With less hail, flood risk may rise in Rockies

U. COLORADO-BOULDER (US) — Climate changes may mean summertime hail could disappear from the eastern flank of Colorado’s Rocky Mountains by 2070, a new study shows.

While less hail damage could be good news for gardeners and farmers, a shift from hail to rain can also mean more runoff, which could raise the risk of flash floods.

“In this region of elevated terrain, hail may lessen the risk of flooding because it takes awhile to melt,” says Kelly Mahoney, a research scientist at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado-Boulder. “Decision makers may not want to count on that in the future.”


In the summer in Colorado’s Front Range above about 7,500 feet, precipitation commonly falls as hail. Decision makers concerned about the safety of mountain dams and flood risk have been interested in how climate change may affect the amount and nature of precipitation in the region. (Credit: NOAA)

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For the study, published in the journal Nature Climate Change, Mahoney and colleagues used “downscaling” techniques to try to understand how climate change might affect hail-producing weather patterns across Colorado.

The research focused on storms involving pea-sized and smaller hailstones on Colorado’s Front Range, a region that stretches from the foothill communities of Colorado Springs, Denver, and Fort Collins up to the Continental Divide.

Colorado’s most damaging hailstorms tend to occur further east and involve larger hailstones not examined in the study.

In the summer in Colorado’s Front Range above about 7,500 feet, precipitation commonly falls as hail. Decision makers concerned about the safety of mountain dams and flood risk have been interested in how climate change may affect the amount and nature of precipitation in the region.

Researchers began exploring that question with results from two climate models, which assumed that levels of climate-warming greenhouse gases will continue to increase in the future, from about 390 parts per million in the atmosphere today to about 620 parts per million in 2070.

But the weather processes that form hail, like thunderstorms, occur on much smaller scales than can be reproduced by global climate models. So the team “downscaled” the global model results twice: first to regional-scale models that can take regional topography and other details into account, then again to weather-scale models that can resolve individual storms and even the cloud processes that create hail.

The regional-scale topography step was completed as part of NCAR’s North American Regional Climate Change Assessment Program.

Finally, the team compared the hailstorms of the future, from 2041 to 2070, to those of the past, from 1971 to 2000, as captured by the same sets of downscaled models. Results were similar in experiments with both climate models.

“We found a near elimination of hail at the surface,” Mahoney says.

In the future, increasingly intense storms may actually produce more hail inside clouds, but because those relatively small hailstones fall through a warmer atmosphere, they melt quickly, falling as rain at the surface or evaporating back into the atmosphere. In some regions, simulated hail fell through an additional 1,500 feet of above-freezing air in the future as compared with the past.

The researchers also found evidence that precipitation events over Colorado will become more extreme in the future, while changes in hail may depend on the size of the hailstones—results that will be explored in more detail in ongoing work.

Mahoney’s research was supported by the Postdocs Applying Climate Expertise administered by the University Corporation for Atmospheric Research and funded by CIRES Western Water Assessment, NOAA, and the U.S. Bureau of Reclamation.

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