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

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 m 2 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.

Microstructure of Snow and Its Link to Trace Elements and Isotopic Composition at Kohnen Station, Dronning Maud Land, Antarctica

“…Picard et al, 2012), specific surface area (e.g. Carlsen et al, 2017), or crystal fabric orientation (e.g. Calonne et al, 2017).…”

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

“…From the irradiance measurements, the spectral snow surface albedo was obtained (Wendisch et al, 2004). The spectral resolution is 2 to 3 nm between 0.3 and 1.0 µm and 15 nm up to 2.2 µm wavelength; the temporal resolution is in the order of 1 s. Combining the errors associated with the signalto-noise ratio (1.3-3.0 %), the accuracy of the dark correction (0.1 %), the wavelength calibration (1.0 %), the accuracy of the cross-calibration (1.0-4.5 %), the non-ideal cosine response of the optical inlets (3.5 %), and the horizontal stabilization (1.0 %), the surface albedo measurements with SMART have an estimated uncertainty between 4.1 % and 8.1 % depending on wavelength (Carlsen et al, 2017).…”

“…The optical-equivalent snow grain size is defined as the radius R opt of a collection of spheres with the same volume-tosurface ratio compared to the actual non-spherical snow grains. The retrieval of R opt from spectral surface albedo measurements is described in Carlsen et al (2017). In principle, the snow grain size and pollution amount (SGSP) algorithm (Zege et al, 2011) was extended to spectral ratios of surface albedo at 1280 and 1100 nm wavelength.…”

“…Note that this inversion yields the parameters of a BRDF model derived from a HDRF measurement as the BRDF is not measurable under atmospheric conditions. However, the atmospheric influence is assumed to be small due to the high surface elevation and the dry, aerosol-free atmosphere on the Antarctic plateau: the mean AOD at Kohnen station was 0.015 between 2001(at 500 nm, Tomasi et al, 2007 and the mean integrated atmospheric water vapor was 1.1 kg m −2 between December 2013 and January 2014 (Carlsen et al, 2017). Simulations with the library for radiative transfer libRadtran (Mayer and Kylling, 2005) of the mean direct fraction of the global irradiance f dir at Kohnen station between 10 December 2013 and 31 January 2014 indicate rather weak atmospheric scattering effects.…”

Spectral albedo measurements over snow-covered slopes: theory and slope effect corrections

“…Spectral albedo measurements are less frequent than broadband albedo, but provide richer information, enabling not only to establish the shortwave radiative budget, but also to investigate if and how surface albedo is driven by snow microstructural properties (Gallet et al, 2011;Carmagnola et al, 2013;Libois et al, 2015;Carlsen et al, 2017), liquid water (Dumont et al, 2017), impurities (Skiles et al, 2018;Tuzet et al, 2017), or algea (Painter et al, 2001). As is the case with broadband albedo, spectral albedo measurements are affected by slope.…”