Hysteresis and Surface Shear Stresses During Snow-Particle Aeolian Transportation

Abstract

Surface shear stresses produced by wind and particle collision play a key role in aerodynamic entrainment and splash processes. The fluid shear stress at the surface during aeolian transport has been researched for decades; however, the equilibrium property reported in the literature, numerical simulations, and experiments is inconsistent. To discuss this discrepancy, this study investigates fluid and particle shear stresses at the surface during the aeolian transport of snow particles using a two-dimensional random-flight model of drifting snow. The simulations are performed for various friction velocities on a loose snow bed. By varying the wind conditions in stages, the transport hysteresis is confirmed, and the impact threshold is estimated from the particle transport rate ($$0.206\,\hbox {m\,s}^{-1}$$ 0.206 m s - 1 ). The friction velocity at the surface during transport decreases marginally with an increase in wind speed caused by the impact threshold, revealing that our results do not contradict Owen’s second hypothesis. The total shear stress, which is calculated by summing the fluid and particle shear stresses, is vertically uniform in the equilibrium state; thus, the increase in the particle shear stress decreases the fluid shear stress at the surface. The equilibrium property of the fluid shear stress near the surface changes significantly with height (from a decreasing trend to an increasing trend) because the particle shear stress decreases rapidly in the height range of 1–10 mm. Our findings suggest that it is difficult to accurately measure the fluid shear stress in the surface vicinity using anemometers, and a new methodology is needed.

Understanding snow saltation parameterizations: lessons from theory, experiments and numerical simulations

“…For instance, an increase of the streamwise and vertical velocities with the increase of the friction velocity is only seen above approximately 1.5 cm height above the surface. Below this height, the particle velocity is mainly invariant with respect to u * , which is in agreement with several saltation models (e.g., Kok and Renno, 2009;Niiya and Nishimura, 2022) and measurements carried out over sand (Creyssels et al, 2009;Ho et al, 2011), discussed in Sect. 2.1.2 and 2.2.2.…”

“…In the snow saltation model of Niiya and Nishimura (2022), u * ,s is found to decrease slightly with the increase of the friction velocity from 0.24 to 0.3 m s −1 . This differs from the increase of u * ,s found in Fig.…”

“…In this way, Owen's hypothesis is expected to oversimplify the saltation dynamics (Kok et al, 2012). Nonetheless, the assumption that τ s does not vary with u * , but depends solely on the bed characteristics, is regarded as an acceptable first order approximation (Walter et al, 2014;Niiya and Nishimura, 2022).…”

“…In addition, numerical models of snow saltation that represent the surface processes of aerodynamic entrainment, rebound and splash have also revealed ejection and impact velocities that are mainly invariant with respect to u * (Melo et al, 2022;Niiya and Nishimura, 2022).…”