MODELING THE ATMOSPHERIC DUST CYCLE .1. DESIGN OF A SOIL-DERIVED DUSTEMISSION SCHEME

A soil-derived dust emission scheme has been designed to provide an ex plicit representation of the desert dust sources for the atmospheric t ransport models dealing with the simulation of the desert dust cycle. Two major factors characterizing the erodible surface are considered: (1) the size distribution of the erodible loose particles of the soil which controls the erosion threshold and the emission strength and (2) the surface roughness which imposes the efficient wind friction veloc ity acting on the erodible surface. These two parameters are included in a formulation of the threshold wind friction velocity by adapting a size-dependent parameterization proposed by Iversen and White (1982) and by applying to the rough erodible surfaces a drag partition scheme derived from Arya (1975). This parameterization of the threshold fric tion velocity has been included in an horizontal flux equation propose d by White (1979). This allows to attribute a specific production rate to each soil size range for each type of surface. The dust flux F is then considered as a fraction of the total horizontal flux G, the valu e of the ratio F/G being imposed, at this time, by the soil clay conte nt. In summary, the computed mass fluxes depend on the soil size distr ibution, the roughness lengths, and the wind friction velocity. The di fferent steps of this scheme have been independently validated by comp arison with relevant experimental data. Globally, the agreement is sat isfying, so that the dust fluxes could be retrieved with less uncertai nties than those observed in previous simulations of the desert dust c ycle.