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The Dischma experiment: how does interaction with the atmosphere determine the depositing and melting of snow?

 

The depth of the snow cover in various regions of the mountains plays a crucial role not only in avalanche formation, it also influences the reach of available water resources. Precise estimates of the amount of water contained in the snow remains difficult, however, because little is known about the mechanisms that influence snow distribution and melting. Thus within the framework of the Dischma experiment, scientists are examining the following questions:

  • Which atmospheric processes determine snow deposits in wintertime Alpine terrain?
  • How does the energy balance of a thinning snowpack shift in the spring? And why do some patches of snow persist longer than others?

For this purpose, precipitation fields, snow distribution and the melt dynamics of the Alpine snowpack are analysed, supported by field experiments and numerical modelling in the Dischma valley (Davos district, Fig. 1), a high-altitude Alpine catchment zone. In particular, the interaction which takes place between the near-ground atmosphere and the snowpack is examined.

Fig. 1: The Dischma valley in spring. The landscape becomes very patchy because some areas of ground are bare while others are covered with snow. In view of its U-shape (thermal winds) and good accessibility, as well as the only marginal influence of human activity, it is an ideal location for conducting snow hydrological and meteorological investigations.

Project details

Project duration

2014 - 2017

Project lead

Dr. Rebecca Mott-Grünewald

 

Snow Hydrology

Snow and Atmosphere

mott(at)slfto make life hard for spam bots.ch

Davos

 

What are the factors determining the spatial distribution of the winter snowpack?

Investigations have shown that the distribution of precipitation a few hundred metres above the ground is much more even than the distribution of snow on the ground. In order to examine the near-ground processes that give rise to this disparity, scientists taking part in the Dischma experiment are tracing the path of snow particles from their formation to their moment of contact with the ground. The specific questions being addressed by the research are:

  1. What influence does the terrain exert on the formation of precipitation (cloud microphysics and dynamics) and the distribution of precipitation in an Alpine catchment zone?
  2. How does preferential depositing of precipitation on lee slopes contribute to precipitation variability?
  3. How does snow transport contribute to precipitation variability?
  4. How much snow is deposited on steep rock faces and what effect does a turbulent windfield have on the depositing of snow there (Fig. 2)?
Fig. 2: Rock face in the Dischma valley. The accumulation of snow on the rock face differs markedly from the pattern of accumulation on the windward lower slopes.
 

What happens when the snowpack is partially thinning, and why do some patches of snow persist longer than others?

Fig. 3: Weather radar for measuring the concentration of precipitation in the air.

When the winter snowpack thins in springtime and individual patches of bare ground appear, the energy balance shifts significantly. Near-surface layers of air become warmer above bare ground than above snow. The wind transports this warmer air over the areas still covered with snow, raising the temperature of the snowpack there. While upslope winds can arise above the bare areas, downslope winds occur at the same time above the larger snow patches. This gives rise to a complex wind system that can, as a result, substantially influence the heat exchange between snow and atmosphere. In addition, local differences in the micrometeorology enable individual patches of snow to persist much longer than others. In the Dischma experiment, researchers are analysing this energy balance and the melt dynamics of the thinning snowpack by way of extensive measuring campaigns. They are also investigating how the various processes influence the longevity of individual snow patches, and quantifying the hydrology of the entire catchment zone. This aspect of the project gives rise to the following research questions:

  1. How much energy, in the form of sensible heat, is transported from bare areas to the snow, and how does this quantity change, depending on the snow cover?
  2. How does the turbulent heat flux above the snow change as a consequence of atmospheric stability (e.g. atmospheric decoupling)?
  3. What complex wind systems arise from the simultaneous occurrence of downslope and upslope winds, and how do these influence the energy balance of the snowpack?

What are the benefits of these investigations?

The goal of this research project is to understand fundamental processes to the extent necessary to allow their parameterisation and integration in hydrological and meteorological models. Among other outcomes, this can lead to improvements in the management of Alpine water resources, related to hydropower for example, and in the forecasting of floods. Furthermore, such models can indicate how water resources are likely to be affected by climate change.

 
Fig. 4: Ultrasonic anemometer to measure atmospheric turbulence and turbulent heat transfer.

Numerical models

The researchers are using the Weather Research and Forecasting (WRF) model to analyse the effect on the precipitation field of the atmospheric flow and cloud microphysics. They are examining the particle dynamics with the aid of large eddy simulation (LES), in particular by simulating the interaction between local atmospheric turbulence and particle movement in the air. For the numerical analysis of the melt dynamics they are using the snowpack and surface process model ALPINE3D, which is driven by high-resolution atmospheric fields (3D fields: temperature, humidity, wind) from the atmosphere model ARPS (Advanced Regional Prediction System).

Field experiments

In collaboration with the Environmental Remote Sensing Laboratory of the Swiss Federal Institute of Technology (EPF), Lausanne, scientists are measuring the precipitation field above the Dischma valley with high-resolution weather radar (Fig. 3). In order to establish how the precipitation is actually distributed on the ground, they are also measuring the snow distribution before and after major precipitation events by terrestrial and airborne laser scanning.

In order to capture the boundary layer flow and heat exchange above the thinning snowpack, the wind and temperature are being recorded at different altitudes with a high temporal resolution of 20 Hertz (Fig. 4), which also reveals their high-frequency fluctuations. These fluctuations provide insights into turbulent structures within the boundary layer and the turbulent heat exchange above individual snow patches. A large number of mobile weather stations measure the large-scale wind and temperature fields (www.sensorscope.ch).

The researchers also record ground temperature with an infrared camera. High-resolution temperature fields are provide insights into the dynamics of the warming and cooling of the patchy bare and snow-covered areas. This will also enable scientists to identify areas of persistent cold air, which foster the survival of snow patches.