Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Three sets of snow specimens with different initial densities ranging from 311 to 486 kg m⁻³, which were collected from depths of 7, 40 and 57 cm at Summit, Greenland (72°35′ N, 38°25′ W) in June 2017, were subjected to the temperature gradient (TG) of ~160 K m⁻¹ in opposing directions. The snow metamorphism was characterized using both X‐ray micro computed tomography and optical microscopy. The formation of depth hoar is evident as the snow grains transformed from the initial approximately rounded particles to needles, broad‐plates, columns and cup‐shapes. The de‐densification process of snow is also reflected in various microstructural parameters, viz., the density and the structure thickness (the feature size of ice particle) generally decreased with time, while the area‐equivalent circle diameter (the feature size of pore), the total porosity, the specific surface area and the structure model index (a measure of convexity/concavity of ice surface) increased. Based on an ice‐sphere pair of equal size, a vapour transport model, which depends on the initial particle radius, rp$$ {r}_p $$, and the ratio of the initial particle‐bond radius to the initial particle radius, α$$ \alpha $$, was constructed to simulate the main characteristics of change in the density with time under the TG metamorphism. We found critical values of rp$$ {\mathrm{r}}_p $$ = 0.1 mm and α$$ \alpha $$ = 0.1 that determine how fast the geometric coefficient, ψ$$ \psi $$ (a tuning parameter used in this model at the cone angle θ=45°$$ \theta =45{}^{\circ} $$ between the two ice spheres), changes. We propose a conceptual model in which an ice‐sphere pair can be regarded as a volume element of morphogenesis of depth hoar. This model can be used to explain the formation of the needle‐like, broad‐plate and column depth hoar by combining particle‐by‐particle with inter‐layer vapour transport.