Scientists tackle mystery of thunderstorms that strike at night
Researchers will spend six weeks this summer probing nighttime thunderstorms on the Great Plains
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Thunderstorms that form at night, without a spark from the sun's heat, are a mysterious phenomenon. This summer, scientists will be staying up late in search of some answers.
From June 1 through July 15, researchers from across North America will fan out each evening across the Great Plains, where storms are more common at night than during the day.
The effort, co-organized by numerous collaborating institutions, will use lab-equipped aircraft, ground-based instruments and weather balloons to better understand the atmospheric conditions that lead to storm formation and evolution after sunset.
What the scientists find may ultimately help improve forecasts of these sometimes damaging storms.
The Plains Elevated Convection at Night (PECAN) field campaign will involve scientists, students and support staff from eight research laboratories and 14 universities.
The $13.5 million project is largely funded by the National Science Foundation (NSF), which contributed $10.6 million. Additional support is provided by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Energy.
Aloft in the night
Thunderstorms that form during the day are less puzzling than nighttime storms.
The sun heats the surface of Earth, which in turn warms the air directly above the ground. When that warm air is forced to rise, it causes convection--a circulation of warm updrafts and cool downdrafts--and sometimes creates a storm.
But the formation of thunderstorms at night, when the sun is not baking the land, is less well understood.
"At night, the entire storm circulation is elevated higher off the ground," said National Center for Atmospheric Research (NCAR) scientist Tammy Weckwerth, a PECAN principal investigator.
"This makes observations of the conditions leading to nighttime thunderstorms much more challenging because that part of the atmosphere is not well covered by the network of instruments we normally rely on."
The vast array of instruments available to PECAN researchers will allow them to collect data higher in the atmosphere.
The data will help scientists characterize the conditions that lead to individual storm formation as well as to the clustering and organizing of these storms into large-scale systems, which can result in significant precipitation.
"Nighttime thunderstorms are an essential source of summer rain for crops, but are also a potential hazard through excessive rainfall, flash flooding and dangerous cloud-to-ground lightning," said Ed Bensman, a program director in NSF's Division of Atmospheric and Geospace Sciences, which funded the research.
"Weather forecast models often struggle to accurately account for this critical element of summer rainfall on the Great Plains," said Bensman. "The PECAN field campaign will provide researchers and operational forecasters with valuable insights into thunderstorms at night and improve our ability to model them more accurately."
Deploying in the dark
The campaign, based in Hays, Kan., will begin each day at 8 a.m. when a crew of forecasters starts developing a nightly forecast.
At 3 p.m., the scientists will use the forecast to determine where across northern Oklahoma, central Kansas or south-central Nebraska to deploy their mobile resources.
Moving dozens of people around the Great Plains each night will be a challenge for PECAN, but it's also what distinguishes it from past field projects.
"Previous severe weather campaigns focused mostly on daytime storms, for largely practical reasons, as it's more difficult to set up instruments in the dark," said Bart Geerts, an atmospheric scientist at the University of Wyoming (UW) and a PECAN principal investigator.
"But the large thunderstorm complexes traveling across the Great Plains at night really are a different beast."
Scientists believe that several factors may interact to contribute to nocturnal storm formation and maintenance: a stable layer of air at the surface; a strong wind current above that layer, known as a low-level jet; and atmospheric waves, some of which are called "bores," that ripple out from the storms themselves.
"But we don't really know how they interact," Geerts said. "That's what PECAN is about."
A better understanding of these storms will have relevance for areas beyond the Great Plains. Clustered nighttime thunderstorms are common in regions scattered across the globe.
Adds Howie Bluestein, an atmospheric scientist at the University of Oklahoma who is participating in PECAN, "Thunderstorms that occur during the middle of the night over the central Plains in the late spring and early summer have been enigmatic. Data collected during PECAN will help us better understand and predict these rain systems."
A fleet of instruments
PECAN will use three research aircraft, two of which--a UW's King Air and NASA's DC-8--will fly in clear air away from storms.
Only the third, a NOAA P-3, which is widely used in hurricane research and reconnaissance, will be able to fly into the trailing regions of storms.
The researchers also will rely on a number of ground-based instruments, known as PECAN Integrated Sounding Arrays, or PISAs.
Six PISAs will operate from fixed locations around the study area and four will be mobile, allowing them to be repositioned each night depending on where storms are expected to form.
The instruments within each PISA vary, but collectively they will give each array the ability to measure temperature, moisture and wind profiles, as well as launch weather balloons.
Among the instruments are several that were developed at NCAR's Earth Observing Laboratory, including one that uses an innovative laser-based technique to remotely measure water vapor, and an advanced wind profiler.
Finally, the scientists will have a fleet of mobile and fixed radars.
In all, PECAN researchers will have access to more than 100 instruments brought to the effort by partner institutions.
"PECAN will be using mobile radars, traveling weather stations on vans and trucks, and other systems to probe inside severe nighttime storms," said scientist Karen Kosiba of the Center for Severe Weather Research, a PECAN participant.
"We want to understand more about when, where and why winds, hail and flooding rains occur," Kosiba said. "That will allow us to better forecast these damaging events."
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