Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara J. H. Marsham1, D. J. Parker1, C. M. Grams2, B. T. Johnson4, W. M. F. Grey3, and A. N. Ross1 1University of Leeds, UK 2Universität Karlsruhe and Forschungszentrum Karlsruhe, Germany 3Climate and Land Surface Systems Interaction Centre, School of the Environment and Society, Swansea, SA2 8PP, UK 4The Met Office, 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.
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
Such anomalies are expected to affect the
vertical mixing of the dusty and weakly stratified Saharan Residual Layer
Mesoscale variations in winds
are also shown to affect dust loadings in the boundary layer.
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.
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.
Citation: Marsham, J. H., Parker, D. J., Grams, C. M., Johnson, B. T., Grey, W. M. F., and Ross, A. N.: Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara, Atmos. Chem. Phys., 8, 6979-6993, doi:10.5194/acp-8-6979-2008, 2008.