Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observat ory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albe do of the region was calculated at 305 and 380 nm for different solar zenit h angles, ground reflectances, and solar azimuth angles. To determine geome trical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corre sponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to di minish the reflected components. This diminishing effect was compensated fo r, however, by the scattered irradiance. Finally, simulations of real condi tions (nonhomogeneous ground reflectivities) were performed for different s now lines in the Sonnblick region. The average albedos obtained when all th e surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle. (C) 2000 Optical Society of America OCIS codes: 010.0010, 010.3920, 260.7190, 260.0260.