Effect of inhomogeneous surface albedo on diffuse UV sky radiance at a high-altitude site

Situated at the very edge of the Aletsch Glacier and thus at the boundary between snow-covered and snow-free terrain, the High Alpine Research Station Jungfraujoch (Switzerland, 3576 m above sea level) represents an ideal site for experimental quantification of surface albedo effects on solar UV radiation. Monochromatic measurements of directional radiance distribution in the sky, performed with a 1degrees aperture lens optics at 310 nm and 350 nm under cloudless sky conditions, are compared with discrete ordinate model calculations of corresponding data over homogeneously snow-covered terrain, taking into account polarization effects for an aerosol-free atmosphere. The model agrees within +/-5% with radiance measurements at the forward scattering direction of the principal plane toward southeast, where snow-covered terrain dominates. Toward northwest, the effect of snow-free terrain causes a significant reduction of UV sky radiance at the backward scattering direction of the principal plane relative to the modeled situation with homogeneous snow cover. Depending on the actual solar zenith angle, this reduction amounts to between 20% and 40%.