Tomography-based characterization of ice-air interface dynamics of temperature gradient snow metamorphism under advective conditions

Abstract

Snow at or close to the surface commonly undergoes temperature gradient metamorphism under advective flow, which alters its microstructure and physical properties. A functional understanding of this process is essential for many disciplines, from modeling the effects of snow on regional and global climate to assessing avalanche formation. Time-lapse X-ray microtomography was applied to investigate the structural dynamics of temperature gradient snow metamorphism exposed to an advective airflow in controlled laboratory conditions. Experiments specifically analyzed sublimation and deposition of water vapor on the ice structure. In addition, an analysis of the ice-air interface dynamics was carried out using a macroscopic equivalent model of heat and water vapor transport through a snow layer. The results indicate that sublimation of the ice matrix dominated for flow rates < 10 À6 m 3 s À1 while during increased mass flow rates the water vapor deposition supplied by the advective flow counteracted sublimation. A flow rate dependence of water vapor deposition at the ice interface was observed, asymptotically approaching an average estimated maximum deposition rate on the whole sample of 1.05 · 10 À4 kg m À3 s À1 . The growth of microsized whisker-like crystals on larger ice crystals was detected on microscope photographs, leading to an increase of the specific surface area and thus suggest a change of the physical and optical properties of the snow. The estimated values of the curvature effect of the ice crystals and the interface kinetic coefficient are in good agreement with previously published values.The growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover [Colbeck, 1983]. The whole ice matrix is continuously recrystallizing EBNER ET AL.TEMPERATURE GRADIENT SNOW METAMORPHISM 2437

Experimental observation of transient &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O interaction between snow and advective airflow under various temperature gradient conditions

“…Therefore, the recrystallization rate increases and causes the change in the δ 18 O of the air. For experiment (2) there is a complex interplay between sublimation and deposition of water molecules into the interstitial flow (Ebner et al, 2015c), while for experiment (3) there is deposition of molecules carried by the interstitial flow onto the snow crystals (Ebner et al, 2015b). Furthermore, in the beginning of each experiment there is a tendency to sublimate from edges of the individual snow crystals due to the higher curvature.…”

“…Only conditions deeper than 1 cm inside a snowpack are considered. Previous work showed that (1) under isothermal conditions, the Kelvin effect leads to a saturation of the pore space in the snow but does not affect the structural change (Ebner et al, 2015a), (2) applying a negative temperature gradient along the flow direction leads to a change in the microstructure due to deposition of water molecules on the ice matrix (Ebner et al, 2015b), and (3) a positive temperature gradient along the flow had a negligible total mass change of the ice but a strong reposition effect of water molecules on the ice grains (Ebner et al, 2016). Here, we continuously measured the isotopic composition of an airflow containing water vapour through a snow sample under both isothermal and temperature gradient conditions.…”

Tomography-based observation of sublimation and snow metamorphism under temperature gradient and advective flow

“…Small fluctuations of the measured inlet and outlet temperature were due to temperature regulation both inside the cold chamber and inside the micro-CT (Ebner et al, 2014). A shift of ∆t < 10 min between inlet and outlet temperature indicated that a fast equilibrium between the temperature of the snow and the 15 airflow was reached (Albert and Hardy, 1995;Ebner et al, 2015b). The morphological evolution was similar between all four experiments and only a slight rounding and coarsening was visually observed, shown in Fig.…”

“…If vapor is advected from a warmer zone into a colder zone, the air becomes supersaturated, and some water vapor deposits onto the surrounding ice grains. This leads to a change in the microstructure creating whistler-like crystals (Ebner et al, 2015b). The flow rate dependence on the deposition 10 rate of water vapor on the ice matrix was observed, reaching asymptotically a maximum rate of 1.05 × 10 −4 kg m −3 s −1 (Ebner et al, 2015b).…”

“…This leads to a change in the microstructure creating whistler-like crystals (Ebner et al, 2015b). The flow rate dependence on the deposition 10 rate of water vapor on the ice matrix was observed, reaching asymptotically a maximum rate of 1.05 × 10 −4 kg m −3 s −1 (Ebner et al, 2015b). Contrarily, if the temperature gradient acts in the opposite direction of the airflow, the airflow through the snow brings cold and relatively dry air into a warmer area, causing that the pore space air becomes undersaturated, and surrounding ice sublimates.…”