Evolution of individual snowflakes during metamorphism

The morphological changes of individual snowflakes evolving within a dry snow aggregate have been studied using X-ray computed microtomography (micro-CT). Fresh dry snow was collected during a snowfall, sealed, and stored in a -5 degrees C cold room between periodic observations using micro-CT. Time series 3-D images were used to examine the structural evolution of an individual snowflake within the aggregate over a 2-month period, after which the snowflake had lost its original dendritic structure. Analysis of the aggregate showed that the fraction of large ice particles increased over this period while the total number of particles decreased, presumably to lower the free energy of the snow specimen. This approach enables the study of metamorphism of individual snowflakes in a local environment close to that found in nature. The evolution of structural parameters, including the volume fraction of ice, the surface-to-volume ratio of the ice matrix, the thickness and separation of the ice structure determined by the distance transform of the ice and pore space, were monitored and analyzed using coarsening theories. The computed growth exponent was smaller than the values obtained in the earlier work by Legagneux et al. (2004) and Kaempfer and Schneebeli (2007), who also interpreted the isothermal metamorphism in terms of coarsening theories.