Abstract
ABSTRACTSnow metamorphism and settlement change the microstructure of a snowpack simultaneously. Past experiments investigated snow deformation under isothermal conditions. In nature, temperature gradient metamorphism and settlement often occur together. We investigated snow settlement in the first days after the onset of temperature-gradient metamorphism in laboratory experiments by means of in-situ time-lapse micro-computed tomography. We imposed temperature gradients of up to 95 K m−1 on samples of rounded snow with a density of ~230 kg m−3 and induced settlement by applying 1.7 kPa stress with a passive load on the samples simultaneously. We found that snow settled about half as fast when a temperature gradient was present, compared with isothermal conditions. The change in specific surface area after 4 days caused by temperature-gradient metamorphism was only a few percent. The viscosity evolution correlated with the amount of the temperature gradient. Finite element simulations of the snow samples revealed that stress-bearing chains had developed in the snow structure, causing the large increase in viscosity. We could show that a small change in microstructure caused a large change in the mechanical properties. This explains the difficulty of predicting snow mechanical properties in applications such as firn compaction or snow avalanche formation.
Highlights
As soon as snow reaches the ground, its morphology changes quickly by snow metamorphism and deforms due to the overburden stress
In experiments with a constant temperature gradient the strain increase was slower compared with isothermal conditions, independent of the direction of the temperature gradient (Fig. 2a)
With our laboratory experiments we showed that settlement is strongly affected by temperature-gradient metamorphism directly after onset of metamorphism
Summary
As soon as snow reaches the ground, its morphology changes quickly by snow metamorphism and deforms due to the overburden stress. Snow deforms as a non-linear viscoelastic material (Scapozza and Bartelt, 2003a), for which constitutive laws similar to polycrystalline ice and viscous fluids were formulated based on field and laboratory measurements (Shapiro and others, 1997). With these experimental laws, snow settlement can be described as a creep process. For primary creep in snow, Glen’s flow law for polycrystalline ice can be simplified as the following power law, relating the strain rate ε_ of snow and the constant uniaxial stress σ linearly by the viscosity η, similar to a viscous fluid (Schleef and Löwe, 2013): ε_
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