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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 9 | Copyright
Atmos. Chem. Phys., 17, 5789-5807, 2017
https://doi.org/10.5194/acp-17-5789-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 11 May 2017

Research article | 11 May 2017

Clouds over the summertime Sahara: an evaluation of Met Office retrievals from Meteosat Second Generation using airborne remote sensing

John C. Kealy1,a, Franco Marenco1, John H. Marsham2,3, Luis Garcia-Carreras2, Pete N. Francis1, Michael C. Cooke1, and James Hocking1 John C. Kealy et al.
  • 1Met Office, Exeter, UK
  • 2School of Earth and Environment, University of Leeds, Leeds, UK
  • 3National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • anow at: College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK

Abstract. Novel methods of cloud detection are applied to airborne remote sensing observations from the unique Fennec aircraft dataset, to evaluate the Met Office-derived products on cloud properties over the Sahara based on the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on-board the Meteosat Second Generation (MSG) satellite. Two cloud mask configurations are considered, as well as the retrievals of cloud-top height (CTH), and these products are compared to airborne cloud remote sensing products acquired during the Fennec campaign in June 2011 and June 2012. Most detected clouds (67% of the total) have a horizontal extent that is smaller than a SEVIRI pixel (3km × 3km). We show that, when partially cloud-contaminated pixels are included, a match between the SEVIRI and aircraft datasets is found in 80±8% of the pixels. Moreover, under clear skies the datasets are shown to agree for more than 90% of the pixels. The mean cloud field, derived from the satellite cloud mask acquired during the Fennec flights, shows that areas of high surface albedo and orography are preferred sites for Saharan cloud cover, consistent with published theories. Cloud-top height retrievals however show large discrepancies over the region, which are ascribed to limiting factors such as the cloud horizontal extent, the derived effective cloud amount, and the absorption by mineral dust. The results of the CTH analysis presented here may also have further-reaching implications for the techniques employed by other satellite applications facilities across the world.

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Using novel methods of cloud detection from aircraft data over the Sahara desert, we evaluate the performance of the Meteosat satellite in measuring cloud properties: namely, the cloud mask and the cloud-top height. We find that the cloud mask can justifiably be used for many applications (such as creating a detailed Saharan cloud climatology), and we also discuss its limitations. As for the cloud-top height, we show that the dataset cannot yet be considered robust in this part of the world.
Using novel methods of cloud detection from aircraft data over the Sahara desert, we evaluate...
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