Characteristics of the UV radiation field in the Alps

Informazioni relative al progetto

ID dell’accordo di sovvenzione: ENV4970575

Progetto chiuso

Data di avvio 1 Febbraio 1998

Data di completamento 31 Gennaio 2000

Finanziato da

• Specific programme of research and technological development in the field of environment and climate, 1994-1998

Costo totale € 0,00

Contributo UE Nessun dato

Coordinato da

LEOPOLD-FRANZENS-UNIVERSITAET INNSBRUCK
Austria

Exploitable results

Improvements of measurement techniques

Newly developed narrow band precision filter radiometers with filters at 305 nm, 311 nm, 318 rim and 332 nm for measurements of direct solar irradiance were tested during CUVRA against spectroradiometers and general agreement within 50 % was a first good result, taking into account the completely different and independent methods of absolute calibration and the great dynamic range of compared data. At two mountain stations, a setup with a CCD-camera and an UVA-detector was used for measurements of directional ground reflectivity, from which the effective albedo could be derived. Within CUVRA the available spectroradiometric measurement techniques of the partners were further improved, two partners established newly bought double-monochromator spectroradiometers. Routines for better interpretation of the spectral data and better quality control were implemented, like better correction of wavelength misalignments or better monitoring of short term variations of solar irradiance during a scan. This supports also for the future the continuation of highest competence of European scientist in this field of research.

Spectral UV measurements

Spectral and broadband measurements of solar UV irradiance at 8 stations in the Alps at elevations between 600 m and 3600 m are available from the partners of CUVRA. Furthermore, a co-ordinated campaign with 7 spectroradiometers and a great number of ancillary instruments was carried out in the surrounding of Garmisch-Partenkirchen in spring 1999. Within a distance of 70 km the instruments were located at altitudes between 600 m and 3000 m.. In this campaign, the great natural variability of measured UV irradiance was additionally caused by ozone variations between 300 DU and 450 DU, by variations of aerosol optical depth by a factor of about 1 0 between 0. 1 and 1 (valid for 320 rim) and by rising of the snow line from 700 m to 1200 m during the campaign. These data will be submitted to the European SUVDAMA database, which was built up in the last years in the framework of the ECproject SUVDAMA, co-ordinated by IFU, who is also a partner in the CUVRA project. This process of data submission is partly finished and will be continued in the next months, in close co-operation with the managers of the database in Finland.

Interpretation using radiative transfer modeling

From detailed analyses of measurements with one dimensional radiative transfer models an 'effective albedo' was derived, characterizing the Overall effect of snow-covered terrain on UV irradiance. Depending 'on the site and on the local conditions of topography and ground vegetation, the effective albedo for snow-covered terrain varied between -about 0.3 and 0.8, whereas for snow-free terrain values below 0.05 are valid. The value of the effective albedo could be related to the height of the snow line, the snow covered surface as derived from satellite pictures and the local variability of snowfall. The strongest enhancement of UV irradiance due to changing round albedo was found to be around 320 nffl, showing an increase of about 20% for an increase of effective by 0.5, Using three-dimensional models allowed to get estimates of the 'radius of importance', which extends significantly up to 30 km. Following the path of reflected photons with a Monte-Carlo code shows that - with some variation depending on the specific distribution of ground reflectance and atmospheric characteristics - about 40% of the reflected photons come from an area within 5 km radius.