Heterogeneous grain growth and vertical mass transfer within a snow layer under temperature gradient

Abstract. Inside a snow cover, metamorphism plays a key role in snow evolution at different scales. This study focuses on the impact of temperature gradient metamorphism on a snow layer in its vertical extent. To this end, two cold-laboratory experiments were conducted to monitor a snow layer evolving under 100 K m⁻¹ using X-ray tomography and environmental sensors. The first experiment shows that snow evolves differently in the vertical: at the end, coarser depth hoar are found in the center part of the layer, with covariance lengths about 50 % higher, compared to the top and bottom areas. We show that this heterogeneous grain growth could be related to the temperature profile and the associated crystal growth regimes, and to the profile of vapor supersaturation. In the second experiment, a non-disturbing sampling method was applied to enable a precise observation of the basal mass transfer in the case of dry boundary conditions. An air gap, characterized by a sharp drop in density, developed at the base and reached more than 3 mm after a month. The two reported phenomena, heterogeneous grain growth and basal mass loss, create heterogeneities in snow – in terms of density, grain and pore size, and ice morphology – from an initial homogeneous layer. Finally, we report the formation of hard depth hoar associated with an increase of SSA observed in the second experiment with higher initial density. These micro-scale effects may strongly impact the snowpack behavior, e.g. for snow transport processes or snow mechanics.