Why Rain on Snow in California’s Mountains Concerns Scientists

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In an aerial view, a person shovels along a snowy road lined with snow banks piled up by recent and past storms in the Sierra Nevada mountains March 12, 2023 in Mammoth Lakes, California, following an atmospheric flow event.

After storms in January and February 2023 that dumped record amounts of snow, another round of powerful atmospheric flows is hitting California. The storms are warmer this time, and they are issue flood warnings when they bring the rain higher up in the mountains – on the snowpack.

professor Keith Musselmanwho studies water and climate change at the University of Colorado Institute for Arctic and Alpine Research, explained the complex risks of rain on snow and how they might change in a warming climate.

What happens when rain falls on a snow cover?

In much of the United States, heavy rain storms can coincide with seasonal snow cover. When that happens, the resulting water runoff can be much greater than what is generated by rain or snowmelt alone. The combination made it happen some of the nation most destructive and costly floods, including the 1996 Midwest floods and the 2017 flood damaged California’s Oroville Dam.

Contrary to popular belief, rain itself has limited energy to melt snow. Rather, it is the warm temperatures, strong winds and high humidity that can transport considerable energy in form latent And noticeable heat, which mainly drive the snowmelt at rain-on-snow events.

The snowpack has air spaces through which water can move. When the rain falls, the water can flow through the layers of snowpack relatively quickly to reach the ground below. How streams respond to this runoff depends on how much water is already flowing and how saturated the soil is.

If the soil is not yet saturated, it can dampen or delay a flood response by soaking up rain and melting snow. But when the ground is saturated, snowmelt combined with rain can cause rapid and devastating flooding.

One of the challenges in managing these rain-on-snow events is that flood risk is difficult to predict.

Knowledge of weather and hydrological conditions is required to predict whether flooding will occur. It requires knowledge of pre-storm soil moisture and snowpack conditions, the elevation at which rain turns to snow, precipitation rate, wind speed, air temperature and humidity, and estimates of how these factors contribute to snowmelt. Additionally, each factor varies over time and in complex ways during a storm, especially in a mountainous environment.

For this reason, rain-on-snow floods are referred to as composite extreme events. Despite the tremendous damage they can do, it may come as a surprise how little is known about how they vary in temporal, spatial extent, and intensity.

California gets another atmospheric flux, with more rain on snow expected. How does the rain-on-snow effect differ depending on the altitude in the mountains there?

In the mountains of California right now, it’s the mid-elevations that people need to watch out for.

It has rained more than snowed in the lower elevations, so less snow can melt. And at the highest elevations, colder temperatures favor the continued accumulation of deep snowpack, and precipitation is less likely.

In the middle transition zone – where significant rainfall or snowfall can occur – rain-on-snow events are most common, causing both melting and the risk of roof collapse.

If all storms were generated equally, there would be well-defined zones of rain and snow, and the risk of rain-on-snow flooding would be low. But that’s not what happens. Instead, the height of the snow zone varies not only during an event, but also significantly from one storm to the next.

The most destructive rain-on-snow events occur when rivers are already flowing high and soils are saturated, which can occur in response to a series of warm atmospheric flows interacting with deep snowpack — as California’s mountains are currently doing . The order in which these storms occur – or the order of the storms – is particularly important for assessing flood risk because these events are caused in part by rapid alternations between cold periods of snow accumulation and subsequent warm precipitation events.

What does research show about the future risk of rain-on-snow events in a warming climate?

Even less is known about how rain-on-snow flood risk might respond to the planet’s warming.

In a warmer climate, there is less risk of rain falling on snow at lower elevations when the snow cover decreasesespecially in warmer regions like the Pacific Northwest.

But at higher altitudes frequent rain on snow Events are expected. While warmer temperatures While precipitation intensity is projected to increase, research shows that this is not the most important factor in this risk. Much of the expected increase in rain-on-snow flood risk is a result of the Rain-snow transition zone that extends higher in altitude to include alpine areas that historically received predominantly snowfall.

Flood defense and reservoir management systems in these mountain regions must account for these future changes in rain-on-snow events—in addition to changes in precipitation intensity and storm order fully understand and prepare the local risk of flooding as the planet warms.

So, will the projected increase in precipitation extremes and winter precipitation increase the occurrence of rain on snow and the associated risk of flooding? Or will less snow cover and greater soil moisture deficits reduce rain-on-snow flood risk in a warmer climate?

In a future climate, the response to rain-on-snow flood risk is expected to change in complex and often contradictory ways. The projected changes are probably vary B. by region, season, climate model, emission scenario and future time horizon. It is a costly risk that requires more research.

By Keith MusselmanAssistant Professor of Geography, Mountain Hydrology and Climate Change, University of Colorado Boulder

This article is republished by The conversation under a Creative Commons license. read this original article.