| Summary: | The snow-covered ice sheets of Antarctica and Greenland reflect most of the incoming solar radiation. The reflectivity, commonly called the albedo, of snow on these ice sheets has been observed to vary in space and time. In this thesis, temporal and spatial changes in snow albedo is found to depend mostly on variations in the size of the snow crystals on the surface. Firstly, a radiative transfer model is developed and adapted in order to study the propagation of solar radiation through an atmosphere containing absorbing gases, clouds and aerosols, that is bounded below by a snowpack. The effects of varying solar zenith angle, snow grain size, the presence and thickness of clouds, aerosol and water vapour content, are faithfully reproduced by the model. Applying the radiative transfer model to series of radiation and albedo measurements from different Antarctic sites reveals that temporal and spatial variation of albedo are dominantly caused by changes in snow grain size. This strong link between snow grain size and albedo is further investigated using field data from Summit, Greenland. By analyzing measurements of solar radiation and analysis of snow crystals, it was found that the spectral albedo of a snowpack is determined by snow grain size of the top layer. For near-infrared radiation, the layer that determines albedo has a thickness of only about a millimeter. As absorbed solar radiation is the largest source of energy for heating and melting of the snow on ice sheets, the role of solar radiation in the energy budget of the snowpack was also studied using the field data from Summit. It was found that net solar radiation is by far the largest source of energy for the snowpack. Absorption of radiation is not confined to the surface, but also happens below the surface. This subsurface absorption leads to an additional heating of the uppermost meter of the snowpack by several degrees Celsius.
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