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Colorado’s disappearing hail?

Study shows small hail dwindling in future storms over high terrain

A slice of hailstone on a black background.

A thin slice of hailstone under polarized light produces a kaleidoscope of color in an NCAR laboratory.

| The effects of a warming climate on hail are largely unknown, as global climate models are too coarse in resolution to simulate hailstorms in detail. But a new modeling study in Nature Climate Change tackles this subject, looking at the future of hail in Colorado’s Rocky Mountains over the coming decades. Results show a near-elimination of small hail in these specific high-elevation locations, suggesting that it will fall as rain instead.

The research team, which is led by Kelly Mahoney (NOAA/CIRES/University of Colorado) and includes NCAR’s Greg Thompson, used NARCCAP (North American Regional Climate Change Assessment Program) analyses from climate models that assume atmospheric greenhouse gases will increase to 620 parts per million by the year 2070. They ran simulations comparing hailstorms during the period 2041–2070 to past hailstorms (1971–2000) that served as controls.

The modeling experiment shows small hail nearly disappearing from future storms over terrain higher than 7,500 feet (2,290 meters). Such storms may actually produce more hail inside clouds, according to the research, but as these relatively small hailstones fall through a warmer atmosphere, they are more likely to fall as rain or evaporate.

The research may have implications for runoff and flooding. In the mountainous terrain featured in the study, where summer precipitation commonly falls as hail rather than rain, scientists think that hail may serve as a buffer against the risk of flash flooding when heavy precipitation falls in a short amount of time.

"In this region of elevated terrain, hail may lessen the risk of flooding because it takes awhile to melt," explains Mahoney, a postdoctoral scientist whose research program, Postdocs Applying Climate Expertise (PACE), is administered by UCAR. "Decision makers may not want to count on that in the future."

The study did not look at hailstorms that occur further east on the Plains, where larger hailstones are known for causing considerable damage to agriculture.

Mahoney, K., M.A. Alexander, G. Thompson, J.J. Barsugli, and J.D. Scott, “Changes in hail and flood risk in high-resolution simulations over Colorado’s mountains,” Nature Climate Change, 2012; DOI: 10.1038/nclimate1344