Permeability, snowpack

Snow is a porous medium formed of air, ice crystals and small amounts of chemical impurities. Because ice has a high vapor pressure (165 Pa at -15°C, 610 Pa at 0°C), the vertical temperature gradient that is almost always present within the snowpack generates sublimation and condensation of water vapor that change the size and shape of snow crystals. This results in changes in physical variables such as density, albedo, heat conductivity, permeability and hardness. These physical changes have formed the basis for the definition of snow metamorphism. ... 

Variables such as density, albedo, light e-folding depth, specific surface area (SSA), crystal size and shape, heat conductivity, permeability, diffusivity and shear resistance are required for a complete physical description of the snowpack. Not all these variables have major relevance to climatic issues. Albedo, i.e. the fraction of incident light that is reflected, has obvious climatic relevance and is discussed here. It depends in part on crystal size and shape and this dependence can in fact more simply be related in part to the SSA of the snow i.e. the surface area accessible to gases per unit mass. ... 

Snowpack permeability determines air flow within the snowpack, as driven by differences in surface pressure between different parts of the snow surface. Air flow through the snow leads to the exchange of heat and of chemical species between the snow and the atmosphere and is the last physical variable discussed here. 

The snowpack is a porous medium through which air flows if pressure differences exist within the snowpack. Such pressure gradients may be generated by the action of wind on sastrugi. Under most conditions relevant to snowpacks, the flow velocity v is proportional to the pressure gradient and the proportionality factor is the snow permeability Kp... 

In the natural snowpack on the ground, evolving under HGM conditions, depth hoar of density 0.20 g.cm formed rapidly and the permeability in the lower half of the snowpack increased to beyond 500x10 ° m in late March. In contrast, on the tables under QIM conditions, fine-grained snow of density 0.28 g.cm formed and the permeability decreased to values between 30 and 70x10 m in early March. 

Species emitted to snowpack interstitial air must be transported out of the snow into the atmosphere to have an atmospheric impact. The most efficient mechanism is ventilation by wind, which depends on wind speed, snow surface structure and permeability. Hansen et al." predict little surface wind change in polar regions. Snow surface structure depends mostly on wind, but speculating on its evolution is beyond our scope. We discussed that snow permeability will in general decrease with warming. The residence time of species produced in interstitial air will therefore increase under a warmer climate, reducing the chances of reactive species to escape before they react in the snowpack. 

For example, HCHO and H2O2 can photolyze and the HOx radicals produced can react with organic species contained in aerosols that are always present in snow. Other species will be produced, " that can also be released to the atmosphere, so that the nature of snowpack emissions are highly dependent on the residence time of primary products in the snowpack. Complex snow photochemical models, inexistent today, are needed to predict the effect of warming on snowpack emissions, caused by changes in snow permeability. 

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