Extreme value statistics of snowfall in the Swiss Alpine region

The mountain snow cover is an important source of water but also leads to natural hazards, such as avalanches and floods. We use data collected during winters 1999/2000 to 2007/2008 by 239 automatic and manual measurement stations in Switzerland to highlight spatial characteristics of extreme snowfall. With the help of extreme value theory based on a "peaks-over-threshold'' approach and a Poisson point process representation, we analyze spatial patterns and correlation characteristics. Our analyses show that a significant number of stations do not follow the Gumbel distribution. In particular, low altitude stations in the Swiss Plateau are heavy tailed because of rare extraordinary snowfall events. Spatial characteristics of extreme snowfall are compared to those of the mean snowfall. Altitudinal dependence and spatial distribution of mean and extreme snowfall are similar. Both mean snowfall and extreme snowfall show an increase of magnitude between 400 and 2200 m a.s.l. and a constant or slightly decreasing magnitude at higher altitudes. Below 1200 m a.s.l., the increase with altitude is stronger because of the rain-snow transition. Another finding is that the spatial correlation pattern of extreme snowfall is similar to that of mean snowfall, both of which are determined by the main climatological regions of Switzerland. An analysis based on those stations with a long record shows that extreme snowfall was 10% lower in the nine winters investigated than in the long-term period, but the main spatial characteristics of the two periods show no change.