Measuring and modelling spatial variability of snowpack stability

In the winter 2006/07, SLF put the Wannengrat research area into action in the area of Strelapass-Wannengrat-Chörbschhorn. The research area is equipped with a total of seven automatic measurement stations in order to record the detailed windfield and solar radiation and temperature conditions in complex alpine terrain. The research area is a part of Swiss Experiment, which aims to measure and interpret the natural processes and changes thereof using dense networks of sensors.

The Wannengrat measurements are being carried out in order to increase understanding on how avalanches form. Intensive measurement campaigns will be carried out in addition to the automatically measured data, in order tomeasure the snowpack features and their spatial variation. In these campaigns, snowpack stability will be particularly closely observered. The understanding of the creation and temporal development of the spatial stability variations is of utmost importance in avalanche danger level forecasting. With the help of computer models (SNOWPACK, Alpine3D), the snowpack properties and their spatial and temporal variations will be simulated. The measurements and observations in the Wannengrat research area allow the verification and further development of these models, which in future will provide an additional information source in the evaluation of avalanche danger by the avalanche warning service.

Snowpack Stability Research

Spatial variability stands for differences in a snowpack at different scales. The snowpack can vary in a multitude of parameters like snow depths, surface characteristics, and weak layer characteristic and distribution. Reasons are mainly changing meteorological parameters (wind and radiation) due to terrain effects like slope, azimuth or shape but are only known in a qualitative sense. The snowpack also varies at different scales, i.e. in a slope influencing fracture initiation and propagation of slab avalanches in a negative and positive manner and in a region. At this scale it is important to know if weak layers or interfaces are present or not, how they are overlain (slab properties) and if there are patterns related to azimuth or elevation. Buried surface hoar for example, which is often related with slab avalanches, shows a big spatial variation. Reasons are that the fragile crystals can be easily destroyed by wind or the initiation conditions can vary because of small catabatic wind systems or solar radiation differences. In that project the focus is on the regional scale. In order to detect these reasons field measurements will be done to record snowpack and stability characteristics (cp. fig. 1).

Fig. 1: Rutschblock test (J. Schweizer)

This data set will be used for verifying a model complex called ALPINE3D which consists mainly of a snowpack model SNOWPACK, an energy balance and a wind drift model. The modelled incoming shortwave radiation can be seen in fig. 2, which shows a large variability.

Fig. 2: Modelled shortwave radiation (M. Schirmer)

A validation of the combination of the different models is presently lacking. The question, if ALPINE3D is able to model stability related patterns which are recognised in the field work, has to be solved. A study site is chosen in the surrounding of Davos and is provided with seven automatic weather stations (fig. 3) to assess first model input parameters and second verification data.

Fig. 3: Automated weather station (M. Schirmer)

A terrestial laser scanner (fig. 4) is used for mapping the spatial distribution of snow depth to control modelled snow transport output.

Fig. 4: Terrestrial laser scanner (M. Schirmer)

If the model is found to work accurate, the output can help finding reasons for the spatial variability since many important parameters cannot be measured in a high resolution in space an time. A stability evaluation for the region derived from the modelled snowpack will be related to predicted avalanche danger degree, avalanche activity and observed stability assessment to conclude if ALPINE3D can be a helpful tool for the operative avalanche forecast.

Participants:

• Michael Schirmer
• Sascha Bellaire
• Jürg Schweizer
• Michael Lehning
• Charles Fierz

• Michael Lehning