Spatial and temporal analysis of snowpack strength and stability and environmental determinants on an inclined, forest opening
Avalanche hazard evaluation relies on snowpack stability observations. Because snowpack properties can vary extensively over time and space, estimating slope-scale stability is difficult. This study addressed these challenges by implementing a methodology that 1. quantified spatial and temporal patterns of snowpack stability, 2. identified spatial associations between the strength and stability of a weak layer and slab load, and radiation properties, 3. identified internal associations between weak layer thickness, shear strength, microstructural properties, and slab load. An instability associated with a buried surface hoar weak layer was examined on an inclined forest opening at Lionhead, southwest Montana, during February and March, 2005. During five sampling days, 824 snow depth and SnowMicroPen resistance profiles and 352 shear frame tests were performed. An objective texture-based stratigraphic sampling approach was developed to obtain microstructural estimates of a stratigraphic weakness and instability from SnowMicroPen profiles, utilizing the coefficient of variation of rupture force. Spatial models of hemispheric sky visibility, and incoming long- and shortwave radiation were generated for the surface hoar formation period using a Geographic Information System and independent optical observations. Despite relative topographic uniformity, in a distance of 30 m, the buried surface hoar weak layer thicknesses varied between 3 - 21 mm. Before burial, the surface hoar persisted despite moderate winds and above freezing air temperatures. Spatial patterns of modeled incoming longwave and shortwave radiation explained the large variation in weak layer thickness and strength properties. Areas exposed to large amounts of radiation contained a strong, thin buried surface hoar layer, while in areas with limited incoming longwave (due to high sky visibility) and shortwave radiation (due to shading), the layer was thicker and possessed low shear and microstructural strengths. Over time, the shear frame stability index and SnowMicroPen-derived microstrength of the surface hoar layer increased and values became spatially more variable (divergence): it became harder to predict stability as the snowpack became more stable. A loading event then decreased stability and micro-strength and caused spatial uniformity (convergence), thereby increasing predictive strength of observations. The findings illustrate the usefulness of the SnowMicroPen for evaluating spatial patterns and load-related changes in snowpack stability.