International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
After some initial reliability problems were resolved, the SM4 was able to measure the snow
depth with acceptable accuracy for avalanche forecasting. From the data, it is easy to
distinguish the sensors buried in snow from the ones above the snow surface. Figure 2 shows
data from all the sensors of a SM4 unit. The graph shows very little fluctations for the sensors
buried in snow, while the sensors above the snow surface display greater fluctation in
temperature. The blue line shows an example of a thermistor that is above the surface in the
beginning of the period, but becomes covered by snow on March 6 th . Figure 4 shows the
temperature profile from an SM4 sensor located in Kistufell by Ísafjörður. The snow depth is
at the upper brake of the gradient. During this period, the snow depth increased from 180 cm
to 250 cm.
Figure 4. Temperature profiles measured by SM4 with five days interval.
An algorithm that calculates the snow depth from a time-series of temperature profiles
through the snowpack has been developed, and gives promising results. Figure 3 shows the
calculated snow depth and compares it with snow depth data from the ultrasonic sensor. (Note
that the timespan is the same as for Figure 2.) The gap in the data from the ultrasonic sensor is
considered to be due to icing. The challenge regarding the algorithm is greatest when the
temperature of the atmosphere approaches the temperature of the snowpack.
For IMO it is of special interest to test the reliability of SM4 during icing periods since the
ultrasonic sensors do not work well under those circumstances. The following picture (Figure
5) was taken on December 9 th and shows the instruments with an icing coat. The ultrasonic
sensor had not been working in the days before, and it is considered very likely that the reason
was icing. The data from SM4 seems correct from those same days (Figure 6), and therefore,
it can be concluded that icing in this magnitude does not interfere with the air temperature
measurements of SM4.
154 The SM4 snowpack temperature and snow depth sensor