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.
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.
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).
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.
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. >>
