Snow Precipitation, Distribution and Transport in Alpine Terrain

Knowledge and forecasting of mountain precipitation and mountain snow water resources has become a major issue given the fact that large lowland communities depend on the steady supply of fresh water from mountains. Competing uses of water for renewable energy production, irrigation and drinking on the one hand and climate change on the other hand cause an increasing need to accurately understand the processes that influence the mountain water storage.

However, not even total precipitation amounts are well constraint in mountains because of scarce measurement infrastructure and the inherent difficulty in measuring precipitation, which is particularly grim for snowfall. Additionally, lateral transport processes such as drifting and blowing snow and an unknown amount of sublimation add to the uncertainty around snow water storage in mountains.

This contribution first presents new evidence from airborne and ground-based laser scans at several mountains worldwide to show typical patterns of snow distribution and elevation gradients. A novel result is that snow amounts typically decrease at summit/crest level. Thus, assuming a monotonically increasing snowfall rate, as it is often done for hydrological (snow) modelling, leads to a wrong representation of the distribution of snow amounts in mountains at least if no redistribution mechanisms are considered.

The major part of the presentation then discusses the processes that lead to observed small-scale snow distribution patterns. On a scale of hundreds of meters, cloud processes such as the seeder-feeder mechanism already lead to spatially inhomogeneous snowfall. The deposition of snow is subsequently altered by preferential deposition, which is caused by the interaction of flow characteristics such as speed-up or separation and turbulence with the (snow) particles. After deposition, redistribution through wind and gravity play an additional and major role especially for steep terrain. We show the relative importance of all these processes for selected environments and cases. Using spatial observations from a polarizing radar together with model simulations with ARPS and Alpine3D, we show a case which suggests that seeder-feeder effects, preferential deposition and snow transport all contribute with a similar magnitude to observed snow distribution.