The radiative properties of snow at Summit, Greenland

The amount of solar radiation reflected at the earth surface is one of the primary parameters controlling the surface radiation balance which determines the heat and water exchange processes. Snow and ice cover a seasonally varying area of up to 15% on this planet. Therefore a profound knowledge of the snow radiative processes is needed to calculate the planetary radiation balance. Among all surfaces snow in particular shows high reflectance and complex absorption features. However, snow properties influencing the magnitude of reflectance and absorbtion are not constant over time. For example the anisotropic reflectance of a snow cover over the hemisphere varies with the solar incident angle. Presently, most general circulation models use a fixed snow albedo value over the globe or they apply a simple parametrization to calculate the surface reflectance based on two broadband radiation ranges (e.g. visible and near infrared). In remote sensing on the other hand, the broadband hemispherical albedo has to be extrapolated from a discrete and small number of confined spectral bands, and usually measured from a single viewing direction. Both applications generally fail to resolve the fine spectral and directional reflectance properties of snow, which in fact determine the broadband albedo. As a first step in mitigating this unsatisfying situation and improving the understanding of the snow radiative properties, the Summit Environmental Observatory on the ice sheet in the dry snow zone of Greenland has been chosen to carry out detailed studies on absorption, transmittance and reflectance properties of snow. The research focuses on the penetration depth of solar radiation into the snow and on the directional reflectance of solar radiation back to the hemisphere. The specially developed measuring instrumentation is presented which was used during three field campaigns, in 2001, 2004 and 2005. In the first part of this work the climatological conditions at the measuring site are shown. The relevant snow radiative processes are then summarized. A detailed literature review demonstrates the complexity of the involved problem and shows the present state of research. The experiments of 2001, investigating on the penetration and extinction of solar radiation within the snow cover, confirm the selective spectral absorption of snow and the exponential pattern of the decreasing intensity with depth. Results of this study show that a large portion of the absorbed radiation does not penetrate deep into the snow and it is assumed to be removed in the form of latent heat flux. However, processes at the snow-atmosphere interface are still not well understood. The second part of this work presents the new IACETH field Gonio-Spectrometer developed at the Institute for Atmospheric and Climate Science. It measures directional reflected radiation with high spectral and spatial resolution and a hemispherical directional reflectance factor can be calculated. The goniometer is fully automatic and the two robotic arms are controlled with step motors. A spectrometer equipped with a 3◦ foreoptic is used for taking spectrums of the reflected radiation in the range of 350 to