Comparison between the Structural Evolution of Dry Snow under Quasi-isothermal Conditions and in a Temperature Gradient
Abstract
The structural evolution of dry snow - snow without free water - begins as soon as snow reaches the ground. Both the development of crystal morphologies and the evolution of structural parameters of snow aggregates strongly depend on the thermal conditions encountered in the snowpack, particularly the presence or absence of a macro temperature gradient. To understand the influence of thermal conditions on structural changes, we examined three natural snow specimens as they underwent evolution under three different well-controlled conditions: a macro uniform temperature (-5 ± 0.2°C) and two temperature gradient conditions (50°C m-1 and 140°C m-15, with the top of the specimens maintained at -2.3 ± 0.2°C). An X-ray computed microtomography (micro-CT) scanner was employed at regular time intervals to acquire both three-dimensional renderings and structural parameters, including relative density, specific surface area (SSA), structure thickness (Sr.Th), structure model index (SMI), and degree of anisotropy (DA). A scanning electron microscope (SEM) was used immediately after periodic micro-CT observations to examine fine scale structural features. For the isothermal conditions at -5 ± 0.2°C, the structure became less complex and coarser with increasing time, which was reflected by an exponential decline in SSA and an increase in Sr.Th, respectively. Additionally, the SMI value increased and approached 3, indicating the formation of rounded structures. This rounding process was confirmed by SEM observations. Under a temperature gradient, although the crystals grew much faster, the mean Sr.Th did not increase, probably due to the formation of kinetic forms, such as the staircase-like structures on the edges and flat thin ice layers on the surfaces of crystals revealed by SEM observations. The SMI value declined over time, with a higher rate observed for the specimen subjected to the higher temperature gradient, indicating the formation of faceted crystals. An increase in DA value over time was observed for both specimens under isothermal conditions or a temperature gradient. However, the images obtained from mutually perpendicular planes show that while small ice chains aligned horizontally under isothermal conditions, the structure was aligned vertically along the temperature gradient.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.C33D0569B
- Keywords:
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- 0736 CRYOSPHERE / Snow