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Research Highlights

November 2016

Getting at the Root of Rain

More frequent, more intense and longer-lasting storms cause heavier spring rain in central US

shelf cloud looms over Kansas, ready to dump rain on the Plain
Large “shelf” cloud looms over the Kansas prairie. These cloud types can bring large and enduring thunderstorms with heavy rain. The PNNL study focused on these storms and their origin, which is pinned to the contrasting environment of the changes over the Southern Great Plains and the nearest ocean, pulling wind-driven and moisture-laden clouds from the Gulf of Mexico to the Great Plains in spring and summer. Graphic courtesy of Rachel Miles, licensed by Creative Commons. zoomEnlarge Image.

Results: The central United States is getting clobbered with spring rain. Researchers at the Pacific Northwest National Laboratory (PNNL) studied the last 35 years and report that large intense storms are now more frequent and long-lasting, even as lighter rain storms have decreased. Investigating the source, they found that changing climate in the Southern Great Plains states in contrast with the nearest ocean produces winds that carry moisture from the Gulf of Mexico to the Great Plains. They published their findings in Nature Communications.

"These storms are impressive," said Dr. Zhe Feng, atmospheric scientist at PNNL and lead author of the study. "A storm can span the entire state of Oklahoma and last 24 hours as it propagates eastward from the Rocky Mountain foothills across the Great Plains, producing heavy rain along the way."

Why It Matters: Farmers and ranchers rely on understanding the timing, frequency, and amount of rain for survival of their businesses. Heavy rainstorms at the wrong time in a crop cycle can be devastating. Knowledge of routine cycles of storms and the amount of expected rainfall is essential for agricultural and municipal planning. Researchers in this paper found that understanding historical storm changes is an important step toward projecting future changes. Researchers are working to improve climate models so they can accurately capture these large and complicated rainstorms, for better understanding and planning.

"These storms bring well over half of the rain received in the central United States in the spring and summer," said PNNL's Dr. Ruby Leung, a coauthor with Feng and others at on the publication. "But almost no climate model can simulate these storms. Even though these storms are big enough for the models to capture, they are more complicated than the smaller isolated thunderstorms or the larger frontal rainstorms that models are wired to produce."

Methods: The PNNL team worked out a way to identify storms called mesoscale convective systems. This type of storm develops from smaller convective storms that aggregate to form the largest type of convective storms on Earth. They are best detected using satellites with a bird's eye view from space. Feng transformed well-established satellite detection methods into a new technique that he then applied to rainfall measured by radars and rain gauges for the past 35 years. This allowed the researchers to identify thousands of the large convective storms and their rainfall east of the Rocky Mountains.

The results showed the frequency of very long-lasting storms increased by about 4 percent per decade, most notably in the northern half of the central region-just below the Great Lakes. The researchers rated the storms that produced the top five percent of rainfall as extreme events and saw that extreme events have become more frequent in the last 35 years.

Mesoscale storms low level jet great plains states
Rain on the Plain. Surface warming over the Rockies increases the pressure gradient across the central United States, strengthening the southerly low-level jet and associated moisture transport, and favoring more frequent intense and long-lived mesoscale convective systems that dump large amounts of rain on the Plains. Image by Nathan Johnson, PNNL

The researchers then analyzed the region's meteorological environment. They found that the Southern Great Plains warms more than the ocean does creating a pressure gradient between the Rocky Mountains and the Atlantic Ocean. This difference induces stronger winds that push moisture up from the Gulf of Mexico, converging in the Norther Great Plains and falling in massive storms.

For more information, read PNNL news release Where the rains come from.

What's Next? Continued work on how and when storms form and how to depict them in climate models will provide more accurate information for reservoir and flood management, as well as land use decisions.


Sponsor: This research was supported by the U.S. Department of Energy Office of Science, Office of Biological and Environmental Research as part of the Regional and Global Climate Modeling Program.

Research Team: Zhe Feng, L. Ruby Leung, Samson Hagos, Robert Houze, Casey Burleyson, and Karthik Balaguru, PNNL.

Research Area: Climate and Earth Systems Science

Reference: Feng Z, LR Leung, S Hagos, RA Houze, CD Burleyson and K Balaguru. 2016. "More Frequent Intense and Long-lived Storms Dominate the Springtime Trend in Central US Rainfall." Nature Communications 7:13429. DOI: 10.1038/ncomms1342

Related Research: The Short and the Long of Storms

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In short...

In 108 characters: Climate changes cause moisture-laden wind from Gulf of Mexico to push big intense rainstorms on Great Plains @PNNLab

In one sentence: Though lighter rainstorms have decreased, researchers at Pacific Northwest National Laboratory found that large intense storms are now more frequent and long-lasting because of a changing climate that produces winds carrying moisture from the Gulf of Mexico to the Great Plains.