ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-6979-2008 Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara Marsham J. H. 1 Parker D. J. 1 Grams C. M. 2 Johnson B. T. 4 Grey W. M. F. 3 Ross A. N. 1 University of Leeds, UK Universität Karlsruhe and Forschungszentrum Karlsruhe, Germany Climate and Land Surface Systems Interaction Centre, School of the Environment and Society, Swansea, SA2 8PP, UK The Met Office, UK 04 12 2008 8 23 6979 6993 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/6979/2008/acp-8-6979-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/6979/2008/acp-8-6979-2008.pdf

Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations) induced mesoscale circulations. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Residual Layer (SRL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary layer. <br><br> Using the aircraft observations and data from the COSMO model, a region of local dust uplift, with strong along-track winds, was identified in one low-level flight. Large eddy model (LEM) simulations based on this location showed linearly organised boundary-layer convection. Calculating dust uplift rates from the LEM wind field showed that the boundary-layer convection increased uplift by approximately 30%, compared with the uplift rate calculated neglecting the convection. The modelled effects of boundary-layer convection on uplift are shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur. <br><br> Both the coupling of albedo features to the atmosphere on the mesoscale, and the enhancement of dust uplift by boundary-layer convection are unrepresented in many climate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parametrise.

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