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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.

The Alps are one of the regions in Europe showing very high erythemal UV irradiance (UVery) which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Effective albedo plays a key role in understanding the radiative transfer over an Alpine terrain. The difficulty in determining the effective albedo is predominantly due to inhomogeneous surfaces in Alpine regions and a number of interrelated parameters. Most of these parameters can be corrected or normalized by simple methods. The effect of total ozone on different components of the erythemal UV irradiance and the influence of aerosols have been largely investigated and are determined by model calculations. After these atmospheric corrections or normalizations the albedo of the surrounding surfaces within a radius of about 25 km is the only remaining influence to be taken into account on the ratio between direct and diffuse UVery radiation. Satellite data from selected days allowed us to determine the fraction of snow coverage within this radius and to weight the effect of the surrounding surfaces on UVery radiation. With this information from space and data from local albedo measurements in the erythemal UV, it is possible to calculate effective albedo values for these selected days. A correlation between effective albedo and the ratio direct to diffuse was found to be linear for constant solar elevations. Hence this correlation allows us to determine effective albedo at any Alpine location with accurate measurements of direct and diffuse UVery radiation. It is shown from this investigation that the albedo effect can enhance diffuse UVery radiation by up to 57% and global UVery radiation by 30%, especially in spring.

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.

Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One 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 sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Alps are one of the regions where the highest UV levels are measured in Europe, which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Solar UV radiation increases with altitude mainly due to decreasing amounts of air molecules, ozone, aerosols and clouds in the atmosphere. The altitude effect plays a key role in the understanding of the UV radiation field in mountainous terrain. Therefore simultaneous measurements of erythemal UV radiation (broadband) on three different heights were performed in the Swiss Alps during more than four years. Under clear-sky conditions, the altitude effect of daily noon-time yearly mean values of direct, diffuse and global erythemal UV radiation results in 17.4%/1000 m (direct), 8.5%/1000 m (diffuse) and 10.7%/1000 m (global). Seasonal variations of the altitude effect are mainly influenced by changes of solar elevation, albedo values and turbidity levels during the year. In addition, measured altitude effects are shown and compared to calculated altitude effects obtained by application of the MODTRAN radiative transmission model.

The Alps are one of the regions where the highest UV levels are measured in Europe. Solar UV radiation increases with altitude mainly due to decreasing amounts of air molecules, ozone, aerosols, and clouds in the atmosphere as well as due to snow covered surfaces. The altitude effect plays an important role in the understanding of the UV radiation field in mountainous terrain. Therefore simultaneous measurements of erythemal UV radiation (broadband) on three different heights were performed in the Alps over more than four years. Under clear-sky conditions, the altitude effect of daily noon-time yearly mean values of direct, diffuse, and global erythemal UV radiation results in 17.4%/1000 m (direct), 8.5%/1000 m (diffuse), and 10.7%/1000 m (global). Seasonal variations of the altitude effect are mainly influenced by changes of solar elevation, albedo values, and turbidity levels during the year. Measured altitude effects are also compared to calculated values obtained by the MODTRAN radiative transmission model.

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.

UV radiation measured on normal-to-the-sun-oriented surfaces can show significantly higher global UV irradiance values compared to measurements on horizontal receivers. The direct component is amplified by the inverse cosine of the zenith angle, but over surfaces with high local albedo this accounts for only about half of the signal rise of global irradiance. The signal rise of the diffuse component, however, is strongly related to local albedo and solar elevation, which is demonstrated by 2 years of measurements of direct, diffuse, global, reflected and global normal-to-the-sun erythemal effective UV irradiance (UVery). Global UVery signal rises, on normal-to-the-sun-oriented versus horizontal receivers, of up to 65% were measured on fresh snow and solar elevation angles below 30 degrees. An empirical expression has been deduced from the measurements relating the ratio of normal-to-the-sun versus horizontal measurements of global UVery to surface albedo and solar elevation. This allows one to calculate the maximum global UVery irradiance levels which are to be expected on normal-to-the-sun-oriented surfaces with respect to horizontal measurements or model calculations.