Initial situation

A knowledge of the snowpack structure is crucial for avalanche forecasting purposes. Thus far, information on the various layers has been obtained from snow profiling, but this is a time-consuming exercise that cannot be performed in every location and is sometimes ruled out because of the avalanche danger. Furthermore, snow profiles do not capture changes in the snowpack over time because the layers are destroyed when the pit is dug.

For this reason, researchers from Heidelberg University (in collaboration with the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven), the JOANNEUM University of Applied Sciences in Kapfenberg, Austria, and the SLF have installed special radars in the ground at the Weissfluhjoch test site and in the vicinity of an avalanche path on the Dorfberg. The radars conduct a non-destructive investigation of the snowpack from underneath. Looking ahead, it is conceivable for such a measuring system to be buried directly in the path of an avalanche without exposure to the danger of being swept along in an avalanche event. Since avalanche prone slopes can be remote and not always accessible, the radars must operate autonomously. They are powered by solar energy from a safe distance and deliver data to the SLF by radio.

Fig. 1: Radar		Fig. 2: Radars are powered by solar energy from a safe distance.

Initial results

These ground-based radar systems were operational throughout an entire season for the first time in the winter of 2010/2011. As illustrated by the data (Fig. 3), they are capable of measuring the absolute snow depth (green line) and tracking the layers that exist within the snowpack and their change over time. After the snowfall of January 26, for example, the graph shows how the 30 cm of fresh snow settled during the ensuing 2 weeks, while the thickness of the deep layers remained more or less constant.

The radar data also clearly reveal the penetration of the snowpack by moisture (Fig. 4). The surface first becomes moist on March 22. From March 23 until April 1, the water accumulates 1.4 m above the ground. Thereafter, the moisture penetrates the deeper layers.

Outlook

The radar is the first system capable of measuring the internal snow layers. The two radars are operating again throughout the winter of 2010/2011. The aim is to use the data to establish the density, and the liquid water content and its change in the course of a day, for each of the layers. The researchers also intend the use the radar data to update and verify the SNOWPACK model in future. If the radar observes discrepancies caused by snow transportation, for example, it may be necessary to correct the model's snow depth parameters.

The project is being sponsored within the framework of a three nations programme by the German Research Foundation (DFG), the Austrian Science Fund (FWF), and the Swiss National Science Foundation (SNSF).

Fig. 3: Processed radar data, collected at the Weissfluhjoch test site from 26.1. until 22.2.2011. The green line shows the snow depth measured by the radar. The reddish lines represent the snow depth measured by two conventional instruments. The graph shows two snowfalls and the settling of the snow between the two events.
Fig. 4: Processed radar data, collected at the Weissfluhjoch test site from 22.3. until 8.4.2011. The green line shows the snow depth measured by the radar. The reddish lines represent the snow depth measured by two conventional instruments. The transition from dry to wet snow is indicated by a strong reflection, so that the graph clearly shows the progressive penetration of the snow (from the surface to the ground) by moisture.