations within the ranges given by the measurement uncertainties. However, SSA retrieved from the airborne SMART data slightly underestimated the ground-based results.

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The optical size of snow grains (ropt) affects the reflectivity of snow surfaces and thus the local surface energy budget in particular in polar regions. The temporal evolution of ropt retrieved from ground-based, airborne, and spaceborne remote sensing could reproduce optical in situ measurements for a 2-month period in central Antarctica (2013/14). The presented validation study provided a unique testbed for retrievals of ropt under Antarctic conditions where in situ data are scarce.
The optical size of snow grains (ropt) affects the reflectivity of snow surfaces and thus the...
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