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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-421
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-421
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Oct 2020

26 Oct 2020

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This preprint is currently under review for the journal AMT.

Relative sky radiance from multi-exposure all-sky camera images

Juan C. Antuña-Sánchez1, Roberto Román1, Victoria E. Cachorro1, Carlos Toledano1, César López2, Ramiro González1, David Mateos1, Abel Calle1, and Ángel M. de Frutos1 Juan C. Antuña-Sánchez et al.
  • 1Group of Atmospheric Optics, Universidad de Valladolid (GOA-UVa), Valladolid, 47011, Spain
  • 2Sieltec Canarias S.L., La Laguna, 38230, Spain

Abstract. All-sky cameras are frequently used to detect cloud cover; however, this work explores the use of these instruments for the more complex purpose of extracting relative sky radiances. An all-sky camera (SONA202-NF model) with three colour filters, narrower than usual for this kind of cameras, is configured to capture raw images at seven exposure times. A detailed camera characterization of the black level, readout noise, hot pixels and linear response is carried out. A methodology is proposed to obtain a linear high dynamic range (HDR) image and its uncertainty, which represents the relative sky radiance map at three effective wavelengths. The relative sky radiance (normalized by the sum of all radiances) is extracted from these maps and compared with the sky radiance measured at different sky points by a sun/sky photometer belonging to the Aerosol Robotic Network (AERONET). The camera radiances are in line with photometer ones excepting for scattering angles below 10º, which is probably due to some light reflections on the fisheye lens and camera dome. Camera and photometer wavelengths are not coincident, hence camera radiances are also compared with sky radiances simulated by a radiative transfer model at the same camera effective wavelengths. This comparison reveals an uncertainty on the normalized camera radiances about 3.3 %, 4.3 % and 5.3 % for 467, 536 and 605 nm, respectively, if specific quality criteria are applied.

Juan C. Antuña-Sánchez et al.

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Juan C. Antuña-Sánchez et al.

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Short summary
This paper presents a new technique to exploit the potential of all sky cameras. The sky radiance at three effective wavelengths are calculated and compared with alternative measurements and simulated data. The proposed method will be useful for the retrieval of aerosols and clouds properties.
This paper presents a new technique to exploit the potential of all sky cameras. The sky...
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