Temporal evolution of weak layer and slab properties in view of snow instability
Abstract. If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests on seven sampling days during a two-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, showed a distinct pattern of temporal evolution that differed from the trend of other mechanical properties suggesting that it is not possible to assess crack propagation propensity by simply monitoring some of the relevant mechanical properties. A simple sensitivity study showed the complex interplay between these properties. Traditional and newly-developed metrics of snow instability describing either the failure initiation or the crack propagation propensity, calculated from simulated snow stratigraphy (SNOWPACK) or derived from the SMP signal, did partially reproduce the observed temporal pattern. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.