Abstract
Abstract. Snow on the ground impacts climate through its high albedo and affects atmospheric composition through its ability to adsorb chemical compounds. The quantification of these effects requires the knowledge of the specific surface area (SSA) of snow and its rate of change. All relevant studies indicate that snow SSA decreases over time. Here, we report for the first time three cases where the SSA of snow increased over time. These are (1) the transformation of a melt-freeze crust into depth hoar, producing an increase in SSA from 3.4 to 8.8 m2 kg−1. (2) The mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmented them. This formed a surface snow layer with a SSA of 61 m2 kg−1 from layers whose SSAs were originally 42 and 50 m2 kg−1. (3) The sieving of blowing snow by a snow layer, which allowed the smallest crystals to penetrate into open spaces in the snow, leading to an SSA increase from 32 to 61 m2 kg−1. We discuss that other mechanisms for SSA increase are possible. Overall, SSA increases are probably not rare. They lead to enhanced uptake of chemical compounds and to increases in snow albedo, and their inclusion in relevant chemical and climate models deserves consideration.
Highlights
The snow cover is an interface that probably has one of the highest impacts on the exchanges of energy and chemical species between the atmosphere and the surface of the Earth (Domine et al, 2008)
The three examples where the specific surface area (SSA) of snow increased that we report here are: (1) the transformation of a melt-freeze crust into depth hoar; (2) the mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmentation; (3) the sieving of blowing snow by an unperturbed snow layer, leading to the deposition of the smallest crystals inside the layer
The transformation of a melt-freeze crust into depth hoar indicates that any snow type subjected to a sufficiently strong temperature gradient will transform into depth hoar
Summary
The snow cover is an interface that probably has one of the highest impacts on the exchanges of energy and chemical species between the atmosphere and the surface of the Earth (Domine et al, 2008). The SSA is closely related to the surface (S) to volume (V ) ratio of snow crystals used in models that describe the interactions between snow and optical radiation (Warren, 1982; Kokhanovsky and Zege, 2004). If snow crystals are assumed to be spheres, SSA=S/ρV =3/(ρr), where ρ is the volumic mass of ice and r the radius of the spheres representing the snow crystals. The high albedo of snow helps cool the Earth’s surface. In current climate warming scenarios snow cover is expected to decrease (Dye, 2002; Stone et al, 2002; Pielke et al, 2004). The replacement of snow by darker surfaces (exposed vegetation, soils or ground cover) explains why polar regions are most affected by warming (Hall, 2004)
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