Metamorphism of snow near the surface

The snow layers near the surface play a crucial role both in the subsequent formation of weak layers as well as for photochemical processes within the snowpack. At the snow surface, not only the temperature gradient changes, but also its direction. In other words, it is sometimes warmer at the surface than deeper in the snowpack and a few hours later, it is vice versa. Until now there have been no experiments that examined the influence of the direction of the temperature gradient on the snow metamorphism. We analyzed these processes using high-resolution tomography.

60% of the snow mass recrystallized

A cyclic temperature gradients of ± 90 degrees per meter was applied to the new snow, as he might appear on a clear winter day in about 10 cm depth. Within 12 hours 60% of the snow mass recrystallized. This recrystallization led to larger, and less complex, structures. The new snow crystals formed rounded, branch-link structure, but no facetted shapes (Figure 1).

The transformation into more elongated structures makes the snow as soft as fresh snow, even though its structure is now quite different. Thus, to the skiers the snow seems almost like new snow. If the snow had recrystallized into more spherical shapes, it would settle to a much harder material. The elongated structures are less connected than new snow. Later, when buried by new snow falls, they may become a weak layer, on which slab avalanches can form.

The project "Dynamics of snow metamorphism: observations of physical and chemical processes and micro structural multi-phase numerical simulation (200021-108219)" was supported by the Swiss National Science Foundation.

Movie

© AGU

Publication

Pinzer, B. R., and M. Schneebeli (4 December 2009), Snow metamorphism under alternating temperature gradients: Morphology and recrystallization in surface snow, Geophysical Research Letters >>

Contact

• Martin Schneebeli