Snow spectral albedo at Summit, Greenland: comparison between in situ measurements and numerical simulations using measured physical and chemical properties of the snowpack
1Météo-France – CNRS, CNRM – GAME URA 1357, Centre d'Etudes de la Neige, Grenoble, France
2CNRS, UJF Grenoble, LGGE, Grenoble, France
3Takuvik Joint International Laboratory, CNRS and Université Laval, Québec (QC), Canada
4University of Houston, Houston (TX), USA
5Georgia Institute of Technology, Atlanta (GA), USA
6University of New Hampshire, Durham (NH), USA
Abstract. The albedo of surface snow is determined both by the near-surface profile of the physical and chemical properties of the snowpack and by the spectral and angular characteristics of the incident solar radiation. Simultaneous measurements of the physical and chemical properties of snow were carried out at Summit Camp, Greenland (72°36´ N, 38°25´ W, 3210 m a.s.l.) in May and June 2011, along with spectral albedo measurements. One of the main objectives of the field campaign was to test our ability to predict snow albedo comparing measured snow spectral albedo to the albedo calculated with a radiative transfer model. To achieve this goal, we made daily measurements of the snow spectral albedo in the range 350–2200 nm and recorded snow stratigraphic information down to roughly 80 cm. The snow specific surface area (SSA) was measured using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement, Gallet et al., 2009). Samples were also collected for chemical analyses including black carbon (BC) and trace elements, to evaluate the impact of light absorbing particulate matter in snow. This is one of the most comprehensive albedo-related data sets combining chemical analysis, snow physical properties and spectral albedo measurements obtained in a polar environment. The surface albedo was calculated from density, SSA, BC and dust profiles using the DISORT model (DIScrete Ordinate Radiative Transfer, Stamnes et al., 1988) and compared to the measured values. Results indicate that the energy absorbed by the snowpack through the whole spectrum considered can be inferred within 1.35%. This accuracy is only slightly better than that which can be obtained considering pure snow, meaning that the impact of impurities on the snow albedo is small at Summit. In the visible region, the discrepancies between measured and simulated albedo are mostly due to the lack of correction of the cosine collector deviation from a true cosine response. In the near-infrared, minor deviations up to 0.014 can be due the accuracy of SSA measurements and to the surface roughness, whereas deviations up to 0.05 can be explained by the vertical resolution of measurements of surface layer physical properties. At 1430 and around 1800 nm the discrepancies are larger and independent of the snow properties; they may be due to the uncertainties in the ice refractive index at these wavelengths. This work contributes to the development of physically-based albedo schemes in detailed snowpack models, and to the improvement of retrieval algorithms for estimating snow properties from remote sensing data.