PNNL

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.

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.

Acknowledgments

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.

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