Crystals depth hoar

The Taiga snowpack covers cold forested regions in North America and Eurasia, as represented in detail in Sturm et al. It is typically 50 cm thick (Figure 1) and covers the ground from October-November to April. In mid-winter, it is composed of a thick basal layer formed of centimetric depth hoar crystals, that has a very low mechanical strength, and a density near 0.2 g.cm It is topped by a layer of faceted crystals 1 to 2 mm in size, that eventually transform into depth hoar."" Layers of decomposing crystals and of fresh snow are observed after snow falls. All of these snow layers have a low density, typically < 0.2 g.cm as shown in Figure 1. 

In dense windpacks, crystals do not appear to have the space required to grow to large sizes and this has been invoked to explain why depth hoar does not grow in dense snow." However, we found depth hoar of density up to 0.4 g.cm on the tundra, so that space is probably not the only factor. As detailed below, windpacks have a high heat conductivity that hinders the establishment of a high temperature gradient across them and this may also explain why little transformation is observed in such snow layers. 

In the absence of wind, snow density increases because of loading by subsequent layers and of destruction of small structures by sublimation, leading to the collapse of crystals higher up. Without wind and melting and under QTM conditions, snow density is around 0.35 g.cm at 1 m depth. Under HGM conditions, compaction is compensated by upward vapor fluxes,limiting the density of basal depth hoar layers to about 0.2 g.cm. ... 

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. 

If the southern tundra turns into shrub tundra or taiga the transformation of the tundra windpack into faceted crystals or depth hoar of lower SSA, longer e-folding depth and higher permeability will lead to enhanced release of adsorbed and dissolved species, greater photochemical activity (modulated by tree shading) and more efficient release to the atmosphere, so that emissions may in this case increase. 

Crystal shapes of each layer were observed and classified into three types the initial crystal type (lightly compacted snow (Class 2dc)), solid type depth hoar (Class 3mx, 4a) and skeleton type depth hoar (Class 5cp), Snow crystal photomicrographs of each layer were taken and then the projected area of the crystals were obtained. 

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