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Register a new userSize-independent susceptibility to transport in aeolian saltation
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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 non-interacting 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 non-interacting 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.
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Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. However, it remains unclear how the sand flux scales with wind speed, largely because models do not agree on how particle speed changes with wind shear velocity. Here, we present comprehensive measurements from three new field sites and three published studies, showing that characteristic saltation layer heights, and thus particle speeds, remain approximately constant with shear velocity. This result implies a linear dependence of saltation flux on wind shear stress, which contrasts with the nonlinear 3/2 scaling used in most aeolian process predictions. We confirm the linear flux law with direct measurements of the stress-flux relationship occurring at each site. Models for dust generation, dune migration, and other processes driven by wind-blown sand on Earth, Mars, and several other planetary surfaces should be modified to account for linear stress-flux scaling.
Aeolian transport of sand and dust is driven by turbulent winds that fluctuate over a broad range of temporal and spatial scales. However, commonly used aeolian transport models do not explicitly account for such fluctuations, likely contributing to substantial discrepancies between models and measurements. Underlying this problem is the absence of accurate sand flux measurements at the short time scales at which wind speed fluctuates. Here, we draw on extensive field measurements of aeolian saltation to develop a methodology for generating high-frequency (25 Hz) time series of total (vertically-integrated) saltation flux, namely by calibrating high-frequency (HF) particle counts to low-frequency (LF) flux measurements. The methodology follows four steps: (1) fit exponential curves to vertical profiles of saltation flux from LF saltation traps, (2) determine empirical calibration factors through comparison of LF exponential fits to HF number counts over concurrent time intervals, (3) apply these calibration factors to subsamples of the saltation count time series to obtain HF height-specific saltation fluxes, and (4) aggregate the calibrated HF height-specific saltation fluxes into estimates of total saltation fluxes. When coupled to high-frequency measurements of wind velocity, this methodology offers new opportunities for understanding how aeolian saltation dynamics respond to variability in driving winds over time scales from tens of milliseconds to days.
We conduct numerical simulations based on a model of blowing snow to reveal the long-term properties and equilibrium state of aeolian particle transport from $10^{-5} hspace{0.5 ex} mathrm{m}$ to $10 hspace{0.5 ex} mathrm{m}$ above the flat surface. The numerical results are as follows. (i) Time-series data of particle transport are divided into development, relaxation, and equilibrium phases, which are formed by rapid wind response below $10 hspace{0.5 ex} mathrm{cm}$ and gradual wind response above $10 hspace{0.5 ex} mathrm{cm}$. (ii) The particle transport rate at equilibrium is expressed as a power function of friction velocity, and the index of 2.35 implies that most particles are transported by saltation. (iii) The friction velocity below $100 hspace{0.5 ex} mumathrm{m}$ remains roughly constant and lower than the fluid threshold at equilibrium. (iv) The mean particle speed above $300 hspace{0.5 ex} mumathrm{m}$ is less than the wind speed, whereas that below $300 hspace{0.5 ex} mumathrm{m}$ exceeds the wind speed because of descending particles. (v) The particle diameter increases with height in the saltation layer, and the relationship is expressed as a power function. Through comparisons with the previously reported random-flight model, we find a crucial problem that empirical splash functions cannot reproduce particle dynamics at a relatively high wind speed.
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 the fluid threshold, existence of a separate lower impact threshold complicates the prediction of wind-driven transport. Here, we derive the first field-based estimates of distinct fluid and impact thresholds from high-frequency 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, we find that the time-averaged saltation flux is primarily governed by impact threshold.
Aeolian-erosion is a destructive process which can erode small-size planetary objects through their interaction with a gaseous environment. Aeolian-erosion operates in a wide range of environments and under various conditions. Aeolian-erosion has been extensively explored in the context of geophysics in terrestrial planets. Here we show that aeolian-erosion of cobbles, boulders and small planetesimals in protoplanetary-discs can constitute a significant barrier for the early stages of planet formation. We use analytic calculations to show that under the conditions prevailing in protoplanetary-discs small bodies ($10-10^4 rm{m}$) are highly susceptible to gas-drag aeolian-erosion. At this size-range aeolian-erosion can efficiently erode the planetesimals down to tens-cm size and quench any further growth of such small bodies. It thereby raises potential difficulties for channels suggested to alleviate the metre-size barrier. Nevertheless, the population of $sim$decimetre-size cobbles resulting from aeolian-erosion might boost the growth of larger (>km size) planetesimals and planetary embryos through increasing the efficiency of pebble-accretion, once/if such large planetesimals and planetary embryos exist in the disc.
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