The daytime cycle in dust aerosol direct radiative effects observed in the central Sahara during the Fennec campaign in June 2011
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Abstract© 2014. American Geophysical Union. All Rights Reserved. The direct clear-sky radiative effect (DRE) of atmospheric mineral dust is diagnosed over the Bordj Badji Mokhtar (BBM) supersite in the central Sahara during the Fennec campaign in June 2011. During this period, thick dust events were observed, with aerosol optical depth values peaking at 3.5. Satellite observations from Meteosat-9 are combined with ground-based radiative flux measurements to obtain estimates of DRE at the surface, top-of-atmosphere (TOA), and within the atmosphere. At TOA, there is a distinct daytime cycle in net DRE. Both shortwave (SW) and longwave (LW) DRE peak around noon and induce a warming of the Earth-atmosphere system. Toward dusk and dawn, the LW DRE reduces while the SW effect can switch sign triggering net radiative cooling. The net TOA DRE mean values range from -9 Wm<sup>-2</sup> in the morning to heating of +59 Wm<sup>-2</sup> near midday. At the surface, the SW dust impact is larger than at TOA: SW scattering and absorption by dust results in a mean surface radiative cooling of 145Wm<sup>-2</sup>. The corresponding mean surface heating caused by increased downward LW emission from the dust layer is a factor of 6 smaller. The dust impact on the magnitude and variability of the atmospheric radiative divergence is dominated by the SW cooling of the surface, modified by the smaller SW and LW effects at TOA. Consequently, dust has a mean daytime net radiative warming effect on the atmosphere of 153Wm<sup>-2</sup>.
CitationBanks JR, Brindley HE, Hobby M, Marsham JH (2014) The daytime cycle in dust aerosol direct radiative effects observed in the central Sahara during the Fennec campaign in June 2011. J Geophys Res Atmos 119: 13,861–13,876. Available: http://dx.doi.org/10.1002/2014jd022077.
SponsorsWe would particularly like to thank Azzedine Saci, Abdelkader Ouladichir, Bouziane Ouchene, and Mohammed Salah-Ferroudj of the Office National de la Meteorologie of Algeria (ONM) for managing and running the Fennec supersite at BBM, and Benyakoub Abderrahmane, Mohammmed Limam, and Diali Sidali (ONM) for their assistance. Further thanks is due to all the members of the Fennec ground-based field campaign team and especially to Martin Todd (University of Sussex) for access to the AERONET data. The Royal Meteorological Institute of Belgium provided the GERB HR data used in the DRE retrievals. The MODIS emissivity data product was obtained through the online Data Pool at the NASA Land Processes Distributed Active Archive Center (LP DAAC). The ERA-Interim meteorological data used in the AOD and DRE<INF>LWTOA</INF> retrievals were produced by ECWMF, and access to the data set was provided by the British Atmospheric Data Centre. The SEVIRI AOD and DRE data used in this study are processed and archived at Imperial College London; contact Jamie Banks (email@example.com) for more details and to request data access. Research reported in this publication has been supported by the King Abdullah University of Science and Technology (KAUST). Finally, we would like to thank the three anonymous reviewers and the editor for their valuable comments which greatly improved this paper.
PublisherAmerican Geophysical Union (AGU)