The Critical Role of the Boundary Layer Thickness for the Initiation of Aeolian Sediment Transport

Here, we propose a conceptual framework of Aeolian sediment transport initiation that includes the role of turbulence. Upon increasing the wind shear stress t above a threshold value tau(t)', particles resting at the bed surface begin to rock in their pockets because the largest turbulent fluctuations of the instantaneous wind velocity above its mean value (u) over bar induce fluid torques that exceed resisting torques. Upon a slight further increase of t, rocking turns into a rolling regime (i.e., rolling threshold tau(t) similar or equal to tau(t)') provided that the ratio between the integral time scale T-i proportional to delta/(u) over bar (where delta is the boundary layer thickness) and the time T-e proportional to root d/[(1 - 1/s)g] required for entrainment (where d is the particle diameter and s the particle-air-density ratio) is sufficiently large. Rolling then evolves into mean-wind-sustained saltation transport provided that the mean wind is able to compensate energy losses from particle-bed rebounds. However, when T-i/T-e is too small, the threshold ratio scales as tau(t)/tau(t)' proportional to T-e/T-i proportional to sd(2)/delta(2), consistent with experiments. Because delta/d controls T-i/T-e and the relative amplitude of turbulent wind velocity fluctuations, we qualitatively predict that Aeolian sediment transport in natural atmospheres can be initiated under weaker (potentially much weaker) winds than in wind tunnels, consistent with indirect observational evidence on Earth and Mars.