Impact of snow cover and topography on ultraviolet irradiance at the Alpine station of Briançon

Abstract

The enhancement of the UV global irradiance due to snow cover on the ground has been observed at the station of Briançon, in a high‐altitude Alpine valley. The analysis relies on a three‐dimensional (3‐D) model, using an elevation map of the area. Without snow, comparison with the results of a 1‐D model shows no detectable effect of topography, within the uncertainty of modeling (2–3%). The 3‐D model relates the enhancement due to snow to the altitude of the snow line. The enhancement is shown to depend on the snow distribution around the site and not on the topography itself. The enhancement was measured at Briançon for nine cloudless days in winter 2002. As expected, it increases with the decrease of the snow line. In erythemal UV the enhancement reaches a maximum of about 22% in the beginning of March, in agreement with the results of the 3‐D model, assuming a snow albedo of 0.3 above the snow line and below the tree line and 0.8 above the tree line. Retrieving an effective surface albedo is a very challenging problem. Very small uncertainties in enhancement (±2%) lead to large uncertainties (±0.05) in effective albedo. Using the snow distribution with a contribution function does not give good results when the snow line is high; this is explained by the low resolution of the map with the rapid variation of the contribution near the site.