Parameterization of the shear strength of faceted crystals during equi-temperature metamorphism

Abstract

AbstarctChanges in the shear strength of faceted crystals were measured and parameterized under equi-temperature conditions in the laboratory. The air temperature and bottom temperature of the snow layer were controlled to create a large temperature gradient for 8.8 days to make faceted crystals. Subsequently, the temperature gradient was eliminated and the snow was kept under equi-temperature conditions (–5°C and –10°C) for 55 days. During the experiments, the snow density and shear strength were measured six times. The measured shear strength was compared with two empirical relationships based on density. One of these relationships addresses the strength of round grains, whereas the other refers to faceted grains. The measured shear strength approached the calculated value for faceted crystals when the temperature gradient was large, whereas it approached that for rounded grains after the temperature gradient was removed. The dry snow metamorphism factor (DSM factor), which expresses the developmental stage of faceted crystals, was used to model the shear strength increase under equi-temperature conditions. The DSM factor indicates shear strength using an empirical equation based on rounded grains or faceted crystals. It is approximately 0 for rounded grains and 1 for faceted crystals. It decreased from 1.04 to 0.84 and 0.79 at –10°C and –5°C, respectively, in 55 days. These results were incorporated into the numerical snowpack model SNOWPACK, which successfully reproduced the experimentally observed increase in shear strength under equi-temperature conditions.