Figure 1: Strain and strain rate pattern in a specimen deformed at ˙ e$_{ext}$ = 5 . 77 × 10 − $^{3}$s − 1 to 9.94 % overall compressive strain; left: photographic image showing the compaction front, the arrow indicates the direction of front propagation; center: corresponding strain distribution as obtained by DIC; right: corresponding strain rate distribution; note that the outermost edges of the sample are not covered by the DIC analysis. To model compressive deformation of snow, we start out from a model proposed by Zaiser et al. [9] who generalize the compressible plasticity model formulated by Deshpande and Fleck [10] for deformation of solid foams to include non-locality and strain localization phenomena. We modify this model to account for the interplay between shear softening of snow associated with the breaking of bonds between ice granules upon shear deformation [11], and rapid sintering processes which re-establish intergranular links and restore strength over time. Pore pressure effects, which may influence dynamic snow compaction under impact loads [1], are not relevant at the presently investigated compaction rates and are therefore not taken into account. The stress tensor is denoted as σ with components σ$_{ij}$ . It fulfils the momentum balance equation which for quasi-static deformation processes as considered here is ∂$_{i}$σ$_{ij}$ = 0 where we sum over repeated indices. The first invariant of the stress tensor is Tr( σ ) = σ$_{ii}$ , and the deviatoric stress is σ$_{dev}$ = σ − (1 / 3)Tr( σ ) 1 with components σ dev ij . Our model is based on the Deshpande-Fleck yield criterion which has been shown to give a good description of the deformation of solid foams of low to intermediate density [10] and may thus be appropriate for snow where typical relative densities are in the range of 0.1-0.4. It is given by where σ$_{eq}$ is the equivalent stress and the elastic domain is defined by f < 0 . The plastic strain rate is assumed to derive from an associated flow rule, ˙ ϵ pl = ˙ e ( ∂f /∂ σ ) where the equivalent strain rate ˙ e is determined by imposing that, under temporally changing constraints, the system must not enter the domain f ( σ ) = σ$_{eq}$ − S = 0 , σ 2 eq = 3 σ dev ij σ dev ij 2(1 + α ) + α ( σ$_{ii}$ ) 2 1 + α (1) 3 Constitutive Model 3