Atmos. Chem. Phys. Discuss., 8, 8817-8846, 2008
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Observations of mesoscale and boundary-layer circulations affecting dust uplift and transport in the Saharan boundary layer
J. H. Marsham1, D. J. Parker1, C. M. Grams2, W. M. F. Grey3, and B. T. Johnson4
1School of Earth and Enviroment, University of Leeds, Leeds, UK
2Institut für Meteorologie und Klimaforschung, Universität Karlsruhe (TH), 76128 Karlsruhe, Germany
3Climate and Land Surface Systems Interaction Centre, School of the Environment and Society, Swansea, SA2 8PP, UK
4The Met Office, Fitzroy Road, Exeter, EX1 3PB, UK

Abstract. 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, and that mesoscale and boundary-layer circulations affected dust uplift and transport. These processes 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 parameterise.

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 Air Layer (SAL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary-layer. In a region of local uplift, with strong along-track winds, boundary-layer rolls are shown to lead to warm moist dusty updraughts in the boundary layer. Large eddy model (LEM) simulations suggest that these rolls increased uplift by approximately 30%. The modelled effects of boundary-layer convection on uplift is 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.

Citation: Marsham, J. H., Parker, D. J., Grams, C. M., Grey, W. M. F., and Johnson, B. T.: Observations of mesoscale and boundary-layer circulations affecting dust uplift and transport in the Saharan boundary layer, Atmos. Chem. Phys. Discuss., 8, 8817-8846, doi:10.5194/acpd-8-8817-2008, 2008.
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