Rate of decay of specific surface area of snow during isothermal experiments and morphological changes studied by scanning electron microscopy

Abstract

The quantification of the specific surface area (SSA) of snow crystals and of its variation during metamorphism are essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Therefore, the decay rate of SSA of five fresh snow samples was studied in the laboratory at –4, –10, and –15°C under isothermal conditions in closed systems. The time-evolution of the snow SSA can in all cases be very well described by an empirical law of the form, SSA = – A log(t + Δt) + B, where A, B, and Δt are adjustable parameters. B seems to be closely related to the initial SSA of the snow, and A describes the SSA decay rate. Our preliminary findings at –15°C suggest that a linear relationship exists between A and B, so that it may be possible to predict the decay rate of snow SSA from its initial value. For the first time, images obtained from scanning electron microscopy show that crystal rounding of snow is the main process taking place during isothermal metamorphism. New grain boundaries also form. More surprising, however, was the formation of new basal, prismatic, and pyramidal crystal faces, sometimes with very sharp angles, especially at –15°C. The growth of facets with sharp angles is not fully explained by current theories of snow metamorphism and has not been observed before. PACS Nos.: 68.35Md, 68.37Hk, 81.20Ev, 81.05Rm

Evolution of the Snow Area Index of the Subarctic Snowpack in Central Alaska over a Whole Season. Consequences for the Air to Snow Transfer of Pollutants

“…Photomicrographs of snow crystals at various stages of evolution are shown in the Supporting Information. In contrast to isothermal metamorphism, where crystal shapes remain recognizable after weeks or even months (28), here the strong temperature gradient generates large water vapor fluxes that accelerate metamorphism. Changes in grain shapes are readily observable after a few hours.…”

“…As SSA decreases with snow aging (24,28,32,33), it decreased with depth inside the snowpack. Freshly fallen snow SSA values ranged from about 800 cm 2 g -1 for bullet rosettes and columns to 1171 cm 2 g -1 for very rimed dendritic snow.…”

Influence of the ice growth rate on the incorporation of gaseous HCl

“…Domine et al (2003) may have observed growth steps on surface hoar crystals studied by scanning electron microscopy (SEM), and whose height was about 1 µm, but it is not even totally certain that the structures observed were growth steps, and they did not seem to originate from a crystal edge, but rather from a grain boundary, so that the representativity of this observation may be limited. Indeed, the numerous SEM studies of snow crystals (Wergin et al, 1996;Domine et al, 2003;Erbe et al, 2003;Legagneux et al, 2003) usually do not show any structures that can be convincingly attributed to growth steps, which suggest that these could be undetectable by SEM and much smaller than 1 µm in most cases.…”

3-D imaging and quantification of graupel porosity by synchrotron-based micro-tomography

“…While the formation of spherical and round bubbles was discussed theoretically and on experimental basis already three decades ago (List, 1958;Knight and Knight, 1968;List et al, 1972;Carras and Macklin, 1975;Macklin et al, 1976), little is known about the formation of non-spherical bubbles which must have a higher interface free energy than spherical bubbles. Their Y-shaped morphology is probably related to the void wedges at the triple junctions between frozen droplets retaining approximately their spheroid shape, and does not occur in a preferred orientation.…”

“…Measured graupel densities vary, e.g. from 0.05 to 0.45 g cm −3 (Locatelli and Hobbs, 1974), 0.25 to 0.7 g cm −3 (Zikmunda and Vali, 1972), up to 0.5 to 0.7 g cm −3 (List, 1958). The densities of hailstones were found as low as 0.4 g cm −3 (Knight and Heymsfield, 1983), but generally lie around 0.8 g cm −3 (e.g.…”