Why does snow fall when it rains?

How much water flows out when it rains on a blanket of snow?

In spring it is not uncommon for it to rain on an existing blanket of snow. Usually it can initially absorb part of the rain and thus initially reduces the amount of water that runs off. However, if it rains long and intensely or if the snow cover is already completely wet, as usual in spring, when it starts to rain, snow can also melt and increase runoff.

Critical conditions can also arise in autumn - as in October 2011 when numerous roads and rails were flooded after heavy rainfall and simultaneous melting of the snow that had just fallen before. The floods of October 2011 clearly showed that we still don't know enough about how heavy precipitation affects the snow cover to be able to predict such events precisely. Researchers at the SLF therefore examined over 1000 past events in which rain fell on snow, using data from the IMIS weather stations and precipitation measurements from MeteoSwiss. On the one hand, they used this data to identify where and when it had significantly rained on a blanket of snow over the past 16 years. On the other hand, they also used them to investigate snow cover processes such as melt, changes in the snow cover or water leakage into the soil for these events with the help of the SNOWPACK snow cover model - and thus determine the respective influence of the snow cover on the runoff formation.

Improve flood forecast

In most of the events investigated, rain contributed a large part of the runoff. In individual cases, however, the snowmelt accounted for up to 70% of the runoff. The following factors in particular contributed to increased runoff formation: Long-lasting rain events, spatially homogeneous snow cover properties at the beginning of the event, high air temperatures and wind speeds as well as snow cover properties that favor the rapid onset of snow cover runoff. However, the combination of rain on the snowpack does not necessarily result in critical situations. Rather, the formation of runoff is the result of a large number of processes on different spatial scales. Properties such as the thickness, layering and moisture of the snow cover have a major influence on whether a snow cover retains rain or whether additional meltwater increases runoff.

The results of this study help to improve the models of the snow hydrological service at the SLF and thus to better assess upcoming events in which rain falls on snow. These improvements, in turn, flow directly into the federal operational flood forecast.