Coupling of airborne and in situ ground based measurements of surface albedo, BRDF and snow properties in Antarctica to improve prognostic snow models
Zusammenfassung der Projektergebnisse
The significance of the polar regions for the Earth’s climate system and their observed amplified response to climate change indicate the necessity for high temporal and spatial coverage for the monitoring of the reflective properties of snow surfaces and their influencing factors. Therefore, the specific surface area (SSA, as a proxy for snow grain size) and the hemispherical directional reflectance factor (HDRF) of snow were measured for a 2-month period in central Antarctica (Kohnen research station) during austral summer 2013/14. The SSA 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 analyse 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 1100 nm wavelength were used to reduce the retrieval uncertainty. The retrieval was applied to the groundbased and airborne observations and validated against optical in situ observations of SSA utilizing an IceCube device. The SSA retrieved from CORAS observations varied between 29 and 96 m2 kg^-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 underestimated the ground-based results. The spatial variability of SSA in Dronning Maud Land ranged in the same order of magnitude as the temporal variability revealing differences between coastal areas and regions in interior Antarctica. The validation presented in this study provided an unique test bed for retrievals of SSA under Antarctic conditions where in situ data are scarce and can be used for testing prognostic snowpack models in Antarctic conditions. The HDRF of snow was derived from airborne measurements of a digital 180° fisheye camera for a variety of conditions with different surface roughness, snow grain size, and solar zenith angle. The camera provides radiance measurements with high angular resolution utilizing detailed radiometric and geometric calibrations. The comparison between smooth and rough surfaces (sastrugi) showed significant differences in the HDRF of snow, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven model for the bidirectional reflectance distribution function (BRDF), the snow HDRF was parameterized with respect to surface roughness, snow grain size, and solar zenith angle. This allows a direct comparison of the HDRF measurements with BRDF products from satellite remote sensing.
Projektbezogene Publikationen (Auswahl)
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Airborne and in situ ground-based measurements of surface albedo, bidirectional reflectivity BRDF and snow properties on the Antarctic plateau, Workshop: Arctic snow, sea ice properties and interaction with the atmosphere, Bremerhaven, Germany, 6-7 November 2017
Carlsen, T., M. Belke Brea, G. Birnbaum, A. Ehrlich, J. Freitag, S. Kipfstuhl, G. König-Langlo, M. Schäfer, M. Wendisch
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Observations of snow properties in Antarctica, Conference: International Radiation Symposium, Auckland, New Zealand, 19 April 2016
Carlsen, T.,G. Birnbaum, A. Ehrlich, J. Freitag, G. Heygster, L. Istomina, S. Kipfstuhl, A. Orsi, M. Schäfer, M. Wendisch
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Spatio-temporal variability of snow surface albedo and grain size derived from airborne and ground-based observations in Antarctica, Conference: SCAR 2016 Open Science Conference, Kuala Lumpur, Malaysia, 22-26 August 2016
Carlsen, T., M. Belke Brea, G. Birnbaum, A. Ehrlich, J. Freitag, A. Humbert, S. Kipfstuhl, G. König-Langlo, M. Schäfer, M. Wendisch
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2017: Parameterization of snow directional reflectivity in Antarctica, Workshop: Arctic snow, sea ice properties and interaction with the atmosphere, Bremerhaven, Germany, 6 November 2017
Carlsen, T., G. Birnbaum, A. Ehrlich, M. Schäfer, M. Wendisch
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Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica, The Cryosphere, 11, 2727-2741, 2017
Carlsen, T., Birnbaum, G., Ehrlich, A., Freitag, J., Heygster, G., Istomina, L., Kipfstuhl, S., Orsi, A., Schäfer, M., and M. Wendisch
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2018: Parameterization of snow BRDF measurements in Antarctica, Conference: 27th International Polar Conference, Rostock, Germany, 27 March 2018
Carlsen, T., G. Birnbaum, A. Ehrlich, M. Schäfer, M. Wendisch
