Saltating Snow Mechanics: High Frequency Particle Response to Mountain Wind

Abstract:

Blowing snow transport theory is currently limited by its dependency on the coupling of time-averaged measurements of particle saltation and suspension and wind speed. Details of the stochastic process of particle transport and complex bed interactions in the saltation layer, along with the influence of boundary-layer turbulence are unobservable with classic measurement techniques. In contrast, recent advances in two-phase sand transport understanding have been spurred by development of high-frequency wind and particle velocity measurement techniques. To advance the understanding of blowing snow, laser illuminated high-speed videography and ultrasonic anemometry were deployed in a mountain environment to examine saltation of snow over a natural snowpack in detail. A saltating snow measurement site was established at the Fortress Mountain Snow Laboratory, Alberta, Canada and instrumented with two Campbell CSAT3 ultrasonic anemometers, four Campbell SR50 ultrasonic snow depth sounders and a two dimensional Particle Tracking Velocimetry (PTV) system. Measurements were collected during nighttime blowing snow events, quantifying snow particle response to high frequency wind gusts. This novel approach permits PTV to step beyond mean statistics of snow transport by identifying sub-species of saltation motion in the first 20 mm above the surface, as well as previously overlooked initiation processes, such as tumbling aggregate snow crystals ejecting smaller grains, then eventually disintegrating and bouncing into entrainment. Spectral characteristics of snow particle ejection and saltation dynamics were also investigated. These unique observations are starting to inform novel conceptualizations of saltating snow transport mechanisms.