A NUMERICAL INVESTIGATION OF THE DYNAMICS AND MICROPHYSICS OF SAHARAN DUST STORMS (AEROSOL, AFRICA, EASTERLY WAVES)

Two- and three-dimensional numerical simulations of the spatial and temporal distributions of Saharan dust over the desert and the eastern Atlantic Ocean are presented. A primitive equations dynamical model is used to calculate the meteorological variables. An aerosol model is used to simulate aerosol physical processes including mobilization, advection, diffusion, sedimentation, coagulation, and dry deposition. The dust mobilization parameterization uses the predicted friction wind speed to determine the source strength. 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 timescales considered here. Bimodal size distributions develop when dust is mobilized within a dust plume generated on a previous day. The simulation shows that for August 23-28, 1974 low-level jets are responsible for deflation rather than the middle-level easterly jet. The jets are associated with a shallow easterly wave that eventually decayed in the mid-Atlantic Ocean. Dust mobilized in the Central Sahara on August 25 is slowly transported westward and contributes very little to the final mass load. Dust mobilized along the coast makes up most of the dust remaining in suspension at the final model time. The simulated horizontal aerosol distribution over the ocean closely resembles the distribution of dust seen in satellite imagery. The elevated layer of dust develops over the ocean as the northeast trade winds advect clean air underneath the advancing dust air. The size and spatial distributions of aerosol in the marine layer depend upon the undercutting process, the amount of background mineral aerosol present, and vertical turbulent diffusion across the marine layer.