Universidad de Granada Digibug
 

Repositorio Institucional de la Universidad de Granada >
1.-Investigación >
Departamentos, Grupos de Investigación e Institutos >
Departamento de Física Aplicada >
DFA - Artículos >

Please use this identifier to cite or link to this item: http://hdl.handle.net/10481/32283

Title: Calibration of an all-sky camera for obtaining sky radiance at three wavelengths
Authors: Román, Roberto
Antón, Manuel
Cazorla Cabrera, Alberto
Miguel, A. de
Olmo Reyes, Francisco José
Bilbao, J.
Alados-Arboledas, Lucas
Issue Date: 2012
Abstract: 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.
Sponsorship: The 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).
Publisher: Copernicus Publications; European Geosciences Union (EGU)
Keywords: Aerosol optical properties
Photosynthetically active radiation
Solar irradiance
Technical note
Ultraviolet
Spain
Luminance
URI: http://hdl.handle.net/10481/32283
ISSN: 1867-1381
1867-8548
Rights : Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License
Citation: 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]
Appears in Collections:DFA - Artículos
OpenAIRE (Open Access Infrastructure for Research in Europe)
IISTA - Artículos
RNM119 - Artículos

Files in This Item:

File Description SizeFormat
Roman_Calibration.pdf511.25 kBAdobe PDFView/Open
Recommend this item

This item is licensed under a Creative Commons License
Creative Commons

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

 

Valid XHTML 1.0! OpenAire compliant DSpace Software Copyright © 2002-2007 MIT and Hewlett-Packard - Feedback

© Universidad de Granada