Alpine Turbulence and Blowing Snow

Abstract

Blowing snow in mountainous terrain is a complex nonlinear phenomenon driven by turbulent eddies with length scales ranging from millimetres to kilometres. Turbulent motions across a wide spectrum of sizes are superimposed on each other, interacting through a non-stationary energy and momentum cascade. In cold regions, snow redistribution by these turbulent motions impacts hydrology, glaciology, avalanche safety, and civil engineering. Blowing snow models typically rely on relating time-averaged turbulence statistics, which may oversimplify the complexity of the flow, especially in complex mountainous terrain, to steady-state snow transport. The present research sought to improve the understanding of the dominant structures in ASL turbulence relevant to snow transport, as well as characterize the short timescale response of blowing snow to specific eddy structures. A fundamental experiment was designed utilizing high-speed videography of laser illuminated near-surface blowing snow saltation coupled with adjacent 3D sonic anemometer wind measurements at two heights. The experiments were conducted at Fortress Mountain Snow Laboratory in the Canadian Rockies of Alberta during nighttime blowing snow storms. Novel applications of particle tracking velocimetry and binarization algorithms to blowing snow recordings allowed extraction of time resolved snow particle velocities synchronized with instantaneous wind velocities, as well as time series of volumetric averages of blowing snow density in the first 30 mm above the surface. High-speed blowing snow video and measurements revealed the importance of the often- overlooked creep mode of transport to both transport initiation and flux. Blowing snow velocity and flux profiles were found to be temporally variable and dependent on instantaneous wind speed, with dominant modes of transport varying during turbulent gusts. Sweep and ejection wind events were coupled to blowing snow responses on sub-second timescales, with each quadrant event playing a unique role in transport initiation and sustaining snow fluxes. Finally, large low-frequency turbulent motions, hypothesized to follow a top-down characterization, were found to modulate the amplitude of near-surface turbulence, as well as directly contribute to blowing snow fluxes. The role of intermittent coherent turbulent structures challenges the ability of time-averaged turbulence statistics to represent the complexity of wind-snow coupling, especially in mountainous terrain. The strong relationship found between large-scale turbulence modulating eddies and near-surface turbulence, also challenges the efficacy of applying steady- state laboratory-derived flux relationships to model transport in the ASL. The results presented here, along with recent advances on coherent turbulent structures provide an optimistic semi- deterministic avenue for improving blowing snow models in complex mountainous terrain.