Rain-on-snow

The refreezing of rainfall on a layer of snow can cause wide-ranging impacts.

Rain-on-snow (ROS) events occur when rain falls on a preexisting snowpack. The rain can alter the characteristics of the snowpack, potentially leading to a wide array of hazards depending on the season and location. The severity of these hazards depends on the elevation, extent of the snowpack, and the amount of rainfall. Rain-on-snow is a relatively rare phenomenon for most areas but is relatively common in the Arctic, western Eurasia, western North America, the northeastern U.S., and southeastern Canada where sizable snowpacks develop. A given location may experience up to around 10 ROS events per season. Climate change is expected to increase the frequency of ROS events in the Arctic and the western United States as warming air temperatures increase the likelihood of rain. This increase in ROS frequency is anticipated in mountainous areas and at higher latitudes that are generally colder than the freezing point during the winter; at lower elevations and lower latitudes, the increase in temperatures associated with climate change may decrease the frequency of ROS events by lowering the quantity of snowpack.

Rain can refreeze after falling on snowpack if the temperature drops sharply thereafter, producing a layer of ice atop the snow. This hardened crust can block animals such as muskoxen and reindeer from foraging and grazing upon the vegetation in the snow. In the Arctic, where many regions experience rain-on-snow during mid-winter, ROS events have resulted in the deaths of tens of thousands of animals by inducing mass starvation via ice crust formation. An ROS event in 2003 on Banks Island resulted in the deaths of 20,000 oxen after blocking their access to food. Similarly, 61,000 reindeer on the Yamal Peninsula starved to death following an ROS event in November 2013. Water deposited by rain-on-snow can percolate through the snowpack before refreezing, releasing latent heat which can warm and melt snow and the underlying soil and permafrost. This affects the populations of animals reliant on snow for shelter such as grouse, lemmings, owls, ptarmigan, and voles. The mortality rates for cubs of polar bears and ringed seals can increase due to the destruction of burrows following ROS events. Rain-on-snow can thus greatly affect local ecosystems and people reliant on those animals for their livelihoods. These effects can linger across generations despite ROS events lasting on the order of hours to days. Ice formation induced by rain-on-snow can also lead to icy roads and runways.

On a warmer snowpack, contact of moist air and rain with snow cover releases energy that can accelerate snowmelt. The rate of snowmelt depends on the air temperature and the rainfall amount. The combination of rainfall and snowmelt can substantially increase the amount of runoff, potentially leading to flooding. In warmer rain storms, about 20 percent of the liquid water available to runoff arises from meltwater. Additional factors such as the wind, humidity, and the radiation emitted by the Earth and the Sun influence the outcome of rain-on-snow. ROS events are a major cause of wintertime and springtime flooding at higher latitudes and in mountainous areas. While most ROS events do not produce flooding, those that do can lead to significant flood events. These flood events are more frequent in mountainous regions in mid-latitude coastal areas than in the Arctic or other inland areas. Most extreme flooding observed in areas where rain-on-snow is commonplace is associated with ROS events. Some of the most severe and costly floods in U.S. history have been associated with ROS events, such as the flooding that led to the Oroville Dam crisis in 2017. The increase in runoff can also reduce water availability after the snow season by freeing the water that would otherwise be stored in the snowpack. Because the melting of snow resulting from rain-on-snow is often faster and briefer than melting caused by warmer temperatures alone, the amount of snowmelt entering the soil or becoming groundwater is also decreased, further reducing the eventual availability of water in the ground and streams. Rain-on-snow can also trigger avalanches and landslides by loosening or weakening the overall snowpack.

In the Western United States, rain-on-snow flooding occurs most frequently at elevations of 3,000–5,000 ft (910–1,520 m) above sea level. These areas tend to experience both rain and snow. A snowpack can accumulate during abnormally cold periods, increasing the susceptibility to a rain-on-snow event. Storms accompanied by unusually warm air can also bring rainfall to higher elevations. This mechanism of rain-on-snow production is expected to increase with climate change. In the Sierra Nevada, winter storms associated with atmospheric rivers are 2.5 times more likely to produce rain-on-snow compared to other winter storms. Atmospheric rivers tend to bring a relatively warm influx of moist air; during 1998–2014, ROS-producing atmospheric river storms in the Sierra Nevada were 4 °F (2.2 °C) warmer than other atmospheric river storms and tended to originate from the tropical Pacific. The risk of ROS-induced flood events in parts of the western U.S. and western Canada may double during the 21st century as the frequency of rainfall at higher elevations increases.

References

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