Item: Some observations on snowcover temperature patterns
Title: Some observations on snowcover temperature patterns
Proceedings: Proceedings of the 1980 International Snow Science Workshop, Vancouver, BC, Canada
Authors: Richard L. Armstrong, USDA Forest Service, Fort Collins, CO.
Abstract: The two primary boundaries of a snowcover are the snow-air interface (snow surface) and the snow-soil interface (soil surface). The soil surface is fixed, while the snow surface is a moving boundary. Temperatures found within a mountain snowcover normally range from -40°C to OOC. Air temperatures may fluctuate well below O°C, but soil temperatures beneath a snowcover are generally within a few degrees of OOC. Therefore, temperature gradients at the macroscale are present to some extent unless conditions cause the snowcover to be isothermal at O°C throughout. Gradients between the soil and the snow surface are largest in early winter, when the snowcover is relatively thin and air temperatures are low. In his pioneering work on snow properties, Eugster (1952) recognized that a critical minimum temperature gradient must be exceeded before metamorphism in snow changes from the equitemperature (ET) to the temperature gradient (TG) mode (Sommerfeld and LaChapelle, 1970), but he did not specify the value of this minimum gradient, perhaps because he recognized that it varies over an appreciable range. Because the layers of a dry natural snowcover are always exposed to some degree of temperature gradient, the term "equitemperature metamorphism" is somewhat of a misnomer. Colbeck (in press) has addressed this problem specifically and describes two growth forms in the presence of a temperature gradient, a slower equilibrium form resulting in rounded grains and a more rapid kinetic form resulting in faceted crystals. Field workers often state that gradients in excess of lO°C/m are required to drive TG metamorphism. However, actual grain growth associated with TG metamorphism depends on the vapour pressure gradient resulting from the thermal gradient. Vapour concentration within the pore space of the ice matrix is dependent on the average temperature of the dir in the pore space. Therefore, the magnitude of the vapour pressure gradient is controlled by the magnitude of the thermal gradient as well as by the average temperature of the thermal gradient. This paper introduces the concept of a minimal vapour pressure gradient required to produce faceted TG grains rapidly enough to be important for the avalanche formation problems. Heat sources for the gradients incluae not only the soil but also near-surface snow layers which have been warmed, primarily by solar radiation. This paper is an investigation of temperature conditions in the snow near the ground surface and at the snow surface. Near the ground surface, snow is subject to generally steady conditions which tend to alter snow texture over a period of weeks. At the snow surface, snow is subject to rapidly changing conditions which may alter the texture over a period of a few hours. Other work dealing with this subject includes Benson and Trabant (1972) and Bradley, Brown and Williams (1977), Marbouty (1981), and Armstrong (1981).
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Keywords: temperature gradients, metamorphism, faceted crystals
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