To examine the ways a snowpack strengthens and reduces the avalanche hazard, we observe snow stratigraphy and sintering throughout the depth profile of the snowpack. In previous years, we used optical microscopy and scanning electron microscopy to measure snow grain geometry and chemical composition. Because of the time required for those kinds of observations, we sampled only two locations. We plan to extend our sampling to learn how stratigraphy and sintering vary spatially using a new method, near-infrared digital photography. This will allow for rapid quantitative stratigraphy and evaluation of snow stability and rates of snow metamorphism.
Historically, snow avalanche forecasting has focused on identifiable layers in the snowpack, but our near-infrared images across in-storm avalanche crown faces (the furthest uphill tensile fracture) within one day of avalanching show little discernible difference in the optical properties between the bed surface and the slab. In the maritime environment of western North America, more than 90% of failures occur within the storm layer and fail on non persistent weak layers. Our hypothesis is that no obvious weak layer is needed for avalanches. Instead, we propose that stability of new snow should be treated as a continuum problem where, at any time, the weakness is located where the ratio of stress/strength is lowest. The overburden causes downslope stress, while sintering and compaction increase strength. To test this hypothesis, we will make extensive measurements at crown faces using near-infrared photography combined with measurements of mechanical strength, and we will develop a continuum model of snow stability that includes downslope stress and continuous sintering at all depths in the storm layer.
Evaluating snow stability during storms is difficult, yet in some environments almost all avalanches occur during or shortly after storms. The novel application of near-infrared photography to avalanche hazard evaluation enables the rapid study of snow stratigraphy, with the likely results that the data are correlated with mechanical properties of snow and their application to avalanche hazard evaluation. While the heterogeneity of grain sizes at the snow surface has been investigated with remote sensing, there are many fewer studies of the heterogeneity of snow properties in buried layers, especially at the slope scale. Our experience, extensive instrumentation, site accessibility, and large number of avalanche control records make our field location, Mammoth Mountain, an ideal site to test how near-infrared photography can be used to examine where and when in the snow profile the snowpack fails.
We will develop and make available open source software to process near-infrared snowpit images and to map stratigraphy of the profile. Our software, combined with the low cost and speed of near-infrared digital photography, could make high resolution quantitative stratigraphy available to a broad user group beyond the research community, including guides, ski patrollers, avalanche forecasters, and possibly recreational users. Our analysis of failure during storms will shed light on this important set of avalanches, distinct from those that fail on deep, weak older layers. Ultimately, our results and techniques may reduce the increasing number of avalanche fatalities in North America.
Especially in Washington, Oregon, and California -- and other maritime mountains worldwide -- storm snow avalanches are the most common type of avalanche and responsible for most avalanche fatalities. By "storm snow," we mean snow that fell recently or is still falling. Using near-infrared imaging, we were able to publish the first in situ measurements of weak layers within storm snow. We found three layering combinations: (1) failure at thin weak layers sandwiched between thicker layers; (2) failure at an interface between two thick slabs; and (3) failure within a slab, at neither a sandwiched weak layer nor an interface. In almost all storm snow slabs, near-infrared imaging shoed high variability in grain size within the slab. We therefore find that contrary to what is commonly thought (that a weak layer is required for an avalanche) a specific weak layer or interface is not required for an avalanche in storm snow. This is a transformative finding in snow avalanche research. Although well defined weak layers can form in storm snow, avalanches in such snow often fail within the new snow layer. Because storm snow is so weak and full of flaws, cracks can form in many places throughout the new snow slab. Weakness throughout the slab also explains why storm snow fractures tend to be ragged, rather than clean and planar. Although one can think of storm slabs as being comprised entirely of weak layers, in this case the idea of a single weak layer loses meaning. Our findings have implications for how snow stability is assessed, a significant problem for those living in mountain communities (a geographically underserved population). This improved understanding of snow failure could potentially reduce avalanche accidents, injuries, and fatalities by dissuading the myth that a weak layer is required for an avalanche.
