Summary:

## Compaction by Drifting Snow

The article discusses the process of crystal breaking by drifting snow, which leads to higher snow densities at the surface. This process is particularly important on ice sheets, where destructive metamorphism is slow due to low temperatures, but high wind speeds (katabatic winds) are prevalent.

The article presents a parametrization for the drifting snow compaction process, based on the work of Vionnet et al. (2012). The key equations are:

1. Compaction rate by drifting snow:
   $$C_{R5, i} = \left[\frac{1}{\Delta z_{i}} \frac{\partial \Delta z_{i}}{\partial t}\right]_{drift} = - \frac{\rho_{max} - \rho_i}{\tau_{i}}$$
   where $\rho_{max} = 350$ kg/m^3 is the upper limit for this process, and $\tau_i$ is a depth-dependent timescale.

2. Depth-dependent timescale:
   $$\tau_i = \frac{\tau}{\Gamma_{drift}^i} \quad , \quad \Gamma^i_{drift} = \max\left[0, S_I^i \exp(-z_i / 0.1)\right]$$
   where $\tau$ is a characteristic timescale for drifting snow compaction, set to 48 hours, and $z_i$ is a pseudo-depth that takes into account previous hardening of snow layers above the current layer.

3. Driftability index:
   $$\begin{split}\begin{array}{rcl} S_I & = & -2.868 \exp(-0.085 U) + 1 + M_O \\ M_O & = & -0.069 + 0.66 F(\rho) \end{array}\end{split}$$
   The driftability index $S_I$ reflects how well the snow can be drifted and depends on the mobility of the snow as well as the 10 m wind speed.

The article notes that the equation for the mobility index $M_O$ is a simplification from the original paper by Vionnet et al. (2012), removing the dependency on grain size and assuming spherical grains, as presented in van Kampenhout et al. (2017).