Abstract

Abstract. The broadband 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 spectral albedo by comparing the measured albedo to the albedo calculated with a radiative transfer model, using measured snow physical and chemical properties. 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 dust, 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 1.10%. 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 near infrared, minor deviations in albedo up to 0.014 can be due to the accuracy of radiation and SSA measurements and to the surface roughness, whereas deviations up to 0.05 can be explained by the spatial heterogeneity of the snowpack at small scales, the assumption of spherical snow grains made for DISORT simulations and 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; we propose that they are due to errors 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.

Highlights

  • Surface snow is an important component of the climate system (Flanner et al, 2011)

  • The snow spectral albedo measured at Summit (Greenland) was compared to the albedo computed from the snow density and specific surface area profiles and the impurities’ concentrations, using the DISORT radiative transfer model

  • Up to 0.05, can be instead explained by the spatial heterogeneity of the snowpack: even if we took great care to choose an area over the ASD field of view as spatially homogeneous as possible, the horizontal variability at small scales can be responsible for part of the mismatch between calculations and observations. These discrepancies up to 0.05 can come from the assumption of spherical snow grains made for DISORT simulations, from our sampling protocol and from the functioning of the DUFISSS instrument, which retrieves specific surface area (SSA) from infrared reflectance measurements at 1310 nm, where penetration depth is about 1 cm

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Summary

Introduction

Surface snow is an important component of the climate system (Flanner et al, 2011). One of the most powerful drivers of climate and the main characteristic of snow-covered areas is their high albedo (i.e. the fraction of solar light that is reflected) compared to land cover types on Earth. A reduction in the snow cover will lead to increased absorption of energy that further warms the planet in what is called the snow albedo feedback (Hall, 2004) In this context, research efforts are continuing to find out more about the energy balance of snow-covered surfaces (Lemke et al, 2007), and accurately modelling snow albedo becomes of crucial importance. The albedo depends on snow physical properties, density and optically-equivalent grain size, and on the presence of impurities within the snowpack (Warren, 1982; Flanner et al, 2012). It is affected by the angular and spectral distributions of incoming solar radiation (Warren and Wiscombe, 1980)

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