Cold-to-warm flow regime transition in snow avalanches

Köhler, Anselm; Fischer, Jan-Thomas; Scandroglio, Riccardo; Bavay, Mathias; McElwaine, Jim; Sovilla, Betty

Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (inline-formula M1inlinescrollmathml true T normal ‾ < - normal 1 35pt13ptsvg-formulamathimgc061c33e1332134bcc44ddd2fb44f3ee tc-12-3759-2018-ie00001.svg35pt13pttc-12-3759-2018-ie00001.png inline-formulaC) at high elevations near the surface, but warm (inline-formula M3inlinescrollmathml true T normal ‾ > - normal 1 35pt13ptsvg-formulamathimga42e9edec11b50e6d83c96975babeadb tc-12-3759-2018-ie00002.svg35pt13pttc-12-3759-2018-ie00002.png inline-formulaC) at lower elevations or deeper in the snowpack. The intake of warm snow is believed to be of major importance to increase the temperature of the snow composition in the avalanche and eventually cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete when the deposit at runout is characterized only by warm snow or as partial if there is a warm flow regime, but the farthest deposit is characterized by cold snow. We introduce a transition index inline-formulaFt, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude inline-formulaHs, which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that inline-formulaFt is related to the snow cover properties, i.e. approximately proportional to inline-formulaHs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find that, if inline-formulaHs is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

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Köhler, Anselm / Fischer, Jan-Thomas / Scandroglio, Riccardo / et al: Cold-to-warm flow regime transition in snow avalanches. 2018. Copernicus Publications.

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