Size‐Independent Susceptibility to Transport in Aeolian Saltation

Abstract

Natural wind-eroded soils contain a mixture of particle sizes. However, models for aeolian saltation are typically derived for sediment bed surfaces containing only a single particle size. To nonetheless treat natural mixed beds, models for saltation and associated dust aerosol emission have typically simplified aeolian transport either as a series of noninteracting single particle size beds or as a bed containing only the median or mean particle size. Here we test these common assumptions underpinning aeolian transport models using measurements of size-resolved saltation fluxes at three natural field sites. We find that a wide range of sand size classes experience "equal susceptibility" to saltation at a single common threshold wind shear stress, contrary to the "selective susceptibility" expected for treatment of a mixed bed as multiple single particle size beds. Our observation of equal susceptibility refutes the common simplification of saltation as a series of noninteracting single particle sizes. Sand transport and dust emission models that use this incorrect assumption can be both simplified and improved by instead using a single particle size representative of the mixed bed.

Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold

“…Hence, like Pähtz and Durán (), we hypothesize that u t is the minimal wind shear velocity that is needed to compensate energy losses of rebounding particles during particle trajectories, which is a size‐selective process because larger particles are accelerated less strongly during their hops. Note that the finding of size selectivity does not contradict the field measurements by Martin and Kok (), which showed that the size distribution of particles in saltation is relatively insensitive to the wind shear velocity u * , because the sand beds at these authors' field sites were considerably better sorted ( d 90 / d 50 ≈1.6) than our Samples 5 ( d 90 / d 50 ≈2.2) and 6 ( d 90 / d 50 ≈2.5) and thus likely did not exhibit a significant difference between and (like our Samples 1–4).…”

“…Hence, like Pähtz and Durán (2018a), we hypothesize that u t is the minimal wind shear velocity that is needed to compensate energy losses of rebounding particles during particle trajectories, which is a size-selective process because larger particles are accelerated less strongly during their hops. Note that the finding of size selectivity does not contradict the field measurements by Martin and Kok (2019), which showed that the size distribution of particles in saltation is relatively insensitive to the wind shear velocity u * , because the sand beds at these authors' field sites were considerably better sorted (d 90 ∕d 50 ≈ 1.6) than our Samples 5 (d 90 ∕d 50 ≈ 2.2) and 6 (d 90 ∕d 50 ≈ 2.5) and thus likely did not exhibit a significant difference between u vis t and u z o t (like our Samples 1-4). In summary, for poorly sorted sand beds, there may be three distinct dynamic thresholds: a threshold associated with the cessation of intermittent saltation transport of relatively fine particles (u vis t ), a larger threshold associated with the cessation of intermittent saltation transport of the entire ensemble of particles (u z o t ), and an even larger threshold below which continuous saltation transport becomes intermittent (there is an ongoing controversy about whether this threshold is associated with splash entrainment or aerodynamic entrainment; .…”

“…While the difference between both estimations is small for our well-sorted sands (u z o t ∕u vis t ≃ 1.1), which may well be attributed to a systematic underestimation of u vis t (section 3.2.1), it is quite significant for our poorly sorted sands (u . This is a curious finding because the current consensus is that there is a single threshold associated with the cessation of intermittent saltation transport for polydisperse sand beds , 2019.…”

“…Furthermore, we would like to emphasize that the ability of a certain particle class to rebound is probably strongly tied to its ability to eject particles of the same size class via splash (Pähtz & Durán, 2018a, section 4.2.1). Hence, if very spread 10.1029/2019JF005094 splash-sustained transport required that all size classes are equally susceptible to splash entrainment (as it likely does Martin & Kok, 2019), this assumption would also explain why the relationship between u * and U ∞ tends to be described by equation (5b) at shear velocities u * that are not too far above u z o t , as splash-sustained transport is the origin of the strong particle-flow feedback causing the roughness increase described by equation (5b) (section 3.2.2). However, note that the shift from equation (5a) to equation (5b) does not always happen immediately (e.g., there is an obvious transitional region for Sample 5 in Figure 4), which is also consistent with the rebound hypothesis (in the sustained-rebound picture, the splash entrainment threshold is always larger than the rebound threshold; Pähtz & Durán, 2018a).…”

Conditions for aeolian transport in the Solar System

“…Curiously, the slope of the attrition parameter with grain size does not have a consistent sign across environments. We speculate that negative slopes imply efficient production of dust, if there is equal transport susceptibility of grain-sizes 29 .…”