Articles | Volume 18, issue 11
Research article
26 Nov 2014
Research article |  | 26 Nov 2014

Model simulations of the modulating effect of the snow cover in a rain-on-snow event

N. Wever, T. Jonas, C. Fierz, and M. Lehning

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.

Short summary
We simulated a severe rain-on-snow event in the Swiss Alps in October 2011 with a detailed multi-layer snow cover model. We found a strong modulating effect of the incoming rainfall signal by the snow cover. Initially, water from both rainfall and snow melt was absorbed by the snowpack. But once the snowpack released the stored water, simulated outflow rates exceeded rainfall and snow melt rates. The simulations suggest that structural snowpack changes enhanced the outflow during this event.