A two-dimensional numerical investigation of the dynamics and microphysics of Saharan dust storms

Two-dimensional numerical simulations of the spatial and temporal distributions of Saharan dust size distributions over the desert and the eastern Atlantic Ocean are presented. The simulations show that during mobilization the soil size distribution is modified by either a size-dependent lifting mechanism or by mixing of local soil with aged aerosols or with aerosols originating from nearby soils which have different size distributions. The highest number concentrations encountered were not high enough for coagulation to have a significant effect over the time scales considered here. When the source region is near the coast, as opposed to the central Sahara, the highest mass concentration achieved at Sal Island is more than doubled. However, in the two-dimensional simulations the central Saharan storms seem to be equally as important as coastal sources in terms of the optical properties of an outbreak, since the total surface area of the suspended dust at the end of the coastal and inland source simulations are nearly the same. A background mineral aerosol or increased vertical turbulent diffusion across the marine layer inversion is required for the simulated marine layer size distributions to match the observed distributions at particle sizes under 8 μm. Sedimentation prohibits the direct advection of ultragiant particles very far from the coast, suggesting that the ultragiant particles observed over the ocean are locally generated, perhaps in water clouds.