New technique predicts frequency of heavy precipitation with global warming

January 3, 2017 by Jennifer Chu, Massachusetts Institute of Technology
MIT scientists have found that extreme precipitation events in California should become more frequent as the Earth’s climate warms over this century. Credit: Massachusetts Institute of Technology

On Dec. 11, 2014, a freight train of a storm steamed through much of California, deluging the San Francisco Bay Area with three inches of rain in just one hour. The storm was fueled by what meteorologists refer to as the "Pineapple Express"—an atmospheric river of moisture that is whipped up over the Pacific's tropical waters and swept north with the jet stream.

By evening, record rainfall had set off mudslides, floods, and power outages across the state. The storm, which has been called California's "storm of the decade," is among the state's most events in recent history.

Now MIT scientists have found that such extreme precipitation events in California should become more frequent as the Earth's climate warms over this century. The researchers developed a new technique that predicts the frequency of local, extreme rainfall events by identifying telltale large-scale patterns in . For California, they calculated that, if the world's average temperatures rise by 4 degrees Celsius by the year 2100, the state will experience three more extreme precipitation events than the current average, per year.

The researchers, who have published their results in the Journal of Climate, say their technique significantly reduces the uncertainty of extreme storm predictions made by standard .

"One of the struggles is, coarse climate models produce a wide range of outcomes. [Rainfall] can increase or decrease," says Adam Schlosser, senior research scientist in MIT's Joint Program on the Science and Policy of Global Change. "What our method tells you is, for California, we're very confident that [heavy precipitation] will increase by the end of the century."

The research was led by Xiang Gao, a research scientist in the Joint Program on the Science and Policy of Global Change. The paper's co-authors include Paul O'Gorman, associate professor of earth, atmospheric, and planetary sciences; Erwan Monier, principal research scientist in the Joint Program; and Dara Entekhabi, the Bacardi Stockholm Water Foundations Professor of Civil and Environmental Engineering.

Large-scale connection

Currently, researchers estimate the frequency of local heavy precipitation events mainly by using precipitation information simulated from global climate models. But such models typically carry out complex computations to simulate climate processes across hundreds and even thousands of kilometers. At such coarse resolution, it's extremely difficult for such models to adequately represent small-scale features such as moisture convection and topography, which are essential to making accurate predictions of precipitation.

To get a better picture of how future precipitation events might change region by region, Gao decided to focus on not simulated precipitation but large-scale atmospheric patterns, which climate models are able to simulate much more reliably.

"We've actually found there's a connection between what climate models do really well, which is to simulate large-scale motions of the atmosphere, and local, heavy precipitation events," Schlosser says. "We can use this association to tell how frequently these events are occurring now, and how they will change locally, like in New England, or the West Coast."

Weather snapshots

While definitions vary for what is considered an extreme precipitation event, in this case the researchers defined such an event as being within the top 5 percent of a region's precipitation amounts in a particular season, over periods of almost three decades. They focused their analysis on two areas: California and the Midwest, regions which generally experience relatively high amounts of precipitation in the winter and summer, respectively.

For both regions, the team analyzed large-scale atmospheric features such as wind currents and moisture content, from 1979 to 2005, and noted their patterns each day that extreme precipitation occurred. Using statistical analysis, the researchers identified telltale patterns in the atmospheric data that were associated with heavy storms.

"We essentially take snapshots of all the relevant weather information, and we find a common picture, which is used as our red flag," Schlosser explains. "When we examine historical simulations from a suite of state-of-the-art climate models, we peg every time we see that pattern."

Using the new scheme, the team was able to reproduce collectively the frequency of extreme events that were observed over the 27-year period. More importantly, the results are much more accurate than those based on simulated precipitation from the same climate models.

"None of the models are even close to the observations," Gao says. "And regardless of the combination of atmospheric variables we used, the new schemes were much closer to observations."

"Actionable information"

Bolstered by their results, the team applied their technique to large-scale atmospheric patterns from climate models to predict how the frequency of heavy storms may change in a warming climate in California and the Midwest over the next century. They analyzed each region under two climate scenarios: a "business as usual" case, in which the world is projected to warm by 4 degrees Celsius by 2100, and a policy-driven case, in which global environmental policies that regulate greenhouse gases should keep the temperature increase to 2 degrees Celsius.

For each scenario, the team flagged those modeled large-scale atmospheric patterns that they had determined to be associated with heavy storms. In the Midwest, yearly instances of summer extreme precipitation decreased slightly under both warming scenarios, although the researchers say the results are not without uncertainty.

For California, the picture is much clearer: Under the more intense scenario of global warming, the state will experience three more extreme precipitation events per year, on the order of the December 2014 storm. Under the policy-driven scenario, Schlosser says "that trend is cut in half."

The team is now applying its technique to predict changes in heat waves from a globally warming climate. The researchers are looking for patterns in atmospheric data that correlate with past heat waves. If they can more reliably predict the frequency of heat waves in the future, Schlosser says that can be extremely helpful for the long-term maintenance of power grids and transformers.

"That is actionable information," Schlosser says.

Explore further: Extreme downpours could increase fivefold across parts of the US

More information: Xiang Gao et al. 21st Century Changes in U.S. Regional Heavy Precipitation FrequencyBased on Resolved Atmospheric Patterns, Journal of Climate (2016). DOI: 10.1175/JCLI-D-16-0544.1

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9 comments

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gkam
3.9 / 5 (7) Jan 03, 2017
It may be hard to predict the summation of the interactions between complex systems, as in our weather. They exist as systems in stable states until perturbed. Then, they can start to vary, to oscillate, generating some unexpected extremes before falling into another stable state, but one which may not be favorable to Human habitation.
JIm Steele Landscapes and Cycles
1 / 5 (9) Jan 03, 2017
During the La NIna years when high pressure ridges blocked storms, producing California's drought, climate researchers argued rising CO2 will create permanent droughts. Now with increasing rains they argue rising CO2 will cause more extreme rainfall. It has been well known that atmospheric rivers and hurricanes are naturally responsible for most extreme rainfall events, as they have been doing for millennia. I wrote about such effects of atmospheric rivers 3 years.

Conversely as New England endured record snowfall contradictng climate scientists' predictions of decreasing snow fall, climate scientists reported that CO2 warming was increasing precipitation, and thus snow. Now New England is experiencing drought.

There are natural oscillations that create shifts in extremes every few decades. But climate scientists want to blame every weather event on CO2. Those climate charlatans should ashamed of their fear mongering and constant obfiscation of science.
snoosebaum
1 / 5 (5) Jan 03, 2017
@ jim steele , what do you think of the PDO as in Roy Spenser's idea that its responsible for the larger portion of global temp change. ? Makes sense doesn't it,ie whats the biggest thing that moves over longer time periods and is hot and cold, not the land which just aborbs and radiates on a daily basis.
aksdad
1 / 5 (7) Jan 04, 2017
Or you could just examine tea leaves and make predictions. It's likely to be just as accurate.
antigoracle
1 / 5 (7) Jan 04, 2017
Oh the flood of Pathological lies from the AGW Cult and their Pathological "science".

How much manmade CO2 was responsible for the following--

The Great Flood of 1862 was the largest flood in the recorded history of Oregon, Nevada, and California...... https://en.wikipe..._of_1862

The Los Angeles flood of 1938 was responsible for inundating much of Los Angeles, Orange, and Riverside counties,....... https://en.wikipe..._of_1938

Drink up Chicken Littles.
HeloMenelo
4 / 5 (4) Jan 04, 2017
Or you could just examine tea leaves and make predictions. It's likely to be just as accurate.

Smoking tea leaves like you and your 3 socks askdaddy, snoozelose, antisciencegorilla might let you think that, but rest assured, the truth is quite the opposite ;)
HeloMenelo
4 / 5 (4) Jan 04, 2017
Oh the flood of Pathological lies from the AGW Cult and their Pathological "science".

How much manmade CO2 was responsible for the following--

The Great Flood of 1862 was the largest flood in the recorded history of Oregon, Nevada, and California...... https://en.wikipe..._of_1862

Drink up Chicken Littles.


monkeypoo trying to prove how little he knows about climate change... again with 2 unrelated weak links ? yaaaawn... Here, have another banana ;)
antigoracle
1 / 5 (2) Jan 04, 2017
How much manmade CO2 caused the 5th largest flood in history..

https://www.youtu...w_cgj51w

Drink up Chicken Littles.
humy
not rated yet Jan 11, 2017
Or you could just examine tea leaves and make predictions. It's likely to be just as accurate.

No, because that isn't applying scientific method.

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