General characteristics

After being deposited on the ground or on a previous snow layer, snow crystals accumulate and give birth to a new snow layer. The initial structure of this layer depends on the shape and size of the crystals and on the stress applied to the bonds that link them together. Ice forms a solid matrix that delimits pores filled with humid air and, in the case of wet snow, with liquid water. In snow, most pores are interconnected. Thus, snow belongs to the great family of porous media whose members generally present complex physical properties. Compared to other porous media, the complexity of snow physical properties is increased by the fact that the solid, liquid, and gaseous phases of the principal component of snow - water - may

Physical processes within the snow cover immmmmmm f<

Dry Air Water Vapor

Figure 2.3. Diagram of volume fractions of the four components of snow (from Jordan, 1991).

coexist in the medium. Snow also differs from most other porous media in that the snow grains are bonded together. Because of the high activity of water thermodynamics around the triple point, the solid matrix of snow is continuously and sometimes rapidly evolving, thus making snow a unique, dynamic, and complex component of the earth's surface.

2.2.1 General concepts of a porous medium

Constitution

A general feature of porous media is that the solid matrix presents physical and mechanical properties that significantly differ from the properties of the fluid filling the pores. In the case of snow, the fluid may consist of one phase (humid air) or of two immiscible phases (humid air and liquid water). Here we use volume fractions, 6k, to specify the mixing ratio of the four components in snow, where k becomes i, a, v, and I, for ice, air, water vapor, and liquid water, respectively (see Fig. 2.3 and also Morris, 1983). The volume ratio between the solid matrix and the fluids is a key determinant of the physical and mechanical properties of the medium (Scheidegger, 1974; Dullien, 1992). For that reason, porosity 0 (or 1 — 6;) is the most basic parameter for describing snow. The density ps of dry snow is expressed in terms of porosity as pi(1 — 0), where pi is the density of ice (= 917 kg m—3). Liquid water content is also of primary interest for describing the constitution of a snow sample. Although it can be characterized by mass or volume, here we use the volume fraction 6g or the liquid saturation s, which is the ratio of the liquid volume to the pore volume, or 6g/$. Snow porosity (with the exception of ice layers) generally ranges between 0.40 and 0.98 for seasonal snow covers. Snow density therefore ranges well over an order of magnitude, which leads to a range of over two orders of magnitude for some of its physical or mechanical properties.

Texture

Independent of porosity, the texture (i.e. the distribution of shape and size of the grains, pores, and water menisces present in the medium) affects most physical and mechanical properties of snow (Arons and Colbeck, 1995; Shapiro et al., 1997; Golubev and Frolov, 1998). As a result of metamorphism, snow texture evolves rapidly and shows a very large variability. Radiative properties are particularly influenced by grain shape and size, whereas thermal properties depend primarily on the bond structure and fluid properties on the structure of the pore space. The ratio of bond size to grain size and the coordination number (the average number of bonds per grain) are critical parameters for snow mechanical and thermal properties.

Snow classification

Because of the large variability in porosity and texture observed in snow, the scientific community uses an international classification to describe the different snow types encountered in seasonal snowpacks (Colbeck et al., 1990). Since no unique parameter alone describes a snow sample, the classification is based on a qualitative description of the shape and the size of the grains that may be determined with a microscope or a magnifying lens. Snow is classified into nine classes and various subclasses. The six main classes observed in seasonal snowpacks are described in Table 2.2. The classification criteria make it possible to classify a snow sample from field observations but do not provide quantitative information. Stereological analysis of thick or thin sections of snow samples (Good, 1987) and analysis of three-dimensional high-resolution gray-level images obtained from X-ray micro-tomography (Coleou et al., 2001; Flin et al., 2003) are relevant methods to derive characteristics of snow microstructure such as grain and bond size distribution or the coordination number. Compared to other porous media, snow is very brittle and such analyses require sophisticated preparation processes. It is almost impossible to prepare sections of snow with a porosity larger than 0.90 without disturbing the microstructure. Figure 2.4 presents a thick section showing ice, pores, and frozen liquid menisces of a snow type 6a.

Table 2.2 Six classification schemes for snow types (according to Colbeck et al, 1990).

Class 1 Precipitation Particles

Class 2 Decomposing and Fragmented Precipitation Particles

Class 1 Precipitation Particles

Class 2 Decomposing and Fragmented Precipitation Particles

Class 4 Faceted Crystals

Class 5 Cup-Shaped Crystals and Depth Hoar

Class 4 Faceted Crystals

Class 5 Cup-Shaped Crystals and Depth Hoar

Class 3 Rounded Grains

Class 3 Rounded Grains

Class 6 Wet Grains
Figure 2.4. Image of a thick section showing ice, pores, and frozen liquid menisces of a snow type 6a (from Brzoska et al., 1998).
Figure 2.5. p—T phase diagram for bulk-water, based on integral forms of Equation (2.1).
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