Effective albedo derived from UV measurements in the Swiss Alps

The Alps are one of the regions in Europe showing very high erythemal UV ir radiance (UVery) which is a consequence of low aerosol levels, high altitud e and snow-covered surfaces. Effective albedo plays a key role in understan ding the radiative transfer over an Alpine terrain. The difficulty in deter mining the effective albedo is predominantly due to inhomogeneous surfaces in Alpine regions and a number of interrelated parameters. Most of these pa rameters can be corrected or normalized by simple methods. The effect of to tal ozone on different components of the erythemal UV irradiance and the in fluence of aerosols have been largely investigated and are determined by mo del calculations. After these atmospheric corrections or normalizations the albedo of the surrounding surfaces within a radius of about 25 km is the o nly remaining influence to be taken into account on the ratio between direc t and diffuse UVery radiation. Satellite data from selected days allowed us to determine the fraction of snow coverage within this radius and to weigh t the effect of the surrounding surfaces on UVery radiation. With this info rmation 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 diffu se was found to be linear for constant solar elevations. Hence this correla tion allows us to determine effective albedo at any Alpine location with ac curate measurements of direct and diffuse UVery radiation. It is shown from this investigation that the albedo effect can enhance diffuse UVery radiat ion by up to 57% and global UVery radiation by 30%, especially in spring.