Wind slab formation: How do wind slabs form in snow?
The alpine snow cover is often influenced by wind. It is a major factor for the redistribution of snow (wind drift) and also shapes the surface of the snow cover. Wind creates different erosional and depositional surface features, such as zastrugi or dunes. Moreover, a snow surface subjected to wind influence is sometimes hardened, a wind slab is formed.
This project aims at studying this formation process. What are the physical processes going on and under what conditions are the slabs formed? Furthermore, there seem to be different types of wind slabs. Sometimes they are quite soft and up to a meter thick, sometimes they are very hard but have a thickness of only a few centimeters. Which conditions lead to which type of snow slab and why?
2015 - 2018
Better knowledge of the formation of wind slabs will allow to improve understanding of important processes such as the mass balance in polar regions or the exchange of chemical species between the snow cover and the atmosphere. It will also improve snow cover models (i.e. SNOWPACK) as well as the forecast of avalanche danger.
It is hypothesized that the wind must be humid (close to saturation) to be able to form a wind slab and that collisions between particles and mechanical compaction are mainly responsible for the hardening. Which type of wind slab is formed may depend on whether snow drift takes place or not, and may thus ultimately depend on the wind velocity.
To test these hypotheses a special ring-shaped wind tunnel is being built at SLF (Figure 2). The facility is ring-shaped to be able to simulate a infinitely long snow surface (infinite fetch). Because the formation of a wind slab takes some time, a typical straight wind tunnel is usually too short to observe this phenomenon. The wind tunnel will be equipped with different instruments to measure the relevant quantities, such as wind velocity, air temperature and humidity as well as several properties of the snow surface layer.
The experiments in the wind tunnel will be complemented by measurements in the field to determine under what meteorological conditions wind slabs are formed.