2.1. Origin of the matter transfer inside snow

The basic idea behind this model is the same as presented in Reference Flin, Brzoska, Pieritz, Lesaffre, Coléou, Furukawa and KuhsFlin and others (2007): snow crystals grow (respectively decrease) by the condensation (sublimation) of vapour on their bottom (top) portions because they are colder (warmer) than the surrounding air. In the present model, the main limiting mechanism is considered to be the vapour diffusion through the pore space, which is usually assumed in the literature (e.g. de Reference de QuervainQuervain, 1973; Reference ColbeckColbeck, 1997; Reference Legagneux and DominéLegagneux and Dominé, 2005).

We also make the following assumptions:

1. 1. The latent heat is quickly removed through the highly conductive ice (Reference ColbeckColbeck, 1983).

2. 2. The convection effects in the pore space are negligible (Reference Brun and TouvierBrun and Touvier, 1987; Reference Kaempfer, Schneebeli and SokratovKaempfer and others, 2005).

3. 3. The temperature dependence of the diffusion coefficient can be neglected.

4. 4. Thermodiffusion effects can be neglected, according to de Reference de Quervain and KingeryQuervain (1963).

In order to quantify the matter transfer at the ice–pore interface, we can express the incoming matter flux j (kgm ⁻ 2 s ^{− 1}) on a point of the ice surface using Fick’s law:

where ρvap,D and c are the density (kgm ^{− 3}), diffusion coefficient (m² s ^{− 1}) and the concentration in number (mol mol ^{− 1}) of water vapour close to the ice surface, respectively. n represents the local outward normal vector on the ice interface and ∇ is the gradient operator.

Since both air and vapour can be considered as ideal gases, Fick’s law can be rewritten as follows:

where R is the universal gas constant (Jmol ^{− 1} K ^{− 1}) and is ρ _air the density of air (kgm ^{− 3}). P and T are the vapour pressure and vapour temperature, respectively.

The matter flux j incoming to the ice surface depends on the vapour-pressure and temperature fields, which are not uniform in the considered snow volume. By computing these fields, one can then use Equation (2) to obtain the local matter fluxes incoming to each point of the ice surface.

2.2. Faceting and rounding effects

To address the problem of grain faceting, we used a classical atomistic approach based on the Terrace Ledge Kink (TLK) or Kossel crystal model (Reference MarkovMarkov, 1995; Reference MutaftschievMutaftschiev, 2001). The main results, initially presented by Reference KnightKnight (1966), are summarized as follows (Fig. 1):

Fig. 1. Faceting model based on Kossel crystal explanation. Circles and dots correspond to preferential sublimation and condensation sites, respectively. (a) For convex shapes, condensation leads to faceting while sublimation generates a rounding of the grains. (b) For concave shapes, the reverse behaviour is observed.

1. 1. During the growth of a convex Kossel crystal (Table 1., a), some sites (such as step or kink sites) are energetically more likely to adsorb atoms than flat surfaces. This results in a layer-by-layer process and a production of facets.

2. 2. Conversely, the sublimation of such a convex crystal (Table 1., b) will begin by removing the step and kink sites. This will produce new kink and step sites, and finally lead to an atomically rough surface, i.e. a macroscopically rounded interface.

3. 3. For concave crystals, the preferential adsorption (Table 1., c) or removal (Table 1., d) of kink and step sites will result in opposite behaviours to those obtained for convex crystals.

The validity of Table 1. is confirmed by the following common observations.

1. 1. ‘Negative’ faceted crystals have been obtained by sublimation of concave ice shapes by inserting a hypodermic needle inside ice volumes and connecting it to a vacuum (Reference Knight and KnightKnight and Knight, 1965; Reference Furukawa and KohataFurukawa and Kohata, 1993).

2. 2. The isothermal metamorphism, which results only in condensation on concavities (Table 1.) and sublimation on convexities (Table 1.) due to Kelvin’s law, is characterized by significant surface rounding.

As far as TG metamorphism is concerned, the situation is more complex as the regimes presented in Table 1. can operate at the same time. However, the sign of the flux j can determine the faceting or the rounding of shapes depending on the sign of the local curvature C (m ^{− 1}) of the interface. For a snow structure imaged in three dimensions, one can compute the local incoming flux j with the help of Equation (2). It is then possible to check the aspect of the surface depending on the sign of the computed flux.

In order to check the validity of our model, we submitted snow to a temperature gradient and imaged the obtained samples.

3.1. Sample preparation

A 3 week experiment on TGmetamorphism was run at Col de Porte, French Alps, in order to provide precise microtomographic 3-D data for the validation of TG metamorphism models.

Snow was sieved into a specially designed cool box, which is able to generate several TGs in a single experiment. This insulated cool box uses two thick copper plates to create a quasi-vertical TG, whose values vary linearly in the horizontal direction. A fluid circulation maintains the copper plates at the desired temperature.

Temperature probes were used in order to monitor the temperature at different points of the snow layer. The snow, with density ∼280 kgm ^{− 3}, was submitted to a permanent temperature gradient ranging from 3 Km ^{− 1} at one side of the device to 16 Km ^{− 1} at the other side. After 3 weeks of experiment at an average temperature of –3 ^◦ C, samples 4 cm in diameter were cored and impregnated at –8 ^˚ C with 1-chloronaphthalene. The cold room was then set to –25 ^◦ C in order to completely freeze the samples and allow further machining. Smaller cores 9 mm in diameter were then extracted with a precision hole-saw, sealed inside thin PMMA boxes and kept ready for tomography acquisition at –50 ^◦ C.

3.2. Image obtention

Three-dimensional images of snow samples were obtained at the ID19 beamline of the European Synchrotron Radiation Facility (ESRF), Grenoble, France, by X-ray absorption microtomography using a specially designed refrigerated cell (Reference BrzoskaBrzoska and others, 1999; Reference Coléou, Lesaffre, Brzoska, Ludwig and BollerColéou and others, 2001). All the images were obtained at 18 keV, with a voxel (volume element) size of 4.91 μm. The grey-level images obtained were reconstructed at the ESRF and contoured using a semiautomatic procedure.

To allow fast processing times for our image analysis algorithms, the resolution of the 3-D images was reduced by a factor of two in the three axes. In all the 3-D snow images presented in this paper, one voxel therefore corresponds to 9.82 μm. More information about core preparation and image processing can be found in Reference Flin, Brzoska, Lesaffre, Coléou and PieritzFlin and others (2003).

4.1. Temperature field

Temperature maps (Figs 2b and 3b) can be obtained by modelling the heat transfer into the 3-D snow structure. As mentioned in section 2.1, convection and latent-heat effects in snow are negligible for the considered TGs. Consequently, the temperature field was computed by considering only conduction effects in both the ice and the pores of the snow structure. This was accomplished using a finite volume scheme (Reference PatankarPatankar, 1980). Because of the large meshes used, an iterative approach was employed.

The initial boundary conditions were fixed according to the experimental measurements. The temperatures at the top and bottom of the 3-D images were estimated from the experimental temperature data by linear interpolation. For the model validation, a cube of 3 ×3 ×3mm³ was extracted from the centre of the wider (6×6×6mm³) 3-D field initially obtained, in order to remove possible side effects.

4.2. Vapour-pressure field

Vapour-pressure maps can be obtained using an algorithm recently developed by Reference Brzoska, Flin and BarckickeBrzoska and others (2008) for isothermal metamorphism. On the grain surface S the ice is locally considered to be in equilibrium with the vapour, providing a Dirichlet’s boundary condition to the vapour field:

where

is the position vector. P _sat(C,T) is the local saturation vapour pressure at temperature T for a surface of curvature C (m ^{− 1}). This quantity can be obtained by combining the Kelvin and Clausius–Clapeyron laws to obtain

where P ₀ is the saturation vapour pressure for an infinite plan at some temperature is the molar enthalpy of T ₀, Δh _sub sublimation (J mol^-1), is the surface tension of the interface γ (Jm ^{− 2}) and Ω is the molar volume of ice (Jm ^{− 2}).

Elsewhere, P should obey the stationary equation of diffusion:

If the temperature (see section 4.1) and curvature (Reference Flin, Brzoska, Lesaffre, Coléou and PieritzFlin and others, 2004) fields in the snow microstructure (Figs 2a and b and 3a and b) are known and Equation (5) is solved, one can obtain the vapour pressure map in the pore space of a snow image.

4.3. Results

Once both the temperature and vapour pressure fields are computed, one can estimate the local matter fluxes at the interface by digitizing Equation (2). The flux maps obtained for the snow samples submitted to TG=3 Km ^{− 1} and TG=16 Km ^{− 1} are presented in Figures 2c and 3c, respectively. For each snow sample, the flux map obtained with the present model (Figs 2c and 3c) is compared to the flux map computed for a reaction-limited mechanism of effective sticking coefficient α as modelled by Reference Flin, Brzoska, Pieritz, Lesaffre, Coléou, Furukawa and KuhsFlin and others (2007) (Figs 2d and 3d).

For a better visualization of the faceted shapes, all the images in Figures 2 and 3 are presented upside down. The top (bottom) of the images corresponds to the lowest (highest) and warmest (coolest) side of the physical sample. From both of the diffusive and reactive flux maps, it is clear that, for convex shapes, predicted condensation sites (j > 0, orange colour) correspond to faceted zones while rounded forms are defined by sites where j < 0 (sublimation, green colour).