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

Tim Carlsen,Gerit Birnbaum,André Ehrlich,Johannes Freitag,Michael Schäfer,Larysa Istomina,Manfred Wendisch,Sepp Kipfstuhl,Anaïs Orsi,Georg Heygster

doi:10.5194/tc-11-2727-2017

Abstract

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 1100 nm 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 89 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 slightly underestimated the ground-based results.

Highlights

The cryosphere plays a fundamental role in determining the Earth’s surface radiative energy budget, as snow and sea ice represent surfaces with the highest albedo on Earth. Picard et al (2016) estimated that a hypothetical change in global surface albedo of 1 % would offset a difference in reflected energy comparable to the globally averaged radiative forcing of 1.82 W m−2 caused by the increase in CO2 concentration since the preindustrial time (Myhre et al, 2013)
With mean wind speeds of 4 m s−1 and maximum wind gusts reaching 11 m s−1 at Kohnen station, drifting snow occurred mainly due to creeping or saltation of the ice crystals. This wind-induced transportation of freshly fallen snow grains is superimposed on the signal of snow metamorphism in the temporal evolution of specific surface area (SSA) retrieved from COmpact RAdiation measurement System (CORAS)
The temporal variability of SSA and respective ropt on the East Antarctic plateau were investigated during austral summer 2013/14 utilizing spectral albedo measurements and MODerate Resolution Imaging Spectroradiometer (MODIS) reflectance measurements

Summary

Introduction

The cryosphere plays a fundamental role in determining the Earth’s surface radiative energy budget, as snow and sea ice represent surfaces with the highest albedo on Earth. Picard et al (2016) estimated that a hypothetical change in global surface albedo of 1 % would offset a difference in reflected energy comparable to the globally averaged radiative forcing of 1.82 W m−2 caused by the increase in CO2 concentration since the preindustrial time (Myhre et al, 2013). Picard et al (2016) estimated that a hypothetical change in global surface albedo of 1 % would offset a difference in reflected energy comparable to the globally averaged radiative forcing of 1.82 W m−2 caused by the increase in CO2 concentration since the preindustrial time (Myhre et al, 2013). This change in global surface albedo could be caused either by a variation in snow and sea ice cover or by a change of the snow albedo itself. Munneke et al (2008) found variations

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Conclusion