Axel Kreuter

All-sky imaging: a simple, versatile system for atmospheric research

A simple and inexpensive fully automated all-sky imaging system based on a commercial digital camera with a fish-eye lens and a rotating polarizer is presented. The system is characterized and two examples of applications in atmospheric physics are given: polarization maps and cloud detection. All-sky polarization maps are obtained by acquiring images at different polarizer angles and computing Stokes vectors. The polarization in the principal plane, a vertical cut through the sky containing the Sun, is compared to measurements of a well-characterized spectroradiometer with polarized radiance optics to validate the method. The images are further used for automated cloud detection using a simple color-ratio algorithm. The resulting cloud cover is validated against synoptic cloud observations. A Sun coverage parameter is introduced that shows, in combination with the total cloud cover, useful correlation with UV irradiance.

Stray light correction for solar measurements using array spectrometers

A stray light matrix correction method for array spectrometers is presented that is specifically tailored for solar spectral measurements. A stray light distribution function based on a single laser line measurement is approximated by an analytical function using three parameters only. This function is the basis for the stray light correction matrix. One cutoff filter is then used to adjust an offset parameter such that stray light corrected data are spectrally flat and around zero below the cutoff wavelength. This parameter also accounts for the IR contribution and has to be adjusted individually for each type of spectrum. A solid validation is given by intercomparison of calibrated solar spectra with a double-monochromator spectroradiometer. An agreement of 5% for wavelengths down to 307 nm and solar zenith angle smaller than 70 degrees is achieved.

Comparison of surface UV irradiance in mountainous regions derived from satellite observations and model calculations with ground-based measurements

Several UV data products derived from satellite measurements, 1-D and 3-D radiative transfer modeling are compared with high-quality ground-based measurements. Data products include the UV index, erythemally weighted daily dose and spectrally resolved UV irradiances at 305, 310, 324 and 380 nm. The study focuses on the UV radiation climate in mountainous terrain under cloud-free conditions. The results show, that overall the 3-D- and the 1-D-model agree best with the measurements (average ratio 1.10 and 1.13, range 0.88-1.6). It is also found that snow and local topography have a rather minor impact on ground UV-irradiance, while altitude plays a significant role >5 %). Satellite-retrieved values significantly underestimate irradiance for most of our stations due to erroneous cloud correction (average ratio 0.89, range 0.6-1.35). However, if one compares the uncorrected (cloud-free) satellite-retrieved values to the measurements, the ratios are only slightly larger (average ratio 1.14, range 0.8 - 1.6) than for the 1-D- and 3-D-model. The main deficiencies arise in determining the correct surface height and albedo within the satellite-retrieval algorithm.

Aerosol characteristics at the Alpine site of Innsbruck, Austria

Aerosol optical depth (AOD) is being measured in Innsbruck, Austria. Since 2007 a precision filter radiometer (PFR) is set up at 620 m above sea level in order to characterize the aerosol conditions at this Alpine site. The five year time series is analyzed with respect to AOD, the Angstrom coefficient a and the curvature of the 1n(τ)-1n(λ) relationship γ. Information on the fine mode particle fraction and its modal radius is obtained with a graphical method. For special cases such as Saharan dust events the King inversion algorithm is applied. The PFR measures direct irradiance at 368 nm, 412 nm, 500 nm and 862 nm. It is also employed as a network instrument in the Global Atmosphere Watch (GAW) Program of the World Meteorological Organization. The Innsbruck annual aerosol pattern reveals monthly mean AOD at 500 nm in winter between 0.05 and 0.1 and up to 0.22 in summer. Daily AOD at 500 nm remains below 0.5 for all measurement days. This seasonality in AOD is typical for mid-latitude sites. The overall m...

Effects of Albedo on Solar Irradiance

The effects of ground albedo on solar radiation were investigated in a field campaign around Ny Alesund on Svalbard, Norway. In spring time this site exhibits high albedo gradients at the interface between sea water and snow covered land. Array spectroradiometers measuring global irradiance in the UV and visible wavelength range were installed at three field sites with increasing distance from the coastline towards the snow covered glaciers with a horizontal distance of about 20 km. For 3 weeks, quasi synchronous spectra were collected under clear sky and overcast sky conditions. At 320 nm, an enhancement of up to 15% of the global irradiance for clear sky was observed at the higher albedo site relative to the coastal site. Under overcast conditions this enhancement of irradiance was as high as 30%. The measurements agree well with a 1D radiative transfer model, considering an effective average albedo. Diurnal asymmetries of the irradiance have been observed and require a full 3D model treatment to account for the highly inhomogeneous albedo environment and the non-Lambertian reflectance of water (sun glint).