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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 UV spectroradiometer used at the University of Lille is presented, and its characterization and calibration are described. In order to detect possible shifts between calibrations, two different consistency tests are regularly performed on cloudless days. The erythemal dose rate obtained by integrating the weighted spectrum is compared to broadband measurements and modeling results. A Sun photometer provides the aerosol optical depth necessary as an input to the radiative transfer model. Results of these tests are presented for some cases; the general agreement is within 5%, which corresponds to the expected uncertainties.

The ultraviolet spectral irradiance has been measured in the Alps in Briancon (altitude 1300 m asl) in July 1996 and March 1997, with the spectroradiometer of the University of Lille. The erythemal dose rate has been measured simultaneously with a broadband instrument. The aerosol optical depth, necessary as input for modeling, was measured with a Sun photometer. On cloudy days, irradiance is highly variable, with values exceeding those of clear days when the direct solar beam is not obstructed by clouds. For clear days, measured values are compared with the results of a radiative transfer model, and the influence of altitude and ground surface reflectance is analyzed. For the same solar zenith angle, irradiance in UVA is 8–10% higher in Briancon than in Brussels, half of the difference being due to the higher altitude of Briancon. Comparison of winter and summer values shows an amplification of ∼10–15% in winter owing to the snow reflectance; it can be explained by an effective reflectance of 0.3–0.4.

A measurement campaign was carried out in February 1998 at Briançon Station, French Alps (44.9 degrees N, 6.65 degrees E, 1,310 m above sea level) in order to determine the UV effective snow albedo that was retrieved for both erythemal and UV-A irradiances from measurements and modeling enhancement factors. The results are presented for 15 cloudless days with very variable snow cover and a small snowfall in the middle of the campaign. Erythemal irradiance enhancement due to the surface albedo was found to decrease from approximately +15% to +5% with a jump to +22% after the snowfall, whereas UV-A irradiance enhancement decreased from 7% to 5% and increased to 15% after the snowfall. Thesevalues fit to effective surface albedos of 0.4, 0.1, and 0.5 for erythemal, and to effective albedos of 0.25, 0.1, and 0.4 for UV-A irradiances, respectively. An unexpected difference between the effective albedos retrieved in the two wavelength regions can be explained by the difference of the environment contribution.

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.

[1] Spectral UV global irradiance has been measured regularly during the year 2000, in the French Alps, using two UV spectroradiometers. The total ozone column amount is retrieved from the ratio of irradiances around 305 and 340 nm, using look up tables based on modeling data. During cloudless days, ozone values from direct sun observations agree to within 5 DU. These ground based values compare reasonably well (to within about 4%, or 12 DU) with TOMS ozone data; this is consistent with the uncertainty of both methods which is estimated to 5 DU for TOMS, and 5–7 DU for clear sky ground based measurements. Generally the ground based ozone values seem to be slightly higher than the TOMS ozone values. For a few cases, the difference between TOMS and ground based values, can reach 10 to 15%. Different possible explanations of the discrepancies are considered.

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.

A new method for correcting the wavelength misalignment in measured UV spectra is presented. It is based on a comparison between measured irradiances and irradiances computed from a radiative transfer code for a set of given atmospheric and solar conditions (250 < Dobson units < 450, 30 degrees < solar zenith angle < 75 degrees ). Results of tests run with spectra recorded on a clear-sky day by two spectroradiometers in a French UV spectral network station are analyzed. Applying the method once reduces shift to less than 0.05 nm. The smoothing included in the method enables detection of aberrant irradiance values and then completion of an initial quality control of measured spectra. A technique for assessing the instruments' slit function is also presented. The key algorithms needed to build a computer code based on this method are given.