Using Radar to Track Individual Raindrops

Cold War missile-tracking radar helps UND researchers advance understanding of thunderstorms

by Patrick Miller

A “happy accident” with a Cold War-era U.S. Navy radar is helping UND researchers to better understand thunderstorms, leading to better forecasts and more accurate computer models.

UND research professor David Delene specializes in studying cloud physics in the Department of Atmospheric Sciences of the John D. Odegard School of Aerospace Sciences. In 2015 and 2019, he was the principal investigator for the aircraft research on projects sponsored by the U.S. Navy and led by the Naval Research Laboratory using specially equipped research aircraft in concert with the Navy’s Mid-Course Radar (MCR) along Florida’s east coast.

The projects – known as CAPE15 and CAPEX19 – not only gave researchers detailed insights into thunderstorms, but also provided unique experiences for Delene’s undergraduate and graduate students.

Research goals

“We have two main goals with these projects,” Delene explained. “One is to improve and further understand the Navy’s Mid-Course Radar by looking at meteorological targets, rather than rocket launches and tracking ballistic missiles. We’re proving the radar for weather research purposes.

“The other main objective is to improve the forecasting of weather by better understanding clouds and cloud cover,” he continued. “We want to improve the forecasting of clouds and air currents, with the secondary goal of understanding the radar system better.”

Nicholas Gapp (left) was a team member as an undergraduate and graduate student under UND Research Professor David Delene. They’re viewing an issue of the Bulletin of the American Meteorological Society in which they coauthored an article on their research. Photo by Patrick C. Miller/UND Today.

Fargo native Nick Gapp earned bachelor’s and master’s degrees from UND in atmospheric science in 2016 and 2019 respectively. He was part of both research projects to study thunderstorms along Florida’s east coast.

During his junior year in the summer of 2015, Gapp served as the flight engineer aboard a Cessna Citation II research jet aircraft – then owned by UND – during CAPE2015 project flying out of the Space Coast Regional Airport at Titusville, Fla. In the summer of 2019, Gapp incorporated aircraft position information into the radar system and worked closely with a Navy Research Lab scientist on radar data collection.

“It’s not every day you get to fly around thunderstorms and take measurements to figure out what’s going on inside the massive cloud structure,” Gapp said. “I was getting bumped around in the back of the plane, making sure the instruments were working, the data was coming in and everything was going well.”

Discovery by happy accident

What’s different about these projects is the MCR system, originally designed in the 1960s to track test launches of intercontinental ballistic missiles (ICBMs) from submarines off the coast of California.

“It was developed to track fast-moving, big objects and to see them in great detail,” Delene noted. “The main thing was to track the rockets and see if there was any debris coming off them. It’s very sensitive. They can even see panels on a spacecraft.”

After the space shuttle Challenger disaster in 1986, the MCR was moved to near NASA’s Cape Canaveral facility in Florida to track shuttle launches. Gapp said a “happy accident” led scientists to discover the potential use for MCR in meteorological research. It began with operators observing streaks on the radar.

“As they were seeing these streaks, they went outside the radar control building and saw large raindrops falling on them,” Gapp said. “They surmised it was happening right above them since the radar was pointing straight up.”

A research aircraft equipped with sophisticated instruments records data near a Florida thunderstorm. Photo by Nicholas Gapp.

Delene said it took further investigation to determine if the theory that the radar could detect individual raindrops was correct.

“They’ve validated the theory,” he said. “You have good understanding when the experimental observations match the theory.”

Validating the theory

Delene said Gapp’s data analysis has been instrumental in validating that the radar on the ground and the instruments in plane in the air are both observing the same events inside the thunderstorm.

“We understand the radar, we understand the aircraft and now we’re comparing them to see if they show agreement in reflectivity values,” Delene explained. “What’s also important is to use that reflectivity value to come up with total water content in the storm, which is something not directly sensed by the radar. How the reflectivity is related to total water content is a relationship Nick came up with.”

As Gapp explains, “It’s what the weatherman reports on TV – the fancy colors on the radar. What I’m comparing are the actual values between what I derive from the measurements versus what the radar sees from the reflectivity. They agree pretty well. They agree quite often within their measurement uncertainty. They’re very comparable.”

Delene is interested in using this research to better understand how ice crystals aggregate or clump together in the tops of thunderstorms above 40,000 feet and how to more accurately model the area thunderstorms cover.

“Thunderstorms create high-level clouds that cover a big area – the anvil of the thunderstorm,” he noted. “One thing cloud models don’t get right is the extent of these anvil systems. How big of a cloud do they form in terms of area? That’s one of the problems the models have. They either spread this out and cover the entire earth or get them too small.”

As a result of his undergraduate and graduate research experience, Gapp completed an internship, developed a senior project, used the research as the basis for his master’s thesis and coauthored a paper with Delene and others published earlier this year in the Bulletin of the American Meteorological Society (BAMS) titled “Radar Detection of Individual Raindrops.” In 2019, Gapp again served as a member of the UND research team during CAPEX2019, where he collected and analyzed data gathered in the air and on the ground.

The Cessna Citation II weather research aircraft parked at the the Space Coast Regional Airport at Titusville, Fla., as thunderstorms roll through. Photo by UND Atmospheric Sciences.

Delene said many of UND’s atmospheric science undergraduate and graduate students have the experience of going into the field to gather the data they use in their research.

“Many scientists don’t take the measurements they use,” Delene explained. “They might come from satellites or other people. It’s unique for students at UND to be able to go out to a field project, take the measurements and analyze them. It gives them a different perspective.”

When CAPEX2015 research flights concluded in 2015, Gapp stayed with the project to process and analyze the data, which also involved learning how to write computer code. His master’s thesis served as the basis for another article that’s been submitted to BAMS. He’s now working in the Department of Atmospheric Sciences to analyze the data collected from the CAPEX2019 flights in Florida.

“We need to understand all these measurements provided by the processes and the sophisticated instruments,” Delene said. “It’s complicated. That’s why we take two weeks of measurements and spend four years analyzing them.”

After receiving his master’s degree last year, Gapp planned to accept a position with the Naval Research Laboratory in Monterey, Calif. However, the COVID-19 pandemic intervened, closing the facility and putting his plans on hold. He’s currently working from Grand Forks as a contractor for the lab.