Item: Quantifying the obvious: the avalanche danger level
-
-
Title: Quantifying the obvious: the avalanche danger level
Proceedings: International Snow Science Workshop Proceedings 2018, Innsbruck, Austria
Authors:
- Jürg Schweizer [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Christoph Mitterer [ Avalanche Forecasting Service Tyrol, Innsbruck, Austria ]
- Frank Techel [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Andreas Stoffel [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Benjamin Reuter [ Montana State University, Bozeman MT, U.S.A. ]
Date: 2018-10-07
Abstract: The avalanche danger level is the key communication vehicle when it comes to describing the avalanche situation and issuing public warnings. Yet the foundations of the avalanche danger scale are fragile at best - not only from a scientific point of view, but also from an operational point of view they are rather indicative. Still, the avalanche danger level nicely summarizes key elements of avalanche danger: the release probability, the frequency and location of triggering spots and the potential avalanche size. However, none of these three elements is well defined and it is not fully clear how they are finally combined into one number - the danger level. Moreover, temporal and spatial scale issues further complicate the concept. For example, at the danger level 3-Considerable the release probability is described as possible, which translates into a probability of at least 33-66%. Combined with probabilities for frequency and location of triggering spots, assuming 33-66% corresponds to ‘many steep slopes’, we obtain a probability of 11-44% of triggering an avalanche. Is the forecast wrong if we ski ten very steep slopes and nothing happens? Obviously, the forecast is valid for a region, not a slope. Still, what does the release probability mean at the regional scale? We attempt to quantify the three key elements that define the danger level by evaluating a large data set of manually observed avalanches. The frequency of natural avalanches strongly increases with increasing danger level confirming that not only the release probability but likely also the number of triggering spots increases - non-linearly. However, no clear increase of avalanche size with avalanche danger level was observed, which suggests that the definitions of the danger levels should be revisited. Moreover, the frequency of wet-snow avalanches was found to be higher at some danger levels than the frequency of dry-snow avalanches, which may hint at inconsistent usage. With regard to proportional quantifiers such ‘many’, conclusions are not straightforward, but we suggest that ‘many avalanches’ means on the order of 10 avalanches per 100 km2. Data sets of manually observed avalanches are known to be inherently incomplete so that our results need to be confirmed using other similarly comprehensive data sets.
Object ID: ISSW2018_O11.6.pdf
Language of Article: English
Presenter(s):
Keywords: avalanche forecasting, snow instability, avalanche triggering, avalanche size
Page Number(s): 1052-1058
-