Quantifying Particle Size and Turbulent Scale Dependence of Dust Uplift in the Sahara Using Aircraft Measurements

The first size-resolved airborne measurements of dust fluxes and the first dust flux measurements from the central Sahara are presented and compared with a recent parameterization. High frequency measurements of dust size distribution were obtained from ~0.1-300 ìm diameter and eddy covariance fluxes were derived. This is more than an order of magnitude larger size range than previous flux estimates. Number flux is described by a -2 power law from 1 up to a maximum of 144 μm diameter, significantly larger than the 12 μm upper limit suggested by the parameterization. We found that turbulent scales important for dust flux in this region at the aircraft heights were from 0.1 km up to 1-10 km, with the upper scale increasing during the morning as boundary layer depth and maximum eddy size increase. All locations where large dust fluxes were measured had large topographical variations. These features are often linked with highly erodible surface features, such as wadis or dunes. We also hypothesize that upslope wind and flow-separation over such features enhance the dust flux and allows transport of large particles out of the saltation layer. The tendency to locate surface flux measurements in open, flat terrain means these favored dust sources have been neglected in previous studies.