Wind-packing of snow: How do wind crusts form?

Wind-packing of snow is the process responsible for the formation of wind slabs and wind crusts. These are hard layers of well-sintered snow often found in the snowpack in mountains as well as in polar regions. Wind-packing affects the local mass balance, the avalanche danger and how the snow cover interacts with the atmosphere. Yet, it is a poorly understood process. Many ideas about what wind-packing actually is have been proposed in the literature but there is almost no quantitative information about the involved physical processes available. A closed-circuit wind tunnel simulating an infinite fetch was specifically designed and built for this purpose. Experiments were performed with natural snow that was collected on a pair of wooden trays. The hardness of the snow and how it changed during experiments was measured with a SnowMicroPen. Meteorological parameters such as wind speed, air temperature, etc. were also measured. In the literature, there is conflicting information about the necessity of drifting snow for wind-packing. The wind tunnel experiments showed that drifting snow is a necessary but not sufficient condition for wind-packing. We observed no hardening during experiments without drifting snow, but not all drifting snow events lead to the formation of a wind crust. The spatial variability of the hardness of wind-influenced snow was high and this appeared to be related to the dynamics of erosion and deposition. To analyze the influence of these processes in more detail and quantitatively, a Microsoft Kinect sensor was added to the wind tunnel. This instrument measures the snow depth in the main test section with a high spatial and temporal resolution. These measurements showed that wind-packing only occurs through the deposition of snow. If drifting snow causes the snow to be eroded, there is no hardening effect on the remaining snow. Furthermore, the Kinect data allowed to explain about 50% of the observed variability of the hardness of wind-packed snow with only two parameters. Most important was the wind exposure. The snow became harder in more wind-exposed positions compared to wind-sheltered areas. We used the parameter Sx to characterize how wind-exposed a certain position is based on the upstream snow surface. A wind-sheltered position has a high Sx value and a wind-exposed position has a negative Sx value. To increase the range of Sx values, some experiments were performed with an artificial obstacle in the main test section, which created highly wind-sheltered positions. Sx alone explained almost 40% of the variability. The second parameter was the deposition rate. We found a negative correlation of -0.4 between the deposition rate and the snow hardness. Wind-packing is more efficient at creating a hard layer if the snow is deposited slowly. The effect of other parameters, such as wind speed, air humidity or initial density of the snow on the hardness was studied. However, there was either no robust trend in our data or the addition to the multilinear regression did not significantly increase the explanatory power of the model. This does not necessarily mean that these parameters are not relevant for wind-packing. The wind tunnel experiments provided quantitative information and new insights about wind-packing of snow and under what conditions it occurs. However, the question about which physical processes are most important could not yet be fully answered.