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Content archived on 2024-04-30

Characteristics of the UV radiation field in the Alps


The Alps are one of the areas where the highest UV-B levels are measured in the whole of Europe. This is a consequence of various special characteristics of the UV radiation field in mountainous areas: the atmosphere is mostly free from ind UVB radiation is less in the thinner atmosphere, due to higher elevation; snow surfaces have a high reflection and significantly enhance scattered radiation; lower Iying layers of the atmosphere also contribute to the albedo at mountain sites, as they scatter back part of the incident irradiance. All the above mentioned effects influence the spectrum in quite different ways and results from other studies cannot necessarily be applied to the Alpine region. Although these influences are smaller than the ones of ozone and clouds, they are not at all negligible - they interfere with the influence of the ozone trend on an UV radiation trend. Alpine regions are, in this context, areas that need special attention. Moreover, the knowledge of the UV radiation field there is important for many fields of interest, such as tourism, Alpine agriculture and forestry.
This project addresses these special questions for Alpine areas, where albedo investigations play a key role in the understanding of radiative transfer over Alpine terrain. The project will combine different measurements at various sites in four Alpine countries with radiative transfer models to investigate the W radiation field in the Alps. Investigations will start with the evaluation of spectral global and direct UV-irradiance data, which are already available at nine stations covering a wide range of elevations and different geographic and climatic regions of the Alps. Based on the results of these first studies, the measurement stations will be equipped with ancillary measurements aimed especially at the characterisation of albedo. Broadband meters will be used in addition to the spectroradiometers. Further, recently developed UV-Precision-Filter-Radiometers will assure highest quality, as they will be calibrated by radiometric methods to an accuracy of better than 0,5%. Additional spectral sky radiance measurements will give information about the contribution of reflected radiation from the ground.
A method will be developed to incorporate reflections from complex and mixed surfaces in an area of about 30 km around an observing station in radiative transfer calculations. This leads to an 'effective albedo', which has to be linked to parameters determined from an extensive set of ancillary measurements. A coordinated campaign at a site in a snow-covered Alpine valley will provide the necessary observations to determine the best method for evaluation of the 'effective albedo'. A second approach taking horizontally inhomogeneous environment into account, are three-dimensional models of the terrain and radiative transfer calculations with e.g. Monte Carlo simulations. Furthermore, these model calculations allow to determine possible changes of the 'radius of importance' due to complex terrain.
A large amount of quality-controlled spectral UV measurements collected under a great variety of environmental conditions will be made available to the whole scientific community by means of the SUVDAMA database, which already under development in an ongoing EC-project.
In this study, the Alps are representative for mountainous areas in Europe, and therefore the results are relevant for much larger areas than the Alps as such. The better understanding of the UV-B levels under real atmospheric conditions e. g. in mountainous areas in Europe is a prerequisite for an assessment of the impact of possible changes of the UV flux in relation to stratospheric ozone variations.

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