Faceting around ice crusts

Weak faceted crystals are often observed near hard crust layers in the snowpack. The combination of weak facets adjacent to a hard ice crust often creates dangerous, persistent, and tricky avalanche conditions. During the 2001/2002 winter a faceted snow and crust combination formed in the mountains of northern Utah. This layering was responsible for numerous large avalanches and contributed to three avalanche fatalities. A recent article in The Avalanche Review summarizes this cycle.

Two processes that can create or contribute to this dangerous layering scenario are radiation recrystallization and melt-layer recrystallization (the latter is sometimes called wet layer recrystallization). These near-surface faceting processes are described in this article, as well as Bruce Tremper's widely available book, Staying Alive in Avalanche Terrain. In addition to these processes, it is believed that the denser and more conductive ice adjacent to the less conductive snow might concentrate temperature gradients next to the crust (Adams and Brown, 1988; 1990). This phenomenon could result in the enhanced faceted growth near the ice crust long after near-surface recrystallization has diminished. This layering may also allow faceted growth to continue in a thermal environment that is generally conducive to rounding.

Ethan Greene (Colorado State University), working with Kelly Elder of the USDA-Forest Service Rocky Mountain Research Station, Montana State University's Ed Adams, the Swiss Federal Institute for Snow and Avalanche Research's Martin Schneebeli, and the National Avalanche Center, is conducting laboratory work to carefully analyze the metamorphic processes around buried ice lenses. Ethan sucessfully competed for an National Science Foundation Dissertation Improvement Grant that helped to support this research. He did the bult of his experiments during the 2004-05 winter, then cast his samples and shipped them over to Switzerland packed in dry ice! Then he went to Switzerland to analyze the data with help from Martin Schneebeli. This research will form the basis of his PhD dissertation.

Ethan Greene, working in Montana State University's cold laboratory, conducts initial experiements on faceting around ice crusts.

References

Adams, E.E., and R.L. Brown. 1988. On the effect of strong density layering on metamorphism of seasonal snowcover. Proceedings of the 1988 International Snow Science Workshop, Whistler, British Columbia, 37-40.

Adams, E.E., and R.L. Brown. 1990. A mixture theory for evaluation heat and mass transport processes in nonhomogeneous snow. Continuum Mechanics and Thermodynamics 2, 31-63.