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Cold Regions Science and Technology 30 (1999) 125—133
FlowCapt: a new acoustic sensor to measure
snowdrift and wind velocity for avalanche forecasting
V. Chritin, R. Bolognesi, H. Gubler
IAV Engineering, PSE/B, Ecole Polytechnique Fédérale
de Lausanne, Lausanne, Switzerland
Abstract
Wind can create even greater unstable accumulations of snow in mountainous areas than heavy snowfalls. But knowing wind conditions is not sufficient to predict these accumulations because their formations also depend on the snow quality of the snowpack surface upwind of the release zone. Consequently, assessment of snowdrift is required to improve avalanche forecasting. In accordance with this assumption, a new acoustic sensor was developed. The sensor includes a mechanical part designed to form a closed acoustic enclosure. The acoustic enclosure contains microphones connected to an electrical amplifying and filtering device. Because the output information delivered by the instrument is proportional to the wind velocity and to the flux of solid particles (ice grains) drifted by the wind, the instrument is called an anemo-driftometer. Prototypes of the instrument were first tested in a wind tunnel and then at an experimental site in the Alps. Then an operational version, called FlowCapt, was developed and connected to an automatic weather station at 2700 m in the Aminona ski resort (Switzerland). During the winter, snowdrift is recorded on the test site along with other meteorological parameters, and avalanche activity. to provide extensive on-site calibration and testing of the sensor. The experiment demonstrates that the instrument is a useful component of the avalanche forecasting chain. © 1999 Elsevier Science B.V. All rights reserved.
Keywords: Snow drifting; Snow engineering; Avalanche forecasting; Acoustics; Jet roof
Keywords: Snow drifting; Snow engineering; Avalanche forecasting; Acoustics; Jet roof
1. A new snowdrift sensor to improve avalanche forecasting
To improve the reliability of local avalanche warning systems, parameters directly related to avalanche danger or slab stability have to be measured close to and within the potential avalanche release zones that endanger the area to be protected. Because wind can create even greater unstable accumulations of snow in mountainous areas than heavy snowfalls, snowdrift is a very predictive parameter. Wind speed measurements are not sufficient to predict these accumulations which also depend on the snow quality of the snowpack surface all around measurement stations. Knowing snowdrift by direct measurements is subsequently of high importance for avalanche forecasting.
At present time, snowdrift can be measured by ski patrol men (Fig. 1) with the so-called Driftometer (Bolognesi et al., 1995). This simple instrument makes possible quantitative snowdrift assessments, but it requires the presence of a human observer on the sites. If manual measurements are not possible, snowdrift would have to be estimated from other parameters with significantly lower reliability.
The new FlowCapt™ acoustic sensor gives the possibility of a continuous and automatic recording of the snowdrift (~ kg m-2 s-1).
Fig. 1. The Driftometer catches blown snow particles into a collector through a tube by the combined effects of filter and pressure fall. Weighing the collector directly gives a snowdrift index.
Installed upwind of the release zone, FlowCapt provides additional information on the snow accumulation process within the release zone, deformability of the forming slab and erodibility of the snow surface. Because it was shown that snowdrift data increases the reliability of avalanche predictions (Bolognesi, 1996), this information is used by the decision support system NivoLog™ to establish local avalanche predictions.
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