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Please use this identifier to cite or link to this item: http://hdl.handle.net/10481/48588

Title: Cloud cover detection combining high dymanics range sky images and ceilometer measurements
Authors: Román, Roberto
Cazorla Cabrera, Alberto
Toledano, C.
Olmo Reyes, Francisco José
Cachorro, V. E.
Frutos, A.
Alados-Arboledas, Lucas
Issue Date: Nov-2017
Abstract: This paper presents a new algorithm for cloud detection based on high dynamic range images from a sky camera and ceilometer measurements. The algorithm is also able to detect the obstruction of the sun. This algorithm, called CPC (Camera Plus Ceilometer), is based on the assumption that under cloud-free conditions the sky field must show symmetry. The symmetry criteria are applied depending on ceilometer measurements of the cloud base height. CPC algorithm is applied in two Spanish locations (Granada and Valladolid). The performance of CPC retrieving the sun conditions (obstructed or unobstructed) is analyzed in detail using as reference pyranometer measurements at Granada. CPC retrievals are in agreement with those derived from the reference pyranometer in 85% of the cases (it seems that this agreement does not depend on aerosol size or optical depth). The agreement percentage goes down to only 48% when another algorithm, based on Red-Blue Ratio (RBR), is applied to the sky camera images. The retrieved cloud cover at Granada and Valladolid is compared with that registered by trained meteorological observers. CPC cloud cover is in agreement with the reference showing a slight overestimation and a mean absolute error around 1 okta. A major advantage of the CPC algorithm with respect to the RBR method is that the determined cloud cover is independent of aerosol properties. The RBR algorithm overestimates cloud cover for coarse aerosols and high loads. Cloud cover obtained only from ceilometer shows similar results than CPC algorithm; but the horizontal distribution cannot be obtained. In addition, it has been observed that under quick and strong changes on cloud cover ceilometers retrieve a cloud cover fitting worse with the real cloud cover.
Sponsorship: This work was supported by the Andalusia Regional Government (project P12-RNM-2409) and by the Consejería de Educación, Junta de Castilla y León (project VA100U14).
Spanish Ministry of Economy and Competitiveness (CGL2013-45410-R, CMT2015-66742-R, CGL2016-81092-R, and FJCI-2014-22052).
FEDER funds under the projects CGL2013-45410-R, CMT2015-66742-R, CGL2016-81092-R.
“Juan de la Cierva-Formación” (FJCI-2014-22052) program.
European Union H2020-INFRAIA-2014-2015 project ACTRIS-2 (grant agreement No. 654109)
Publisher: Elsevier
Keywords: Sky camera
Cloud cover
HDR (High dynamic range)
URI: http://hdl.handle.net/10481/48588
ISSN: 0169-8095
Rights : Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License
Citation: Román, R.; et al. Cloud cover detection combining high dynamic range sky images and ceilometer measurements. Atmospheric Research, 196(1): 224-236 (2017). [http://hdl.handle.net/10481/48588]
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