Cold-to-warm flow regime transition in snow avalanches

• Published in: The cryosphere Vol. 12; no. 12; pp. 3759 - 3774
• Main Authors: Kohler, Anselm, Fischer, Jan-Thomas, Scandroglio, Riccardo, Bavay, Mathias, McElwaine, Jim, Sovilla, Betty
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 30-11-2018; Copernicus Publications
• Subjects: Avalanches; Cold; Cold flow; Composition; Entrainment; Flow; Impact loads; Landslides; Powder; Radar; Radar data; Snow; Snow avalanches; Snow cover; Snowpack; Temperature; Temperature distribution; Temperature effects; Thermal properties; Weather forecasting
• ISSN: 1994-0424; 1994-0416; 1994-0424; 1994-0416
• DOI: 10.5194/tc-12-3759-2018

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 (T‾<-1 ∘C) at high elevations near the surface, but warm
(T‾>-1 ∘C) 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
Ft, 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 Hs,
which represents the point where the average temperature of the uppermost
0.5 m changes from cold to warm. We find that Ft is related to
the snow cover properties, i.e. approximately proportional to Hs.
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 Hs 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.