NUMERICAL MODELLING OF SALTATION IN THE ATMOSPHERIC SURFACE LAYER YAPING SHAO and AN LI School of Mathematics, University of New South Wales, Sydney, Australia (Received in final form 19 January 1999) Abstract. A numerical model is developed to study the hopping motion of sand grains (saltation) in neutral atmospheric surface layers. Saltation is considered as a self-limiting process governed by the interaction of four components: aerodynamic entrainment, particle motion, splash entrainment and wind modification. The model comprises a large eddy simulation model for atmospheric surface- layer flows, a Lagrangian model for particle trajectories and a statistical description for aerodynamic entrainment and splash entrainment. The numerical simulation is focused on the aspects of saltation that are not well understood from experimental studies, including the role of splash, the evolution of wind and momentum flux profiles, and the effective roughness length, z 0s . It is shown that for splash to be effective, the surface friction velocity must exceed a critical value for a given particle size. The numerically estimated z 0s is compared with the analytical model of Raupach and the experimental data of Gillette et al. The model is also used to calculate the streamwise sand drift and the numerical results are found to be in agreement with the existing wind-tunnel measurements. Keywords: Atmospheric surface layer, Large-eddy simulation, Saltation, Two phase flow, Wind erosion. 1. Introduction Saltation is the hopping motion of sand grains along a mobile aeolian surface, which is not only responsible for sand drift, but also mainly responsible for dust entrainment into the atmosphere (Shao et al., 1993). The saltation process involves interactions in the atmospheric surface layer between the particulate phase, the fluid phase and the surface. These interactions are not yet well understood. For wind erosion modelling on regional to continental scales (Shao and Leslie, 1997; Marticorena et al., 1997) parameterizations for particle entrainment, trans- port and deposition are necessary because of the complexity of the processes in- volved (Shao et al., 1996). Some of the parameterizations and the associated para- meters are based on assumptions that are not rigorously tested, or based on em- pirical relationships that do not provide an understanding at the microscopic level. The purpose of this paper is to develop a suitably sophisticated numerical model for simulation of saltation and to apply the model to achieve increased understanding of the interactions between particle flow and surface. The theoretical consideration of particle motion in the atmospheric surface layer traces back to the simple model of Bagnold (1941) for the vertically integrated Boundary-Layer Meteorology 91: 199–225, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.