Fig. 1: Wet snow avalanches at the
Dorfberg above Davos (photo: W. Steinkogler/SLF).
Fig. 2: Rain and melt close to the surface form water in the snow cover. When water accumulates at a boundary between layers, the strength of the snow is reduced and avalanches can occur.
Fig. 3: Water can strengthen the connections of snow crystals (capillary forces), as in this picture. However, it can also destroy the bonds and weaken the connections (photo: C. Fierz/SLF).
Fig. 4: Tracer experiments show the complex flow paths of water in the snow cover (Foto: Ch. Pielmeier/SLF)
Wet snow avalanches occur predominantly in the spring. They are
typically released following a period of high air temperatures (above zero
degrees) and strong radiation in the spring, or during rainfall (in mid-winter
as well). Wet snow avalanches can endanger infrastructure, settlements and
people in the mountains. Around one in ten fatal avalanche accidents in the
Swiss Alps is caused by a wet snow avalanche. Unlike avalanches consisting of
dry snow, wet snow avalanches mostly occur naturally; they are seldom triggered
artificially (e.g. by a skier), and avalanche blasting is unlikely to be very
successful.
Lack of previous research
into wet snow avalanches
The mechanisms that give rise to
wet snow avalanches have been the subject of only little research to date. It
is evident that the stability of the snowpack is determined by the penetration
of melt water and rainwater, and by the interaction of the water with the
surrounding snow. In order to predict the timing and magnitude of wet snow
avalanches, consideration must be given to both the meteorological conditions
and the state of the snowpack. Measuring the properties of the snowpack is
difficult because they change rapidly when the snow is penetrated by water. The
unstable conditions arising from such penetration are often short-lived, and
they can differ significantly, depending on aspect, slope angle and altitude
zone.
Radar facilitates new
measurements
It is therefore essential to
know exactly how much water is present and how it is moving through the
snowpack. Given that water can move through layered snow in a consistent or
inconsistent pattern, its complex accumulation and runoff behaviour makes the
positioning of measuring instruments in the appropriate places a difficult
process. This behaviour likewise complicates the numerical simulation of water
movement in the snowpack. Sensors inserted directly in the snow can also
influence the measurement of the water content and
thus create a false picture. Radar devices installed in the ground to
investigate the snowpack from underneath are more suitable. Radars installed at
the Weissfluhjoch test site (2540 m) and on the Dorfberg (2240 m) above Davos
allow researchers to measure the water content and infiltration rate without
either disturbing or destroying the snowpack (see project entitled "Snow
layer observation from the ground with radar systems"). It has not been
possible thus far, however, exactly to calculate the quantity of water
contained in the individual snow layers
Snow profile with wet snow
As with dry snow, digging a snow profile in wet snow is a key investigation
method. Analyses of profiles excavated in rather unstable snowpacks on the one
hand, and in the starting zones of wet slab avalanches on the other, regularly
indicate the existence of an isothermal state (snowpack warmed to zero degrees
throughout) as well as soft, often faceted layers and a high water content. Key
variables, such as the amount of water contained in a snow layer, are difficult
to determine, however, without quantitative measuring instruments. Compared
with the results of objective measuring methods, even experienced "snow
connoisseurs" tend to overestimate the snowpack water content.
The greater the energy, the higher the number
of wet snow avalanches
Snow cannot melt until the
temperature of the snowpack rises to 0°C, and then only if surplus energy is
available. The melting process is thus governed by the snowpack's energy
balance; in other words, by the amount of energy captured by the snow and
subsequently released to atmosphere. The SNOWPACK model facilitates an analysis
of which components of the energy balance influence snow melt most
substantially before and during a period of elevated wet snow avalanche
activity (Fig. 5). The sun, by way of shortwave radiation, is the most
significant energy source, but energy also enters the snowpack through the
exchange of heat from the air. This sensible heat flux is not easy to determine
because it arises from the interaction of air temperature, snow surface
temperature and wind speed. In early spring in particular, when the strength of
the sun is still fairly modest, the energy input from this source can exceed
that being generated by shortwave radiation. But snow emits energy as well, in
particular by way of longwave radiation. If the sky is clear, this outgoing
radiation from the snowpack is lost in space, and the surface of the snowpack
can become much cooler than the air temperature. The latent heat flux – the
energy required to melt or sublimate snow – also eliminates heat from the
snowpack. As a general rule, the more positive the snowpack's energy balance,
the greater the probability of melting and, therefore, of wet snow avalanches
occurring.
Combination of radiation and
air and snow temperature data gives rise to reliable forecasts
As a general rule, models for
forecasting wet snow avalanches make use of either energy balance figures or
conventional meteorological variables. The accuracy of the forecasts produced
by each type of model is similar. Forecasting is most reliable with both models
when data concerning the energy input (shortwave radiation and air temperature)
are combined with data illuminating the heat balance of the snowpack (snow
temperatures). The information about the snowpack is important in order to
establish if the surplus energy is still being consumed in raising the
temperature of the snowpack, or if snow is already melting and producing melt
water.
Fig. 5: Modelled energy balance
(white line) in April 2008 for a 35° south facing slope above Davos. Where the
line is deflected upwards, the energy is entering the snowpack. The orange bars
symbolise the incoming solar radiation, the red bars the sensible heat flux,
the grey bars the longwave radiation, and the blue bars the latent heat flux.
Grey shading highlights periods of high wet snow avalanche activity.
These research findings have
been published in a paper entitled "Analysis of the snow-atmosphere energy
balance during wet-snow instabilities and implications for avalanche
prediction" in the journal "The Cryosphere" and elsewhere (pdf paper).
