The amount of water available for power plants and farming depends on how much snow there is on the ground in the high mountains. Now a research project in the Dischma valley in Davos is providing new findings about the processes that play a role in the accumulation and melting of snow.

Prof. Michael Lehning and Rebecca Mott of SLF in the field at a weather station in Davos’ Dischmatal valley.

Weather permitting, a group of researchers drive to the Dischma valley in Davos early in the morning, from April to June, often on a quad-bike. The evening before, together with the responsible safety officials they have closely studied the weather report, the information from the local weather stations, and the snow situation. They have also checked with the avalanche warning service to make sure fieldwork can be carried out in the Dischma valley without the risk of an avalanche. Nonetheless the researcher never travels alone. For the Dischma experiment, a project of the WSL Institute for Snow and Avalanche Research SLF and EPFL, which is partly funded by the Swiss National Science Foundation (SNSF), the project group includes a scientist, a postdoc, four doctoral students and Master’s degree students, who are participating in the field work.

The test area is a hillside in the higher reaches of the valley that stretches southeast from Davos towards the Engadin. “It is quieter here than for example in the neighbouring Flüela valley”, says Rebecca Mott, scientist at SLF, explaining why the Dischma valley was chosen for the experimental field. What is more, the valley has a simple form, almost a perfect U-shape, which is ideal for a model site. The scientist sets up her measuring instruments together with her colleagues. They mount the measuring head  of a laser scanner on a camera tripod. This is used to record the precise distribution of snow on the surrounding mountain slopes.

The laser scanner measurements are a part of a research project to investigate how interactions with the atmosphere control the deposition and melting of snow in the alpine regions. “We want to know where the snow is deposited in winter, and where it contributes to the melt”, explains Michael Lehning, head of the SLF research unit “Snow and Permafrost” and Professor for Cryosphere Research at the EPFL. There are only a few snow measur-ing stations in the high mountains, and they do not provide very informative data. And studies in the past few years have shown, for example, that in the peak regions rather less snow lies on the ground than expected. “This is a crucial gap in our understanding, when one is trying to assess the total water resources in the high mountain areas”, the scientist says.

Reliable statements about the alpine water resources are important for the generation of electrical power at the hydroelectric plants, as well as for agriculture and for predicting when floods will happen. For especially in spring and summer the high mountains are the source of a particularly large proportion of the total quantity of water. Added to this is the fact that the consequences of global climate change can only be estimated if one knows how much snow is presently lying in the mountains. The experts expect that at the end of this century the snow situation in Davos at 1,500 metres altitude will roughly correspond to that which presently prevails in the Küblis region 700 metres lower down. But no-one yet knows exactly what effect global warming will have on the high mountain regions. “Our project intends to help provide an answer to this question”, says Michael Lehning.

Distribution of precipitation and snow: extreme variations

Thanks to the laser beam, researchers can measure the change in snow  distribution with centimetre accuracy.

The laser beam in the Davos test area measures the precise distance to the target. By using the figures obtained before and after a fall of snow, the researchers can determine how the distribution of snow has changed, to within a couple of centimetres. A map is prepared showing the places where snow was transported away in blue, and where it has accumulated in red. “There are sometimes big differences between one and a half metres more, or half a metre less snow”, the expert Rebecca Mott explains. The map also shows whether a portion has slid down as it does during an avalanche. There is one phenomenon that is of particular interest to the researchers here, which they call “preferential deposition”. For this they compare the data from the laser scanner to the precipitation values that were provided by a high-resolution weather radar system last year. This system is set up so that it faces out towards the test area from Davos Parsenn, and measures the concentration of precipitation in the air. The researchers found that the precipitation above the Dischma valley was quite evenly distributed, but not so the distribution of snow on the ground. “We noticed quite a big difference here, and we want to find out exactly what happens in the intervening period”, says Michael Lehning.

First results: Evidently the snowflakes are caught up by eddies in the air even before landing on the ground, and this carries them to be deposited in certain areas more than others. What is more, it appears that various processes take place in the snow cloud that tend to lead to a concentration of precipitation above a peak or ridge. “Until now people thought that the snow falls quite evenly, and is then carried from the crest of the mountain down to the lee, meaning the side facing away from the wind direction”, explains Rebecca Mott. What this means is that besides the classical processes which blow away the snow that is already lying on the ground and transport it elsewhere, winds can exert a strong influence on the fine particles even before this, so that they tend to get deposited in certain areas more than others.

The weather radar system that the EPFL had initially provided has now been dismantled again and shipped off to the Antarctic, for another measuring project. But MeteoSchweiz plans to install a new system on the Weissfluh peak in 2016, and the SLF researchers hope to use the data it provides to support their hypotheses.

Besides the deposition of snow, as part of the Dischma experiment the team is also studying the melting of snow. For this, Rebecca Mott and her colleagues are installing an infra-red camera on the hill of the test area, next to the laser scanner. The camera faces down below onto an area that thaws, where snowflakes alternate with snow-free regions. She takes a picture every couple of seconds. These can be used to determine how the temperature varies on the ground over a period of several hours, in a high-resolution image. “In this way one can see how the dynamics work across the intermittent snow-covered patches”, the researcher says.

The role played by the wind system

The air above the clear, thawing areas warms up more than it does above the snow. Therefore upwinds can develop at these points, while downwinds occur over the snowy patches. This complex wind system, in turn, has a strong effect on the heat exchange between ground and atmosphere, and therefore on the continued melting of the snow-cover. “With the help of the infra-red camera we can see how pools of cold air and cold regions can form”, Rebecca Mott explains.

To record the wind circulation more precisely, in autumn 2015 the researchers transported a Doppler Lidar device (light detection and ranging) to the Dischma valley, and installed the two-metres by one-and-a-half metre box, which also belongs to the EPFL, on top of a container. Rebecca Mott accompanied and assisted the Lidar experts who had specially come from the USA to carry out the extensive measurements for two weeks. Doppler Lidar devices work in a similar way to radar systems, except that laser beams are used instead of radio waves. The rays are scattered back from particles in the atmosphere, thereby allowing one to determine wind speeds and directions along the laser beam. The technology is usually used for finding suitable locations for wind recording stations.

“The Lidar provides an almost three-dimensional image of the air currents”, Michael Lehning points out. In this way the local thermal wind systems can be recorded, which are caused by the topography and which exert a strong influence on how the snow melts away. By evaluating the data the researchers hope to find out more information about the wind currents that cause the snow to tend to pile up more in certain locations. “It is important to have an accurate picture of this ‘wind field’ in order to understand the preferential deposition”, explains the scientist. “This is why such measurements are very useful.”

A dense network of automatic weather stations complements the measurements that the researchers are performing in the test area using the Lidar, laser scanner and infra-red camera. Michael Lehning likes to recall the day when, early on at the beginning of the project, he ventured into the valley on cross-country skis, to dig a few holes by hand for weather stations. Rather more serious was the mood evoked among the researchers in winter 2015, when they learned that one of the stations had been buried under an avalanche. “We had classed the site as relatively safe, but the facts taught us a different story”, the expert narrates.

The Dischma experiment, which is partly funded by the SNSF, will run from 2014 to 2017. The intention is to feed the gathered data into models which the researchers can use to study and understand the interactions between the snow and the atmos-phere. In this phase, too, the researchers in Davos are working closely with their colleagues at EPFL, to achieve better estimates concerning the alpine water resources.