Including snow redistribution in snow hydrology modelling: challenges and developments to make a research model operational at nation-scale

In mountains, wind- and gravity-driven transport of snow affects the overall distribution of snow and can have a significant effect on snowmelt dynamics. In the context of the Swiss operational snow melt forecasting, a compromise must be found to enable the representation of such small-scale processes over the entire Swiss Alps while maintaining viable computational costs.

To this end, the snow redistribution modules SNOWTRAN-3D and SnowSlide were implemented and adapted within the FSM2oshd physics-based snow cover model. In an earlier study we showed the added value of snow redistribution representations on a 1180 km² domain within the Eastern Swiss Alps when running simulations at 25, 50 and 100 m spatial resolutions. Here, we present the challenges and developments that are needed to apply this research model successfully over the whole Swiss Alps at 100 m resolution in an operational setting. In particular, we discuss the following issues:

- The Swiss Alps include very high elevations, with summits above 4000 m.a.s.l. and glaciers. Transport parameters that were shown to be suitable for terrain at 2500 m.a.s.l. are not applicable in more extreme conditions and need diversification.

- Wind fields, although dynamically downscaled, need further post-processing to mitigate biases that became evident in comparison to wind station measurements, particularly on exposed ridges.

- The representation of snow redistribution and of forest snow processes have to be integrated as both types of processes coexist wherever open alpine terrain interfaces with subalpine forest.

- The snow cover fraction scheme has to be adapted to better account for snow transport processes and sub-grid variability in simulations at high spatial resolution.