The Effects of Temperature Gradient on the Formation of Depth Hoar Structures in Snow

Understanding dry snow metamorphism under an external temperature gradient (TG) is essential for modeling the effects of snow on climate, and understanding avalanche events. Previous experiments and modeling on fresh snow under a low TG (30K/m--150K/m) found both vertical alignment of the snow particles and a decrease in the specific surface area (SSA) during vapor transport. However, a high TG (>250 K/m) may occur in the surface snow layer after an abrupt drop in air temperature. Our aim is to examine the microstructural evolution of different types of snow over a large range of TG (70-430K/m) and investigate the different mechanisms of TG metamorphism. To that end, we installed a cooling stage in an X-ray microtomograph in order to produce a temperature gradient along the vertical direction of specimens. Eleven scans were performed over 48 hours. From low to high TGs, the final microstructures transitioned from faceted to depth hoar structures that have highly stepped surfaces. Although previous research showed that the SSA decreased under TG metamorphism, we found two conditions under which the SSA increases, i.e. when the snow had previously undergone sintering, and under a very high temperature gradient (>400 K/m). These two conditions resulted in higher surface energy, affected the snow surface equilibrium and formed the depth hoar structures with highly stepped surfaces.