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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Jan 2019

Research article | 21 Jan 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Aerosol influences on low-level clouds in the West African monsoon

Jonathan W. Taylor1, Sophie L. Haslett1,a, Keith Bower1, Michael Flynn1, Ian Crawford1, James Dorsey1,2, Tom Choularton1, Paul J. Connolly1, Valerian Hahn3, Christiane Voigt3,4, Daniel Sauer3, Régis Dupuy5, Joel Brito5,b, Alfons Schwarzenboeck5, Thierry Bourriane6, Cyrielle Denjean6, Phil Rosenberg7, Cyrille Flamant8, James D. Lee9,10, Adam R. Vaughan9, Peter G. Hill11, Barbara Brooks12, Valéry Catoire13, Peter Knippertz14, and Hugh Coe1 Jonathan W. Taylor et al.
  • 1Centre for Atmospheric Science, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
  • 2National Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 3Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
  • 4Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität Mainz, Germany
  • 5Laboratoire de Météorologie Physique, Université Clermont Auvergne, Clermont-Ferrand, France
  • 6CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 7University of Leeds, Leeds, UK
  • 8LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France
  • 9Wolfson Atmospheric Chemistry Laboratories, University of York, York, UK
  • 10National Centre for Atmospheric Science, University of York, York, UK
  • 11Department of Meteorology, University of Reading, Reading, UK
  • 12National Centre for Atmospheric Science, Leeds, UK
  • 13Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), Université Orléans-CNRS-CNES, Orléans Cedex 2, France
  • 14Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • anow at: Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockhkolm 11418, Sweden
  • bnow at: IMT Lille Douai, Univ. Lille, SAGE, 59000 Lille France

Abstract. Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of regional, in situ measurements of cloud microphysics, taken during June – July 2016, as part of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign, assessing spatial and temporal variation in the properties of these clouds.

LLC developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Additional sea breeze clouds developed near the coast in the late morning, reaching a maximum extent around 12:00 LST. Regional variation in LLC cover was largely determined by the modulation of the cool maritime inflow by the local orography, with peaks on the upwind side of hills and minima on the leeward sides. In the broad-scale cloud field, no lasting impacts related to anthropogenic aerosol were observed downwind of major population centres.

The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities, but were not statistically significant across the region. The majority of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the southern hemisphere. Consequently, all clouds measured (inland and offshore) had significantly higher CDNC and lower effective radius than clouds over the remote south Atlantic from literature.

A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated CDNC by 13–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the southern hemisphere is likely to dampen any effect of these increased local emissions on cloud-aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African monsoon.

Jonathan W. Taylor et al.
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Jonathan W. Taylor et al.
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Publications Copernicus
Short summary
Low-level clouds cover a wide area of southern West Africa (SWA), and play an important role in the region's climate, reflecting sunlight away from the surface. We performed aircraft measurements of aerosols and clouds over SWA during the 2016 summer monsoon, and found pollution, and polluted clouds, across the whole region. Smoke from biomass burning in Central Africa is transported to West Africa, causing a polluted background which limits the effect of local pollution on cloud properties.
Low-level clouds cover a wide area of southern West Africa (SWA), and play an important role in...