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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 thetemporal evolution of a weak layer of faceted crystals as well as theoverlaying slab layers at the location of an automatic weather station in theSteintälli field site above Davos (Eastern Swiss Alps). We focussed onthe crack propagation propensity and performed propagation saw tests (PSTs)on 7 sampling days during a 2-month period from early January to earlyMarch 2015. Based on video images taken during the tests we determined themechanical properties of the slab and the weak layer and compared them to theresults derived from concurrently performed measurements of penetrationresistance using the snow micro-penetrometer (SMP). The critical cut length,observed in PSTs, increased overall during the measurement period. Theincrease was not steady and the lowest values of critical cut length wereobserved around the middle of the measurement period. The relevant mechanicalproperties, the slab effective elastic modulus and the weak layer specificfracture, overall increased as well. However, the changes with time differed,suggesting that the critical cut length cannot be assessed by simplymonitoring a single mechanical property such as slab load, slab modulus orweak layer specific fracture energy. Instead, crack propagation propensity isthe result of a complex interplay between the mechanical properties of theslab and the weak layer. We then compared our field observations to newlydeveloped metrics of snow instability related to either failure initiation orcrack propagation propensity. The metrics were either derived from the SMPsignal or calculated from simulated snow stratigraphy (SNOWPACK). Theypartially reproduced the observed temporal evolution of critical cut lengthand instability test scores. Whereas our unique dataset of quantitativemeasures of snow instability provides new insights into the complex slab-weaklayer interaction, it also showed some deficiencies of the modelled metricsof instability – calling for an improved representation of the mechanical properties

Temporal evolution of crack propagation propensity in snow in relation to slab and weak layer properties