Assessment of snow transport in avalanche terrain

A local to regional assessment of transported snow during snow storms or subsequent periods of strong winds is a prerequisite to reliably estimate avalanche danger. Despite the fact that it has received continuing attention for decades, the problem of quantifying snow transport persists. Systems from point measurements to full three-dimensional simulations have been tested but all have their respective weaknesses. We present a new drift index, which has been tested and operated with some success in Switzerland. The index requires input from a wind-sheltered automatic weather station and a scaled wind speed from a wind-exposed site. Using the snow cover model SNOWPACK, the meteorological data is extrapolated to the four main aspects and snow cover development is calculated for these aspects. Depending on the measured wind direction and speed, a threshold condition for snow erosion at the upwind aspect is tested: if the wind is strong enough to erode the current snow at the surface of this aspect, the snow layer is eroded, transported and deposited onto the downwind aspect. With this scheme, the virtual, “representative” snow cover on the four main aspects in the vicinity of the meteorological stations are reconstructed for the course of the winter and the mass transport rate is converted to a lee-deposition drift index. A comparison with FlowCapt, an acoustic measurement device, which measures a local mass flux, shows that the measured mass flux correlates well with the amount of lee-slope deposition predicted by the drift index. Also, drifting snow periods are well detected by both the FlowCapt sensor and the SNOWPACK drift index and correspond to drifting snow periods reported by local observers. When comparing regional patterns of strong and weak snow transport as calculated from more than 110 automatic weather stations in the Swiss Alps with corresponding reports from local observers a good correlation is found, too. As opposed to earlier versions of the index, which had been based on flat field simulations of SNOWPACK alone, the new index no longer overestimates intensity and duration of blowing snow events. It is concluded that for the purpose of avalanche warning, the FlowCapt sensor and the SNOWPACK drift index are suitable means to quantify local to regional snow transport.