Global warming intensified Colorado’s deadly 2013 floods

A NOAA map shows the area around Boulder with more than 15 inches of rain during the deadly Sept. 2013 rainstorm.

Detailed modeling helps project how climate change will alter extreme weather

Staff Report

Global warming likely boosted rainfall during Colorado’s deadly 2013 floods by 30 percent, according to new research. The September storms killed nine people and destroyed or damaged about 900 homes. Altogether, some spots saw more than 17 inches of rain, sending the South Platte River to a record high level.

Previous research has suggested that global warming helped intensify the storm, and in the new research, scientists at Lawrence Berkeley National Laboratory used climate models to “hindcast” the pattern. The paper was published July 18, 2017 in the online journal Weather and Climate Extremes.

“The storm was so strong, so intense, that the standard climate models that do not resolve fine-scale details were unable to characterize the severe precipitation or large scale meteorological pattern associated with the storm,” said Michael Wehner, a climate scientist in the lab’s Computational Research Division and co-author of the paper.

In their modeling, the scientists looked at two versions of the storm; one based on realistic current conditions that takes human-induced changes to the atmosphere and the associated climate change into account, and one that removed the portion of observed climate change attributed to human activities. The difference between the results were then attributed to these human activities.

“This event was typical in terms of how the storm sent water to the area, but it was unusual in terms of the amount of water and the timing,” said co-author Dáithí Stone, also of Berkeley Lab. “We know that the amount of water air can hold increases by about 6 percent per degree Celsius increase, which led us to expect that rainfall would have been 9-15 percent higher, but instead we found it was 30 percent higher.”

The results perplexed the team initially as the answers were turning out to be more complicated than they originally postulated – the storm was more violent in terms of both wind and rain.

“We had expected the moist air hitting the mountain range to be ‘pushing’ water out of the air,” said lead author Pardeep Pall. “What we had not realized was that the rain itself would also be ‘pulling’ more air in. Air rises as it is raining, and that in turn pulled in more air from below, which was wet, producing more rain, causing more air to raise, pulling in more air, and so on.”

Christina Patricola, co-author and research scientist in the lab’s Climate and Ecosystem Sciences Division, said understanding extreme weather is important because the way we experience climate tends to be dominated by extreme weather.

“This was a very rare event and remains so, and we’re not making predictions with this work,” Stone said. “The exact event won’t happen again, but if we get the same sort of weather pattern in a climate that is even warmer than today’s, then we can expect it to dump even more rain.”

“Our climate modeling framework opens the door to understanding other types of extreme weather events,” said Patricola. “We are now investigating how humans may have influenced tropical cyclones. Advances in supercomputing make it feasible to run simulations that can reveal what is happening inside storm clouds.”

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