Measurement of snow density and liquid water content As seen in Section 2.2.1, the mixing ratio between ice, air, and liquid water is a key parameter that explains a significant part of the variability in physical and mechanical snow properties. For dry snow, density is an accurate indicator of this ratio, while for wet snow, an additional measurement of the liquid water content is necessary. Density measurements of snow are relatively easy to perform both in the laboratory and in the field, and are generally obtained by weighing a calibrated snow core. Automatic measurements can be performed in the field by measuring the absorption of gamma rays. In the case of dry snow, measurement of the relative permittivity can also be used to estimate density with reasonable accuracy. Snow density in a seasonal snow cover generally ranges from 50 to 550 kg m—3, but densities from 20 to 50 kg m—3 and from 550 to 650 kg m—3 can be observed under particular conditions.
Liquid water content can be measured by various procedures. In the cold laboratory, cold calorimetry is frequently used to obtain accurate measurements. In the field, it is difficult to obtain measurements of liquid water content with an accuracy better than 1% per mass. Dielectric measurements, chromatic elution, cold calorimetry, and even hot calorimetry are the techniques generally used. In a well-drained wet snow layer, liquid water content generally ranges from 0 to 15% per mass, but saturated snow layers (almost 100% of pore volume filled with liquid water) can form inside or at the base of a snowpack above low permeability ice or frozen soil layers. In saturated layers, liquid water content can exceed 75% of the snow mass.
As a result of its importance in the dynamics of snowpacks, snow compaction has been widely investigated in the past. Snow rheological properties are of extreme complexity, showing combined viscous, elastic, and plastic behavior. This behavior is strongly affected by the deformation rate. Here we will consider only the slow deformation of snow within a snowpack under natural conditions.
At the surface or inside a snowpack, compaction of snow occurs under the influence of three main processes that work almost independently with variable efficiency.
• Snow drift. As soon as wind velocity exceeds a threshold depending on the snow type of the surface layer, snow drift occurs. During snow drift, sublimation of snow particles and collision between particles transform snow crystals, which rapidly shrink and become more or less rounded (class 2b or class 3). During this process, density rapidly increases because the packing of small and rather rounded particles is denser than that of the precipitation particles, which often include dendrites and plates. In the case of a strong drift event, a fresh snow layer with a density less than 100 kg m-3 can transform into a drifted snow layer with a density up to 300 kg m-3 within a few tens of hours. This compaction process is of major importance in polar regions where prevailing low temperatures slow down metamorphism (Dang et al., 1997).
• Metamorphism. As soon as precipitation particles (class 1) are deposited and form a snow layer, metamorphism starts. In most cases, crystals initially shrink as a result of curvature effects (dendrites disappear and planar crystals become granular) and turn into class 2. This transformation generally requires between a few hours and a week, depending on temperature and liquid water content. Class 2 snow grains are smaller and less dendritic than class 1 particles and consequently constitute a denser packing. Therefore, metamorphism of class 1 snow induces a rapid compaction. In the next stage of metamorphic compaction, the compaction rate depends on the type of metamorphism. If high temperature gradients prevail, snow turns into faceted crystals (class 4) and eventually into depth hoar (class 5), if the process lasts long enough. The grain shape of these snow classes does not facilitate packing, and thus high temperature gradient metamorphism does not involve a significant compaction of snow, except for the initial compaction of precipitation particles. On the contrary, wet snow metamorphism transforms every type of snow into class 6, which consists of rounded and large grains that pack efficiently and densely. Wet snow metamorphism is a very efficient process to compact moist fresh snow layers, even at the surface where the overburden pressure is low. The density of precipitation particles at the surface can increase by up to 250 kg m-3 within a few hours during rainfall. Similarly, moistening induces a rapid increase in the density of depth hoar.
• Deformation strain. Buried snow layers have to sustain the weight of upper layers. Gravity forces are concentrated in the grain bonds, which break, slide, partially melt, or warp, making the rheological properties of snow very complex (Golubev and Frolov, 1998). These processes are very active in fresh snow, which settles very rapidly during and after snowfall (see Fig. 2.9). Rheological properties of snow have been extensively investigated but, until now, no universal law describing snow rheology is recognized by the scientific community. Deformation strain is very sensitive to the deformation rate. At slow deformation, snow is usually considered as a Newtonian fluid with a viscosity depending on snow density, as well as on snow microstructure and on other parameters, such as
temperature and liquid water content. Snow belonging to classes 1 and 2 has a low viscosity and compacts rapidly under the pressure of the upper layers. The higher the pressure, the faster the compaction. Lower snow layers, which have sustained a higher overburden over a longer time than upper layers, generally show a higher density. This is not always the case, however: viscosity also depends on snow type, and cold depth hoar layers almost do not compact. They can remain at a density close to 300 kg m-3 for several months at the base of a deep snowpack.
Recent papers have reviewed the present knowledge of snow compaction and of the effect of climate on the average density of snowpacks all around the Northern Hemisphere (Sturm and Holmgren, 1998). In most snow models or parameteri-zations, the effects of metamorphism and deformation strain are combined and simulated by way of a Newtonian viscosity. Snow viscosity is described in more or less sophisticated ways to take into account the effects of temperature, liquid water content, and microstructure.
From laboratory experiments, Navarre (1975) established the following equation to describe the Newtonian viscosity of dry snow as a function of temperature and density:
where no is a constant equal to 6.0 x 106 Pa s, ps is snow density, T is snow temperature, T0 is the melting temperature, and f is a constant depending on the snow type, set equal to 0.4 by Navarre. Morris et al. (1997) compile viscosity observations for a range of snow types, densities and temperatures. Recently, some snow models have taken into account the effects of snow drift on the compaction of surface layers (e.g. Brun et al., 1997).
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