Calibration of an all-sky camera for obtaining sky radiance at three wavelengths
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AutorRomán, Roberto; Antón, Manuel; Cazorla Cabrera, Alberto; Miguel, A. de; Olmo Reyes, Francisco José; Bilbao, J.; Alados-Arboledas, Lucas
Copernicus Publications; European Geosciences Union (EGU)
Aerosol optical propertiesPhotosynthetically active radiationSolar irradianceTechnical noteUltravioletSpainLuminance
Román, R.; et al. Calibration of an all-sky camera for obtaining sky radiance at three wavelengths. Atmospheric Measurement Techniques, 5: 2013-2024 (2012). [http://hdl.handle.net/10481/32283]
PatrocinadorThe authors gratefully acknowledge the financial support extended by the Spanish Innovation and Science Ministry for the projects: CGL2011-25363 and CGL2010-12140E. Roberto Román thanks Valladolid University for the support to research short stays and for the PIF-UVa grants for PhD students. This work was also partially supported by the Andalusian Regional Government through projects P08-RNM-3568 and P10-RNM-6299, the Spanish Ministry of Science and Technology through projects CGL2010-18782 and CSD2007-00067, and by the European Union through ACTRIS project (EU INFRA- 2010-1.1.16-262254).
This paper proposes a method to obtain spectral sky radiances, at three wavelengths (464, 534 and 626 nm), from hemispherical sky images. Images are registered with the All-Sky Imager installed at the Andalusian Center for Environmental Research (CEAMA) in Granada (Spain). The methodology followed in this work for the absolute calibration in radiance of this instrument is based on the comparison of its output measurements with modelled sky radiances derived from the LibRadtran/UVSPEC radiative transfer code under cloud-free conditions. Previously, in order to check the goodness of the simulated radiances, these are compared with experimental values recorded by a CIMEL sunphotometer. In general, modelled radiances are in agreement with experimental data, showing mean differences lower than 20% except for the pixels located next to the Sun position that show larger errors. The relationship between the output signal of the All-Sky Imager and the modelled sky radiances provides a calibration matrix for each image. The variability of the matrix coefficients is analyzed, showing no significant changes along a period of 5 months. Therefore, a unique calibration matrix per channel is obtained for all selected images (a total of 705 images per channel). Camera radiances are compared with CIMEL radiances, finding mean absolute differences between 2% and 15% except for pixels near to the Sun and high scattering angles. We apply these calibration matrices to three images in order to study the sky radiance distributions for three different sky conditions: cloudless, overcast and partially cloudy. Horizon brightening under cloudless conditions has been observed together with the enhancement effect of individual clouds on sky radiance.