Abstract
Abstract. In October 2011, the Swiss Alps underwent a marked rain-on-snow (ROS) event when a large snowfall on 8 and 9 October was followed by intense rain on 10 October. This resulted in severe flooding in some parts of Switzerland. Model simulations were carried out for 14 meteorological stations in two affected regions of the Swiss Alps using the detailed physics-based snowpack model SNOWPACK. We also conducted an ensemble sensitivity study, in which repeated simulations for a specific station were done with meteorological forcing and rainfall from other stations. This allowed the quantification of the contribution of rainfall, snow melt and liquid water storage on generating snowpack runoff. In the simulations, the snowpack produced runoff about 4–6 h after rainfall started, and total snowpack runoff became higher than total rainfall after about 11–13 h. These values appeared to be strongly dependent on snow depth, rainfall and melt rates. Deeper snow covers had more storage potential and could absorb all rain and meltwater in the first hours, whereas the snowpack runoff from shallow snow covers reacts much more quickly. However, the simulated snowpack runoff rates exceeded the rainfall intensities in both snow depth classes. In addition to snow melt, the water released due to the reduction of liquid water storage contributed to excess snowpack runoff. This effect appears to be stronger for deeper snow covers and likely results from structural changes to the snowpack due to settling and wet snow metamorphism. These results are specifically valid for the point scale simulations performed in this study and for ROS events on relatively fresh snow.
Highlights
IntroductionThe presence of a snow cover is an important factor in hydrological processes
For mountain regions, the presence of a snow cover is an important factor in hydrological processes
One type of event that is still poorly understood is the behaviour of a snow cover during rainfall. These rain-on-snow (ROS) events are often accompanied by strong snow melt, due to high latent heat exchange and incoming long-wave radiation (ILWR) that reduces the radiative cooling of the snowpack (Marks et al, 2001; Mazurkiewicz et al, 2008)
Summary
The presence of a snow cover is an important factor in hydrological processes. One type of event that is still poorly understood is the behaviour of a snow cover during rainfall These rain-on-snow (ROS) events are often accompanied by strong snow melt, due to high latent heat exchange and incoming long-wave radiation (ILWR) that reduces the radiative cooling of the snowpack (Marks et al, 2001; Mazurkiewicz et al, 2008). These effects increase the water available for outflow from the snowpack, which hereafter we will refer to as snowpack runoff. Rainfall reduces snowpack stability, resulting in stronger wet snow avalanche activity (Conway and Raymond, 1993)
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