Item: STRONG WIND, SOFT SLAB – POSSIBLE EXPLANATIONS
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Title: STRONG WIND, SOFT SLAB – POSSIBLE EXPLANATIONS
Proceedings: International Snow Science Workshop 2024, Tromsø, Norway
Authors:
- Anna Heuberger [ GeoSphere Austria, Avalanche Warning Service Salzburg, Austria ]
- Veronika Krieger [ GeoSphere Austria, Avalanche Warning Service Salzburg, Austria ]
- Ingrid Reiweger [ BOKU University of Natural Resources and Life Sciences, Department of Civil Engineering and Natural Hazards, Institute of Mountain Risk Engineering, Vienna, Austria ]
Date: 2024-09-23
Abstract: Snowfall, strong winds up to 80 km/h, and super powder skiing conditions the next day – a situation which was reported multiple times during the winter season 2023/24 in the province of Salzburg, Austria. Looking at the wind-speeds alone we would have expected hard wind-slabs at the top of the snowpack, possibly increased slab-avalanche activity, and bad skiing. However, conditions in the field showed clear wind effect on the snow surface, but there was little avalanche activity and the slab was soft. Past research validates our observations and states that maximum wind gusts and average unobstructed wind speeds do not necessarily correlate with slab hardness, but near surface wind speed does. These findings point towards the importance of terrain and its influence on wind speeds at the snow surface level. There is, however, still a lack of research in determining factors influencing slab hardness. We therefore took a closer look at the situations of high wind speeds and soft slabs of last winter, comparing snowpack properties and meteorological data. Studying literature of previous research on this subject, we found three possible explanations or at least contributing factors for the combination “strong wind, soft slab”: Firstly, we saw large stellar sizes of up to five millimeters during the snowfall events producing relatively soft slabs despite strong winds. In comparison to smaller fresh snow crystals, the large stellar crystals remain relatively large even after mechanical breakage through transport. Secondly, high wind speeds (especially gusty wind) might lead to more suspensions and less saltation, which leads to less deposition at the snow surface as compared to lower wind speeds. Thirdly, the observed situations all occurred during relatively cold phases. As past research states, other contributing factors to hard wind slabs are temperature and to some extent humidity. The higher the temperature, the higher the sintering process leading to a harder slab. The lower the humidity, the higher the sublimation rate and therefore a less dense slab. Next winter we plan to study the occurrence of wind slabs more closely, making use of our large observer network in the mountain region of Salzburg, Austria.
Object ID: ISSW2024_P3.9.pdf
Language of Article: English
Presenter(s): Anna Heuberger
Keywords: wind slab, slab hardness, wind slab properties, wind speed
Page Number(s): 510 - 514
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