Effective albedo derived from UV measurements in the Swiss Alps

The Alps are one of the regions in Europe showing very high erythemal UV irradiance (UVery) which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Effective albedo plays a key role in understanding the radiative transfer over an Alpine terrain. The difficulty in determining the effective albedo is predominantly due to inhomogeneous surfaces in Alpine regions and a number of interrelated parameters. Most of these parameters can be corrected or normalized by simple methods. The effect of total ozone on different components of the erythemal UV irradiance and the influence of aerosols have been largely investigated and are determined by model calculations. After these atmospheric corrections or normalizations the albedo of the surrounding surfaces within a radius of about 25 km is the only remaining influence to be taken into account on the ratio between direct and diffuse UVery radiation. Satellite data from selected days allowed us to determine the fraction of snow coverage within this radius and to weight the effect of the surrounding surfaces on UVery radiation. With this information from space and data from local albedo measurements in the erythemal UV, it is possible to calculate effective albedo values for these selected days. A correlation between effective albedo and the ratio direct to diffuse was found to be linear for constant solar elevations. Hence this correlation allows us to determine effective albedo at any Alpine location with accurate measurements of direct and diffuse UVery radiation. It is shown from this investigation that the albedo effect can enhance diffuse UVery radiation by up to 57% and global UVery radiation by 30%, especially in spring.