Snowpack evolution

Besides temperature (Figure 5a), the cold and warm scenarios differ by the structure of the snowpack. In both cases, the snow water equivalent have the same temporal evolution 2 cm at the end of October, 11 cm at the end of January and 15.7 cm in late April. Stratigraphies and heat conductivities are very different. In the cold scenario, depth hoar layers of low densities (0.21 to 0.26 g.cm O alternate with denser windpacks (0.38 to 0.48). Transient layers of fresh snow and of faceted crystals are also present. values range from 0.06 W.m K for aged depth hoar to 0.46 W.m K for dense windpacks. In the warm scenario, two melt-freeze layers (densities 0.40 to 0.55) alternate with hard windpacks (0.34 to 0.41) while layers of fresh snow are sometimes included in the mean monthly stratigraphies, kr values are 0.45 and 0.63 W.m K for the melt-freeze layers and range from 0.36 to 0.48 W.m K for dense windpacks. Recent snow has values around 0.2 W.m lC Overall, the warm snowpack has a greater heat conductivity than the cold one. 

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. 

Perrier S, Houdier S, Domine F, Cabanes A, Legagneux L, Stunner AL, Shepson PB (2002) Formaldehyde in Arctic snow. Incorporation into ice particles and evolution in the snowpack Atmos Environ 36 2695... 

---