Spatiotemporal Structure of Aeolian Particle Transport on Flat Surface

Abstract

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 $100 \hspace{0.5 ex} \mu\mathrm{m}$ remains roughly constant and lower than the fluid threshold at equilibrium. (iv) The mean particle speed above $300 \hspace{0.5 ex} \mu\mathrm{m}$ is less than the wind speed, whereas that below $300 \hspace{0.5 ex} \mu\mathrm{m}$ 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

Hysteresis and Surface Shear Stresses During Snow-Particle Aeolian Transportation

“…We simulated the aeolian transport of snow particles using a random-flight model of drifting snow (Nemoto and Nishimura 2004). The calculation process was reconstructed to describe the spatio-temporal transport structure with high accuracy (Niiya and Nishimura 2017). The model calculates the vertical profile of the wind speed and the two-dimensional trajectory of each particle in air.…”

“…Here, we present the governing equations of wind speed and particle trajectory. For further details, please refer to: Nemoto and Nishimura (2004) and Niiya and Nishimura (2017).…”

“…These particle dynamics were simulated using statistical functions measured in wind-tunnel experiments of drifting snow, conducted by Sugiura and Maeno (2000); however, the value of u * was low. For descriptions of both these sub-processes, please refer to Niiya and Nishimura (2017).…”

“…Numerous geophysical models have been applied using Owen's second hypothesis (Raupach 1991;Marticorena and Bergametti 1995;Sauermann et al 2001;Ho et al 2011), owing to its suitability for analytical approaches. Concurrently, numerical simulations without Owen's second hypothesis have also been conducted (Werner 1990;Nemoto and Nishimura 2004;Kok and Renno 2009;Kok et al 2012;Niiya and Nishimura 2017), and several researchers have challenged Owen's second hypothesis. The friction velocity in the saltation layer decreases once the particles are mobilized due to a lesser relative velocity, and the friction velocity in the surface vicinity is less than the impact threshold (i.e., u s * ≤ u i * ).…”

“…This discrepancy, including the particle shear stress acting on the surface, has not yet been discussed in detail. In this study, we aim to investigate the equilibrium property of the fluid shear stress and its relationship with the particle shear stress in the surface vicinity using a random-flight model of drifting snow (Nemoto and Nishimura 2004;Niiya and Nishimura 2017). Sections 2 and 3 describe the governing equations of the random-flight model and the set-up of the numerical simulations, respectively.…”