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Fresh insights into snow metamorphism

In nature, the metamorphism of dry snow is practically always caused by a temperature gradient in the snowpack. Snow metamorphism changes the structure of fresh snow within a few days and plays a significant role in the formation of avalanches. For several years the SLF has been examining the snow structure and its metamorphism in the cold laboratory (in situ) with X-ray micro-tomography.

Depth hoar

When snow metamorphism takes place, snow crystals change and reform. Depth hoar is the most conspicuous type of snow formed on the ground by this process. It is characterised by its large, cup-shaped crystals. Until recently, snow researchers assumed that the largest crystals grew at the cost of small crystals and retained their core, consisting of the metamorphosed snowflake.

Tiefenreifkristall
Fig. 1: A single crystal of depth hoar (approx. 2.5 mm tall) with typical characteristics. The faceted area at the bottom is the growth zone; the rounded part at the top is partly sublimating.

Snow metamorphism made visible for the first time

Using four-dimensional X-ray micro-tomography, researchers B. Pinzer, M. Schneebeli and T. Kämpfer have become the first direct observers of depth hoar formation (see film).

Movie: For a period of three weeks, the temperature gradient was maintained at a constant 50°C per metre – conditions that can also occur naturally in winter. The cold underside of the crystals grew. The ostensible downward movement or flow is attributable to the continuous sublimation at the top of the crystals and the steady growth at the bottom, not by mechanical settling.

They were astonished to discover a mechanism in action that was entirely different from existing assumptions. All the crystals were completely sublimated ("deconstructed") several times and deposited again ("reconstructed") elsewhere. During the three-week experiment, the crystals metamorphosed entirely six to seven times. The lifetime of a snow crystal was therefore 2 to 3 days. 

Lebensdauer der Kristalle
Fig. 2: Lifetime of crystals during an experiment after 9, 18 and 27 days (from top to bottom). Only very little ice is older than 8 days, such as the top part of the depth hoar crystal in the bottom picture. 60% of the ice renews itself within 2 to 3 days.

Vapour flow remains constant

As metamorphism was occurring, the researchers were also able to measure the vapour flow directly for the first time. Although the shape of the crystals changed radically, the amount of transported water vapour remained constant throughout the experiment. This process of vapour transfer from one ice grain to another is described as "hand-to-hand transport"; it was first observed in the 1950s by Z. Yosida and his colleagues. The vapour flow they recorded by way of indirect measurements was too high, however, and gave rise to the enduring debate as to whether the water vapour diffusion coefficient is greater in snow than in air. In this new experiment, the SLF scientists were able to settle the controversy: the diffusion in snow is a physical constant and therefore the same as that which occurs between two plates of ice.

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 3).

Schneemetamorphose Schneemetamorphose  
Fig. 3 (left) snow crystal at the begin and (right) at the end of the experiment . The scale is identical in both photographs. The amplitude of the sinusoidal temperature gradient was constant with 90 degree per meter.

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 was supported by the Swiss National Science Foundation.

Movie

© AGU

Publications

Pinzer, B. R., Schneebeli, M., and Kaempfer, T. U. (2012) Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography, The Cryosphere, 6, 1141-1155, doi:10.5194/tc-6-1141-2012. >>

Pinzer, B. R., and M. Schneebeli (2009), Snow metamorphism under alternating temperature gradients: Morphology and recrystallization in surface snow, Geophys. Res. Lett., 36, L23503, doi:10.1029/2009GL039618. >>


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