From a snowflake to the snow cover: processes that shape polar and taiga snowpacks

Abstract

Snowpacks found in boreal and polar regions are the most widespread types of snow in the world, covering up to 14% of the globe. In both regions, snow accumulates over a long period (6-7 months), transforming the landscape by the presence of a thin snowpack (≤70 cm), affecting the local climate, ecology, and hydrology. In the case of polar snow, wind plays a crucial role in redistributing snow, and shaping the snow surface. But in the case of the taiga snow found in the forests of the boreal regions, micro-topography and vegetation are stronger drivers of snow distribution than wind. In this dissertation, I explore the mechanisms responsible for shaping the snow surfaces in windy and in calm conditions. Collecting data at the plot scale with a terrestrial lidar, I sought explanations of the features geometry visible on the snow surfaces in grain scale physical processes. Because snow is close to its fusion temperature in this environment, its behavior at the grain scale can greatly influence its bulk properties. So finding linkages between processes occurring at the grain scale and the observable features at the plot scale may be key to furthering our understanding of snow distribution. In the first study, I found that the morphology and the occurrence of the seven known types of snow bedforms are dependent on the ability for wind to erode the surface. Erodibility is directly linked to the sintering of wind-slab grains. For this reason, every snow dune eventually turns into sastrugi. In the second study, I studied the effects of underlying topography on the accumulation of snow in calm conditions. I found that processes such as bouncing, cohesion or interlocking of snowflakes can either enhance or inhibit the smoothing of initial bumps. In the third study, I found that plant canopies affect the deposition of snow in the boreal forest. I could differentiate up to five types of canopies for their effects on snow accumulation. Despite the complexity of the canopy structures we observed, over three years, similar accumulation patterns and reactions of canopies to snow loading were seen. I was surprised to find the presence of subnivean cavities associated to plants with a size equivalent to the average snow depth.