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Formation of wet snow avalanches

 

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.

Lack of previous research into wet snow avalanches

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.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. 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.