Water vapor transport in snowpacks

This thesis is an in-depth treatment of water vapor transport in snowpacks and its impacts on snow structure. The aim is to better understand this transport process and to lay the basis for a model representation in physics-based multi-layer snow models. Overall the process should improve the representation of the impact of snow in diverse environments from mountains, ice sheets to sea ice.

The vertical diffusion of water vapor in the snow cover is investigated first by solving a one-dimensional transient diffusion equation in the SNOWPACK model. The implications on the snow layering of this vertical diffusive flux are analyzed for four different regions: Alpine, Subarctic, Arctic, and Antarctic sea ice. The largest impact of diffusion is observed in snow on sea ice in the Weddell Sea and the shallow Arctic snowpack. The simulations show significant density reductions at the base of the snow cover upon inclusion of diffusive water vapor transport.

Diffusion may in some snow covers have a small effect and it is the convection often invoked to explain observed density profiles e.g. of thin tundra snowpacks. The effects of convection on snowpack structure have never been studied systematically by looking at the thermal and phase change regimes for different snowpack conditions. Therefore, this thesis presents for the first time a numerical investigation for convection of water vapor in idealized snowpacks using an Eulerianâ Eulerian two-phase approach. We find considerable impact of the natural convection on the snow density distribution with a layer of significantly lower density at the bottom of the snowpack and a layer of higher density located near the surface. We observe that emergent heterogeneity in the snow porosity results in a feedback effect on the spatial organization of convection cells causing their horizontal displacement. We additionally find that the thermal equilibrium assumption is not valid in our idealized system.

Natural convection is rather not likely to have a large impact in typical Alpine or sub-arctic snow covers if conditions are horizontally homogeneous. Therefore, this thesis also studies numerically the potential impact of heterogeneity induced by shrubs or rocks on convection of water vapor in snowpacks. We find that the convection cells are formed even with sub-critical Rayleigh number as low as 5 due to heterogeneity induced in snow density. We show that contributions of diffusive and convective flux divergence on snow density change show strong variations. We further find that the strongest effect of convection is not for very thin or thick snow covers but for snow covers in between, which allow the formation of convection cells with a scale of 0.5 m.

Among different snow covers, significant convection events are numerically observed by SNOWPACK-OpenFOAM coupling for a herb tundra permafrost at Bylot Island. We find that significant sublimation in downward flow generates a low-density base in the snow cover. W