Raleigh L. Martin

Wind-invariant saltation heights imply linear scaling of aeolian saltation flux with shear stress

Field data show insensitivity of particle trajectories to wind speed and support linear scaling of sand flux with wind stress. Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. However, it remains unclear how the flux of particles in aeolian saltation—the wind-driven transport of sand in hopping trajectories—scales with wind speed, largely because models do not agree on how particle speeds and trajectories change with wind shear velocity. We present comprehensive measurements, from three new field sites and three published studies, showing that characteristic saltation layer heights remain approximately constant with shear velocity, in agreement with recent wind tunnel studies. These results support the assumption of constant particle speeds in recent models predicting linear scaling of saltation flux with shear stress. In contrast, our results refute widely used older models that assume that particle speed increases with shear velocity, thereby predicting nonlinear 3/2 stress-flux scaling. This conclusion is further supported by direct field measurements of saltation flux versus shear stress. Our results thus argue for adoption of linear saltation flux laws and constant saltation trajectories for modeling saltation-driven aeolian processes on Earth, Mars, and other planetary surfaces.

A subordinated advection model for uniform bed load transport from local to regional scales

Sediment tracers moving as bed load can exhibit anomalous dispersion behavior deviating from Fickian diffusion. The presence of heavy-tailed resting time distributions and thin-tailed step length distributions motivate adoption of fractional-derivative models (FDMs) to describe sediment dispersion, but these models require many parameters that are difficult to quantify. Here we propose a considerably simplified FDM for anomalous transport of uniformly sized grains along straight channels, the subordinated advection equation (SAE), which is based on the concept of time subordination. Unlike previous FDM models with time index γ between 0 and 1, our SAE model adopts a value of γ between 1 and 2. This γ describes random velocities deviating significantly from the mean velocity and models both long resting periods and relatively fast displacements. We show that the model quantifies the dynamics of four bed load transport experiments recorded in the literature. In addition to γ, SAE model parameters—velocity and capacity coefficient—are related to the mean and variance of particle velocities, respectively. Successful application of the SAE model also implies a universal probability density for the heavy-tailed waiting time distribution (with finite mean) and a relatively lighter tailed step length distribution for uniform bed load transport from local to regional scales.

Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

Wind-blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind-driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field-based estimates of distinct fluid and impact thresholds from high-frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near-continuous transport conditions. Although both thresholds are thus important for high-frequency saltation prediction, our results suggest that the time-averaged saltation flux is primarily governed by the impact threshold. Plain Language Summary In our article, we address a core issue in studies of Earth and planetary surface processes: What is the minimum or threshold wind speed for moving sediment? Predictions for sediment transport movement by air and water depend sensitively on the choice of thresholds in models. In the case of wind-blown sand particles, the story is complicated by the presence of separate fluid and impact thresholds for the respective initiation and cessation of transport. Using a comprehensive field data set, we provide the first field-based determination of distinct fluid and impact thresholds, and we demonstrate how both thresholds together are needed to predict the instantaneous occurrence of wind-blown sand transport. We also find that models for large-scale processes driven by wind-blown sand—atmospheric dust aerosol generation, transport of wind-borne nutrients, erosion of agricultural soils, and formation of protective coastal dunes—should primarily use the impact threshold rather than the fluid threshold.

Fine dust emissions from active sands at coastal Oceano Dunes, California

Sand dunes and other active sands generally have a low content of fine grains and, therefore, are not considered to be major dust sources in current climate models. However, recent remote sensing studies have indicated that a surprisingly large fraction of dust storms are generated from regions covered by sand dunes, leading these studies to propose that sand dunes might be globally relevant sources of dust. To help understand dust emissions from sand dunes and other active sands, we present in situ field measurements of dust emission under natural saltation from a coastal sand sheet at Oceano Dunes in California. We find that saltation drives dust emissions from this setting that are on the low end of the range in emissions produced by non-sandy soils for similar wind speed. Laboratory analyses of sand samples suggest that these emissions are produced by aeolian abrasion of feldspars and removal of clay-mineral coatings on sand grain surfaces. We further find that this emitted dust is substantially finer than dust emitted from non-sandy soils, which could enhance its downwind impacts on human health, the hydrological cycle, and climate.