Numerical analysis of conditions necessary for near-surface snow metamorphism

Abstract

Faceted snow crystals develop at or near the snow surface due to temperature gradients. After burial, snow avalanches regularly fail on these layers. Generally, surface hoar deposits when the snow surface is cooler than the surrounding environment; near-surface facets form when the subsurface is warmed by solar radiation and the surface is cooled by radiative, convective, and latent heat exchange. Field research stations were established that included daily observations and meteorological data. In two seasons, 14 surface hoar and 26 near-surface facet events were recorded. Statistical analysis of the surface hoar events indicated three factors that were related to surface hoar growth: incoming long-wave radiation, snow surface temperature, and relative humidity. The ideal conditions for each of these parameters were 190-270 W/m², -22 to -11°C, and 45-80%, respectively. For near-surface facet formation, long- and short-wave radiation and relative humidity were statistically linked to the events. The ideal conditions for these parameters ranged from 380-710 W/m², 210-240 W/m², and 23-67%, respectively. Using a thermal model, sensitivity analysis, and Monte Carlo simulations the conditions that lead to facet formation were explored. Based on computed mass-flux, the formation of surface hoar was mainly driven by changes in long-wave radiation, air temperature, wind velocity, and relative humidity. From these terms graphical tools were developed to predict surface hoar; the numerical results matched reasonably well with the field observations. Based on the presence of a specific temperature gradient understood to lead to near-surface facets, three terms were determined to be the most influential: density, thermal conductivity, and incoming long-wave radiation. Using these terms, albedo, and incoming short-wave radiation--a requirement for radiation-recrystallization--a means for predicting the presence of near-surface facets was presented. The physical and analytical data presented indicates that incoming long-wave radiation is the most influential parameter governing the conditions that lead to surface hoar and near-surface facet growth. The analysis suggests that snow with low density and high thermal conductivity may be conducive to the formation of near-surface facets.