Characterisation of the HDRF (as a proxy for BRDF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and inter-comparison of satellite optical data

Abstract

Measurements of the Hemispherical Directional Reflectance Factor (HDRF) of snow surfaces were performed at Dome C, Antarctica, during the Australis Summer 2011–2012 to support the inter-comparison and inter-calibration of satellite optical sensors. HDRF data were collected with the Gonio Radiometric Spectrometer System (GRASS) which performs hyper-spectral measurements of radiance from the same target surface with independent collectors at a number of viewing and azimuth angles in the 0–60° and 0–360° angular ranges, respectively. The radiance collectors, installed on a hemispheric frame and connected to a spectrometer through fibre optics, have an 8° full cone of acceptance and a viewing footprint varying from 0.049m2 at nadir to 0.142m2 at viewing angles of 60°. These relatively small footprints allow for the characterisation of small-scale heterogeneities in the HDRF of observed surfaces. HDRF measurements representative of the Dome C snow surfaces were made at eight different sites along a transect approximately 100m long. All the sites exhibit similar HDRF distributions with inter-site differences explained by small-scale inhomogeneities of the surface. The measured HDRF display marked forward scattering with anisotropy increasing with wavelength in the 400–1600nm spectral region. These data complement those from previous measurements performed in the same area with a different technique. Agreement between the two data sets is shown by differences generally lower than 4% between HDRF distributions derived from a previous study and the spatially averaged HDRF from the various sites along a transect presented in this work.