Rain-on-Snow Flooding and the Sensitivity of Mountain Snowcovers to Temperature, Humidity, and Phase Change in a Warming Climate

Abstract:

Devastating floods in mountain regions of the western US and Canada can result from rapid snowmelt during mid-winter rain-on-snow (ROS) events. Key components of snowmelt flooding during ROS are conditions prior to the storm, the combination of temperature, humidity and wind during the event, and the extent to which the snowcover is exposed to the wind. The critical antecedent condition is the extension of the snowcover to lower elevations. In mountain basins this significantly increases the snow-covered area (SCA) and volume of water stored in the snowcover. During ROS events the elevation of the rain/snow transition can rise, resulting in rain over large snow-covered areas. During typical conditions the mountain snowcover is generally cooled by evaporation (latent heat flux), and warmed by sensible heat flux, such that the turbulent fluxes tend to balance, and have only modest effect on the energy state of the snowcover. However, during ROS higher humidity results in condensation on the snow, increasing melt energy by 50 – 100 times such that most of the energy for snowmelt comes from the combination of sensible and latent heat exchange. If SCA is extensive and exposed to the wind, the surface water input (SWI) can be more than doubled by the addition of melt to the rain. Data indicate that as the climate warms, higher temperatures and more humid conditions during storms may result in more frequent flooding events from mountain regions.