Metamorphism of fine-grained snow due to surface curvature differences

Abstract

The metamorphism of snow in the absence of a significant temperature gradient was investigated. The first part of the study involved analytical modeling of the exchange of mass between grains of differing surface curvature and the process of intergranular sintering. Physical models were developed to evaluate these two processes. For the first process, it was assumed that mass exchange took place primarily by vapor transport between neighboring grains. The principles of mass balance, momentum balance and energy balance were utilized to evaluate time and spatial variations in temperature, vapor velocity, vapor pressure and mass exchange between the two grains. For the second process, mass exchange was also assumed to be dominated by vapor flow from the grain surface to the neck surface. The same variables were solved for in this second process. Results obtained show that, as expected, the exchange rates between grains of different surface curvature depend upon the radii of curvature, pore size and temperature. The rate of sintering, as determined by the rate of vapor deposition on the neck is determined by temperature, grain curvature, and neck curvature. In addition to the physical modeling, an experimental program was undertaken to measure rates of metamorphism in specially prepared snow consisting of fine-grained spherical particles. This snow was made using specialized instrumentation developed in Japan. The mean grain size was 20 μm, which, while very small, allowed the observation of measurable changes in snow microstructure over short time spans. Test results showed that the grain size increased markedly with time and that the small grains were sacrificed as the large grains acquired mass from the smaller grains.