Julian Gröbner

Variability of UV Irradiance in Europe

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used one‐dimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m−2 at 70°N and 5.2 kJ m−2 at 35°N, whereas these values are reduced to 1.5–4.5 kJ m−2 if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

Characterization and calibration of ultraviolet broadband radiometers measuring erythemally weighted irradiance

An ultraviolet calibration center has been established in Davos, Switzerland. It provides a laboratory for characterizing the spectral and angular response of broadband radiometers. The absolute calibration of these instruments is performed through the comparison to the reference spectroradiometer QASUME. We present what we believe to be a novel calibration methodology that explicitly includes the information of the angular and spectral response functions. From the results of the latest broadband intercomparison campaign, the typical uncertainties of these instruments could be obtained. Most radiometers have an expanded uncertainty of approximately 7%. The angular response introduces an uncertainty of 0.9%-7.2%, depending on the cosine error of the radiometer.

Measuring spectral and spatial variations of UVA and UVB sky radiance

Spectral sky radiance measurements have been obtained with a double monochromator using a 1.5° field of view under varying atmospheric conditions at selected sites in Europe. Spatial variations of the intensity of the sky radiance up to a factor of 10 are observed in the UVA, which decrease to a factor of 2 in the UVB. A distinct minimum of the sky radiance is observed, the location depends on the solar zenith angle but not on wavelength. It is located at scattering angles between 90° and 75° for solar zenith angles between 80° and 26°. A comparison between measurements of the sky radiance and radiative transfer calculations with an aerosol free atmosphere shows a strong dependence of UVB sky radiance on aerosol particles in contrast to UVA sky radiance and possibly a dependence on tropospheric ozone as well.

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.

Quality assurance of spectral solar UV measurements: results from 25?UV monitoring sites in Europe, 2002 to 2004

With the transportable reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) routine quality assurance of spectrally resolved solar ultraviolet irradiance measurements were successfully performed at 25 UV monitoring sites in Europe. The absolute scale carried by the QASUME reference spectroradiometer is traceable to the primary irradiance standard of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, and has proved to represent the average scale in use at 25 independent European laboratories; it can thus be taken as a European irradiance reference. Out of the 27 instruments 13 showed deviations relative to the QASUME reference spectroradiometer of less than 4% in the UVB (15 instruments in the UVA) for solar zenith angles below 75 ◦ . The results so far have shown the unique possibilities offered by this transportable reference spectroradiometer for providing on-site quality assurance of solar ultraviolet irradiance measurements. (Some figures in this article are in colour only in the electronic version)

Experimental investigation of spectral global irradiance measurement errors due to a non ideal cosine response

Due to limitations in the diffusers used for global irradiance measurements, the method usually applied to correct global irradiance measurements is investigated. Based on sky radiance measurements, the assumption of a homogeneous sky radiance distribution generally used to calculate the diffuse part of the global cosine error is accurate to within ±1.5% in the UVB for varying atmospheric and geographic conditions. At longer wavelengths, the assumption of a homogeneous sky radiance distribution should not be used for global cosine corrections since it underestimates the true diffuse cosine error by up to 10% at 500 nm. The accuracy of diffuse irradiance measurements using shading disks is investigated and its dependence on different aerosol amounts is shown. The cosine correction of global irradiance measurements is shown to be insensitive to errors in the determination of the fraction of direct to global irradiance. The uncertainty in the cosine correction of global irradiance measurements is therefore less than ±2% in the UVB.

Variability of spectral solar ultraviolet irradiance in an Alpine environment

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.

Intercomparison of aerosol optical depth measurements in the UVB using Brewer spectrophotometers and a Li-Cor spectrophotometer

The first Iberian UV radiation intercomparison was held at “El Arenosillo”-Huelva station of the Instituto Nacional de Tecnica Aeroespatial (INTA) from September 1 to 10, 1999. During this campaign, seven Brewer spectrophotometers and one Li-Cor spectrophotometer measured the total column aerosol optical depth (AOD) at 306, 310, 313.5, 316.75 and 320 nm. The AOD calibration of one Brewer was transferred to all other Brewers using one day of intensive measurements. The remaining days were used to observe the stability and reproducibility of the AOD measurements by the different instruments. All Brewer spectrophotometers agreed to within an AOD of 0.03 during the whole measurement campaign. The differences in AOD between the Li-Cor spectrophotometer and the Brewer spectrophotometers were between −0.07 and +0.02 at 313.5, 316.75, and 320 nm. This investigation demonstrates the possibility of using the existing worldwide Brewer network as a global UV aerosol network for AOD monitoring.

Modeling the effect of an inhomogeneous surface albedo on incident UV radiation in mountainous terrain: Determination of an effective surface albedo

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.

Cloud observations in Switzerland using hemispherical sky cameras

We present observations of total cloud cover and cloud type classification results from a sky camera network comprising four stations in Switzerland. In a comprehensive intercomparison study, records of total cloud cover from the sky camera, long-wave radiation observations, Meteosat, ceilometer, and visual observations were compared. Total cloud cover from the sky camera was in 65–85% of cases within ±1 okta with respect to the other methods. The sky camera overestimates cloudiness with respect to the other automatic techniques on average by up to 1.1 ± 2.8 oktas but underestimates it by 0.8 ± 1.9 oktas compared to the human observer. However, the bias depends on the cloudiness and therefore needs to be considered when records from various observational techniques are being homogenized. Cloud type classification was conducted using the k-Nearest Neighbor classifier in combination with a set of color and textural features. In addition, a radiative feature was introduced which improved the discrimination by up to 10%. The performance of the algorithm mainly depends on the atmospheric conditions, site-specific characteristics, the randomness of the selected images, and possible visual misclassifications: The mean success rate was 80–90% when the image only contained a single cloud class but dropped to 50–70% if the test images were completely randomly selected and multiple cloud classes occurred in the images.