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We compare three different methods for determining an average ‘effective‧ UV albedo. These methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. The first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow-covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present the differences and the correlations between the various methods as well as the results for the different locations.

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch-Partenkirchen, Germany, during spring 1999. The measured clear-sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow-covered stations and the snow-free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single-scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.

The determination of snow grains characteristics (such as type or size) are crucial to validate snow models and interpret data from remote sensing. Until now, the size and type of snow grains are estimated in the fields. Nowadays, the transport of snow sample in iso-octane allows laboratory analyses. An objective analysis of snow grains digital images has been developed at the Centre d'Etudes de la Neige since the mid 80's. The images are taken in a cold room and stored on a videodisk. The calculation of relevant parameters for snow characterisation was developed. Recently, the hardware and software systems were upgraded, allowing quicker and easier analyses. Finally, a procedure designed to determine the snow types from images analysis was developed and calibrated with 65 snow samples. For that purposes, independent experts determined the snow type (as defined by the International Classification) of each sample. The procedure was verified on an independent set of snow images. This semi-automated determination gives 97% of accurate results on granular snow types. However, it is not reliable on precipitation particles.

Le Centre d'Etudes de la Neige a developpe sur SOLAR 16.65 (SEMS) une application en temps reel nommee AIPRA (Assistance Informatise par la Prevision du Risque d'Avalanche). Elle fournit d'une maniere interactive et en temps reel, des donnees necessaires a sa prevision sous forme de tableaux, graphiques, cartes elaborees, etc...