Large-scale motion flow physics regulate dust entrainment rates

Wind-driven hopping of sand particles (saltation) starts when the imposed aerodynamic stress exceeds a fluid threshold. This results in dust emission from the sediment bed, which continues until the surface stress falls below an impact threshold. Elongated regions of relatively high and low momentum (HMR, LMR) aligned with the dominant flow direction are observed in atmospheric surface layer turbulence. High momentum regions impose high stress, thereby mobilizing sand grains, but fail to entrain dust owing to the associated negative vertical velocity. On the other hand, low momentum regions do not initiate saltation, but embody the positive vertical velocity required for entrainment. This entrainment paradox indicates the presence of alternate entrainment mechanisms. Turbulent stress production by sweeps, ejections, inner interactions and outer interactions reveal three modes of dust entrainment. LMRs immediately behind HMRs encounter hysteretic saltation – the remnant of saltation initiated beneath an earlier HMR – and entrain dust; dust-rich fluid from near the base of HMRs is pumped to the adjacent LMRs by interfacial streamwise vorticity; and outer interactions impose high surface stress and directly entrain.