The effect of surface warming on slab stiffness and the fracture behavior of snow

Surface warming is among the most complex contributory factors that need to be considered when forecasting dry-snow slab avalanches. The aim of the present study is to quantify surface warming with respect to the contributing meteorological processes and to investigate in situ crack propagation propensity under conditions of surface warming. The energy fluxes at the snow surface, partly measured and partly modeled with the snow cover model SNOWPACK, were used to determine the energy input into the snow cover. Stiffness of the near-surface layers and its changes with daytime warming were derived from penetration resistance measurements with the snow micro-penetrometer (SMP) and related to the energy input. Changes in fracture behavior were assessed with the propagation saw test (PST). An average reduction in stiffness by a factor of about 2 was observed in near-surface snow layers when the cumulative energy input at the surface exceeded 300 kJ m(-2). At the depth of the weak layer (-40 cm) changes were rather small; in particular for the specific fracture energy no trend was detected with warming. Critical cut lengths tended to decrease with decreasing slab stiffness, suggesting that surface warming increases crack propagation propensity. However, the effect seems to be subtle. It is suggested that a pre-existing weakness and significant energy input are required for surface warming to promote instability. (C) 2012 Elsevier B.V. All rights reserved.