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Boulder Scientist Leads Study Showing Intense Rains, Warming

November 15, 2018

The Summit County Rescue team works to save Suzanne Sophocles, at center, from her severely flooding home on Sept. 13, 2013, on Streamcrest Drive in Boulder. Scientists have found that half of the world's precipitation falls in just 12 days, and they expect that trend to become more pronounced.

Half of the world’s annual precipitation falls in just 12 days, according to a new study published by a scientist at the National Center for Atmospheric Research in Boulder, and that trend is expected to continue.

“Climate scientists, when we think about precipitation, we know it’s kind of uneven and we expect it to become more uneven in response to warming,” said Angeline Pendergrass, an NCAR project scientist and lead author of the study. “This is something we have kind of known, but it hadn’t really been quantified and so we set out to quantify it.

“I found that half of annual precipitation was falling in the wettest 12 days each year,” Pendergrass said. “That really surprised me, because I thought it would be more than that. That is a remarkably small, concentrated period of time.”

And by the end of the century, climate models indicate that the uneven distribution of rain and snow will become more exaggerated, with half of annual precipitation coming down in 11 days.

“What we found is that the expected increases happen when it’s already the wettest,” Pendergrass said. “The rainiest days get rainier.”

She noted that the findings apply even in places with little rain.

“Folks in Arizona, for example, would not be surprised to hear they get their precipitation in just a few days each year,” Pendergrass said. “But in Seattle, that number is still less than a month each year. It is still pretty uneven, in response to warming.

“And this unevenness increases (projecting forward). It increases in 97 percent of the climate models that we looked at. It’s a pretty clear signal.”

Pendergrass developed her analysis with Reto Knutti, of the Institute for Atmospheric and Climate Science in Zurich, Switzerland. Their data came from 185 ground stations for the 16 years from 1999 through 2014, a period when measurements could be validated against data from NASA’s Tropical Rainfall Measuring Mission satellite, according to a news release. The stations were dispersed all over the globe, but most were in North America, Eurasia and Australia.

To project into the future, researchers used simulations from 36 of the world’s leading climate models that had data for daily precipitation. Then, they pinpointed what the climate model projections for the last 16 years of this century would translate to for the individual observation stations.

Scientists found that the total annual precipitations at the observation stations increased slightly in the model runs, but the additional precipitation fell unevenly, with half the extra rain and snow falling over just six days.

Asked whether Colorado’s 2013 flood is an example of what their findings revealed, Pendergrass expressed caution about drawing too firm a conclusion from one specific event.

However, she added, “What these results point to is that for a big flood we would have had in a stable climate in the past, we would expect that same flood to be more intense in the future, due to global warming.”

Pendergrass said her team also looked at projections for different levels of warming, including those where humans did not do much mitigation of climate change.

“In projections where we didn’t do a lot of mitigation, the increased unevenness is even bigger,” she said, adding that “maybe that would motivate someone to putting effort to into mitigating climate change.

“All is not lost, yet,” she added.

The results of the study are published in the American Geophysical Union’s Geophysical Research Letters. The research was supported by the U.S. Department of Energy and the National Science Foundation.

Charlie Brennan: 303-473-1327, brennanc@dailycamera.com or twitter.com/chasbrennan

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