Characterisation of the BRDF (HDRF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and validation of satellite remote sensing products Amelia Marks 1*, Corrado Fragiacomo 2, Alasdair MacArthur 3, Giuseppe Zibordi 4, Nigel Fox 5, Martin King 1 1 Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0TX, UK 2 National Institute of Oceanography and Geophysics, , Trieste, Italy 3 NERC Field Spectroscopy Facility, Grant Institute, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, UK 4 Institute for Environment and Sustainability, Joint Research Centre, Via E. Fermi 2749 I Ispra (VA), Italy 5 National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK Characterisation of the BRDF (HDRF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and validation of satellite remote sensing products Amelia Marks 1*, Corrado Fragiacomo 2, Alasdair MacArthur 3, Giuseppe Zibordi 4, Nigel Fox 5, Martin King 1 1 Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0TX, UK 2 National Institute of Oceanography and Geophysics, , Trieste, Italy 3 NERC Field Spectroscopy Facility, Grant Institute, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, UK 4 Institute for Environment and Sustainability, Joint Research Centre, Via E. Fermi 2749 I Ispra (VA), Italy 5 National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK Key References Hudson, S., Warren, S., Brandt, R., Grenfell, T., & Six, D Spectral bidirectional reflectance of Antarctic snow: Measurements and parameterization. Journal of Geophysical Research, Sandmeier, S Acquisition of Bidirectional Reflectance Factor Data with Field Goniometers. Remote Sensing of the Environment –269. Schaepman-Strub, G., Schaepman, M., Painter, T., Dangel, S., & Martonchik, J Reflectance quantities in optical remote sensing-definitions and case studies. Remote Sensing of Environment –42. Six, D., Fily, M., Alvain, S., Henry, P., & Benoist, J Surface characterisation of the Dome Concordia area (Antarctica) as a potential satellite calibration site, using Spot 4/Vegetation instrument. Remote sensing of Environment. 89, 83–94. Warren, S., Brandt, R., & Hinton, P Effect of surface roughness on bidirectional reflectance of Antarctic snow. Journal of Geophysical Research Introduction Satellites require calibration through surface bi-directional reflectance distribution factor (BRDF) measurements as the reflectance of natural surfaces is not isotropic. Six et al., 2004 identified Dome C, Antarctica as an excellent calibration site for surface BRDF measurements. 8. Conclusions HDRF polar plots show agreement with previous Dome C measurements. Snow is forward scattering which is consistent with previous literature. HDRF values are similar from 400–800 nm but decrease up to 1600 nm. Large differences in HDRF values occur between different sites. BRDF of a surface cannot be directly measured but is measured as a hemispherical-directional reflectance factor (HDRF); a measure of surface reflectance in a given direction with respect to a reference standard, as illuminated from a whole sky hemisphere. 2. Reflectance terminology BRDF describes the ratio of incident light from a known direction to radiance reflected in a another direction. Detector Sun Solar principal plane 0° 180° Figure 1- BRDF measurements taken using azimuth (Φ i ) and zenith (Θ i ) angle of incidence light (sunlight) and the azimuth (Φ r ) and zenith (Θ r ) viewing (detector) angle 4. Measurement location Dome C is high on the Antarctic plateau, with low winds, clear skies and low relief surfaces, making it an ideal place for HDRF measurements. HDRF measurements were taken at 8 sites along a 100 m transect, shown in figure 3. Measurement locations were chosen systematically based on sites that had the smoothest surface features, moving consistently eastwards. Figure 3- Measurement location 7. Average HDRF polar plots for all sites Figure 5 shows average HDRF over all sites. There is excellent agreement with previous Dome C measurements (Hudson et al., 2006), with a forward reflectance peak of The snow surface is strongly forwards scattering, with the forward scattering peak occurring at large zenith angles, ~60°, and a wide range of azimuth angles. Figure 6 shows variation in HDRF with wavelength. From 400–600 nm HDRF values stay fairly constant, however above 800 nm HDRF values significantly decrease, this is most drastic at 1600 nm where values drop to near 0. The prominence of the forward scattering peak also decreases with wavelength, with a greater range of HDRF values seen at shorter wavelengths. Polar plots in figure 4 show HDRF values over a hemisphere. Red colours show high reflectivity and blue colours show low reflectivity. The radius represents zenith angle- the centre being 0°. The circumference represents azimuth angle, with the sun at the base of the plot. All sites show a forward scattering snow surface, but there is a significant variation between sites. 6. HDRF polar plots for individual sites Figure 4- Polar plots of individual sites, 600 nm wavelength 3. Gonio Radio Spectrometer System (GRASS) GRASS, figure 2, enabled HDRF measurements of Dome C snow surfaces over a greater angle range than previous Dome C measurements (Hudson et al., 2006) and also enabled measurement of the same snow surface at each angle and over a larger wavelength range (400–1700 nm) Fibre optics record upwelling radiance, spaced at zenith angles of 15, 30, 45 and 60° three arms situated 30° of azimuth apart Fibre optic for upwelling nadir measurements Downwelling radiation recorded simultaneously by integrating sphere Arms rotate providing measurements covering 360°. Figure 2- GRASS setup C21C-0607 HDRF value Figure 5- Average HDRF, 600 nm Figure 6- Average HDRF, all wavelengths HDRF value