Item: On Modelling the Formation and Survival of Surface Hoar in Complex Terrain
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Title: On Modelling the Formation and Survival of Surface Hoar in Complex Terrain
Proceedings: International Snow Science Workshop Grenoble – Chamonix Mont-Blanc - October 07-11, 2013
Authors:
- Nora Helbig [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Alec van Herwijnen [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
Date: 2013-10-07
Abstract: Surface hoar crystals form on the snow surface during cold nights. Cloudless sky, humid air and low to moderate wind speeds are required meteorological preconditions. When buried by subsequent snowfall, surface hoar crystals can form a weak snowpack layer often associated with snow avalanches. For avalanche forecasting, knowledge about the spatial distribution of surface hoar is therefore of great importance. We investigate if spatial variations of surface hoar in mountainous terrain can be modeled based on terrain characteristics using an ensemble of 1800 simulated topographies, covering a wide range of characteristic length scales. We modeled distributed radiation over this set of topographies using a detailed radiation balance model. Relative humidity was assumed to be favorable for surface hoar formation. Based on a series of cold laboratory experiments, we derived a sky view factor threshold associated with the minimum longwave radiative cooling necessary for surface hoar formation. As a first approach, we further assumed that surface hoar only survives on shaded slopes. Finally, we used a wind sheltering factor to simulate the destruction of surface hoar by wind. Applying these three simple thresholds to our spatial radiation modeling, our results show that the spatial distribution of surface hoar is greatly affected by large-scale terrain roughness and sun elevation angle. Spatial correlation ranges for surface hoar, on the order of several hundred meters, were closely related to the typical width of topographic features. Furthermore, correlation ranges of surface hoar decreased with increasing sun elevation angle. Overall, the modeled spatial patterns of surface hoar were in line with previously published spatial field observations, suggesting that simple terrain parameters can very well be used to describe the predominant surface hoar layer patterns in complex topography. A practical implication of our work is that large-scale field studies can be optimized by carefully considering the underlying topography and the time of the year.
Object ID: ISSW13_paper_P4-06.pdf
Language of Article: English
Presenter(s): Unknown
Keywords: surface hoar, spatial variability, complex terrain characteristics, avalanche forecasting
Page Number(s): 052-056
Subjects: avalanche forecasting spatial variability surface hoar
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