Wind tunnel experiments to quantify the effect of aeolian snow transport on the surface snow microstructure

Abstract. The evolution of the surface snow microstructure under the influence of wind is hardly understood but crucial for polar and alpine snowpacks. Available statistical models are solely parameterized from field data where conditions are difficult to control. Controlled experiments which exemplify the physical processes underlying the evolution of density or specific surface area (SSA) of surface snow under wind are virtually non-existing. As a remedy, we conducted experiments in a cold laboratory using a ring-shaped wind tunnel with an infinite fetch to systematically investigate wind-induced microstructure modifications under controlled atmospheric, flow and snow conditions. Airborne snow particles are characterized by high-speed imaging, while deposited snow is characterized by density and SSA measurements. We used a single snow type (dendritic fresh snow), cover wind speeds from 3 ms−1 to 7 ms−1 (for fixed temperature) and vary temperatures from -24 °C to -2 °C (for fixed wind speed). The measured airborne impact trajectories confirm the consistency of our coefficient of restitution with large scale saltation, rendering the setup suitable to realistically study interactions between airborne and deposited snow. Our measured densification rates in the deposit as a function of wind speed show clear deviations from existing statistical models, but can be re-parameterized through our data. The most drastic changes in densification and SSA rates of deposited snow are observed close to the melting point. This study, as a first of its kind, exemplifies a rich non-linear interplay between airborne and deposited snow particles which is discussed in view of a multitude of involved processes, i.e. airborne metamorphism, cohesion, particle separation and fragmentation.