Determination of an effective spectral surface albedo from ground-based global and direct UV irradiance measurements

Ultraviolet radiation may be enhanced by the presence of snow cover. For areas with snow the surface is often highly inhomogeneous owing to the presence of low-albedo surfaces together with snow-covered surfaces. In order to better understand the coupling between snow cover, surface albedo, and UV radiation, an estimate of an effective albedo for such highly inhomogeneous surfaces is required. In this paper a method to derive an effective wavelength dependent Lambertian surface albedo in the UV and visible part of the spectrum is described. The retrieval method is used to analyze direct and global irradiance measurements performed in the wavelength region 320-500 nm during the winter-spring-summer seasons of 1997 in Tromso, Norway. The derived albedo reached a maximum of 0.57 and 0.78 at 320 and 450 nm, respectively, when the snow depth was at maximum. The corresponding values for completely snow-free conditions were 0.08 and 0.16. The absolute uncertainty in these values is estimated to be 0.1 at the shortest wavelengths and 0.2 at the higher wavelengths. Calculations with a three-dimensional Monte Carlo model indicate that the open waters surrounding Tromso all the year round decrease the effective albedo by approximately 0.2 compared to a completely snow-covered surface.