Snow transport in extreme environments: from small to large scale modeling

The cold regions on Earth, such as the polar and high mountain regions, are snow covered for at least a part of the year. These snow-covered surfaces are highly dynamic, particularly under the influence of strong winds. The aeolian or wind-driven transport of snow occurs when the wind is sufficiently strong to lift the snow particles from the surface. It controls the mass balance of the snowpack under windy conditions and influences the sublimation of snow and ice surfaces. Despite its importance, the representation of snow transport in climate models has high uncertainties because the associated physical processes are complex and highly variable in space and time. One of the main challenges is the accurate representation of snow transport close to the ground, where snow particles are transported by a process called saltation. This shallow layer can comprise most of the horizontal mass flux and sets the lower boundary condition for modeling snow suspension clouds.

In order to investigate the complex wind-particle-bed interactions characteristic of snow particles in saltation, we use a Large Eddy Simulation flow solver coupled with a Lagrangian model that describes the particles trajectory and their interaction with the snow surface. In particular, the model is used to study the influence of snow surface properties on snow saltation dynamics, as well as the validity of the parameterizations used in large-scale models. It is confirmed that interparticle cohesion and grain size heterogeneity can increase the overall saltation mass flux at high friction velocities. Moreover, some simplified models agree reasonably well with the simulations for given bed characteristics, while others clearly do not. In addition, we show that the prevailing parameterizations that describe the saltation system in atmospheric models are based on contradictory assumptions: while some scaling laws are typical of a saltation system dominated by aerodynamic entrainment, others represent a saltation system controlled by splash. Using numerical simulations, we show that both regimes can exist, depending on the friction velocity.

These findings are used to improve the coupled atmosphere-snowpack model CRYOWRF. The parameterizations employed are adjusted so that they become consistent with the current understanding on snow saltation dynamics. By so doing, the comparison between simulation results and mass flux measurements is significantly improved.

This work offers a comprehensive analysis of the snow saltation system and its scaling laws, and showcases how detailed numerical models can be used to improve large-scale models of snow transport and sublimation. These efforts improve the understanding of the physical phenomena taking place at the polar and high mountain regions, and contribute to a more accurate assessment of the surface mass balance at those sites.