Typically, pilots avoid flying where icing conditions can occur, but this week, researchers will fly into extremely cold weather in order to study an Arctic phenomenon known as marine cold air outbreaks (CAOs). CAOs and the clouds they generate may have far-reaching climate impacts and scientists are hoping to understand more about the role they are playing in the rapid warming of the Arctic.

From February 22 to April 7, researchers will travel to Kiruna, Sweden’s northernmost city, for the Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR) field campaign. CAESAR is co-organized by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) and a collaboration of scientists, students, and staff from eight U.S. universities; Stockholm University, Sweden; University of Oslo, Norway; and the U.S. Naval Research Laboratory. 

“The Arctic is changing rapidly, warming at a rate two to four times faster than the global average,” said Paquita Zuidema, a professor of atmospheric sciences at the University of Miami and a CAESAR principal investigator. “A consensus on why and how this is occurring is still lacking, and the role of clouds is uncertain. The more we can learn about Arctic cloud behavior now, the better we can predict the Arctic of the future.” 

Terra Incognita 

During a marine CAO, cold Arctic air meets the relatively warm ocean, and the difference in air and sea temperature causes the ocean to release large amounts of heat and moisture into the air. Shallow convective clouds, reaching 4-5 kilometers (about 2.5-3 miles) in height at most, form. These clouds produce heavy snowfall, especially at the coast, and in the more extreme cases, produce intense hurricane-like polar lows.

Given the low air temperatures, the clouds formed by CAOs are mixed phase, meaning they are a mixture of droplets and ice crystals. In general, mixed phase cloud conditions are harder to characterize with instruments, and harder to simulate, than liquid or ice-only clouds. The marine CAO cloud regime is well recognized in satellite imagery, but surprisingly little is known about how these clouds form and evolve.

Arctic weather, including CAOs, can impact larger-scale weather around the world by influencing how much moisture and heat is transported towards and away from the equator. Despite this impact, cold air outbreaks in the subarctic region have only recently been the subject of study, largely because of the challenging logistics that come with the remote location. The data collected from CAESAR will support improved climate modeling and weather forecasting of the Arctic.  

“These subjectively beautiful clouds serve as a natural lab to study cloud dynamics at a wide range of scales," said Bart Geerts, a professor of atmospheric science at the University of Wyoming and a CAESAR principal investigator. 

Flying Laboratory 

With CAESAR, the main role of NSF NCAR staff and scientists is supporting operations. When NSF NCAR staff were scouting locations for the field campaign, they didn’t initially realize Kiruna was a popular destination for viewing the Northern Lights. 

“Our main criteria was finding an airplane hangar that was large enough for the C-130 and also close enough to the ocean that we wouldn’t spend too much time ferrying over land,” said Cory Wolff, NSF NCAR project manager who is overseeing CAESAR operations.  

While seeing the aurora borealis will be nice, the scientists are most excited about the data they’ll collect on CAOs. A novel feature of CAESAR is a sophisticated suite of radar, lidar, and radiometry instrumentation not previously deployed to the Arctic. NSF NCAR’s C-130, a military transport aircraft that has been transformed into a flying laboratory with a rich array of in situ and remote sensors, provides a great platform to study the cloud formation that occurs over Arctic open waters. The CAESAR team will be able to make one eight to nine hour flight per day, with the C-130 capable of reaching the Arctic sea ice edge. The plane will employ a variety of instruments to collect data from above, beneath, and inside CAOs.

Dropsondes, which are sensing devices designed to be dropped from aircraft, will provide in situ data about the wind, temperature, and humidity as they travel vertically through the atmosphere. Two lidars and two radars will be used on the C-130 to determine the proportions of ice and water as the plane flies above and beneath the clouds by vertically profiling moisture airborne particles as well as determining updrafts and downdrafts. Instruments on the wings will sample cloud properties, and air intakes will bring airborne particles, or aerosols, into the cabin for analysis. All of this data will help the team create a comprehensive view of how atmospheric conditions impact CAOs. 

“The challenge with these clouds in models is that they occur at fine scales and only a coarse representation of them currently exists in climate models,” said Geerts. 

To maximize the value of the data collected, climate modelers from NSF NCAR and other institutions will join the research team in the field as collaborators. The CAESAR team hopes that their observations will help refine the modeling of the Arctic and better represent the circulation and precipitation of CAOs. 

“All field campaigns require some level of flexibility because of the unpredictability of the natural environment, but when you go to remote places to study weather, you will always learn something,” Wolff said.

Natural Classroom

The field campaign also includes several educational opportunities. While in Kiruna, CAESAR will engage in several community outreach projects, including an open house organized by NSF NCAR’s Education, Engagement and Early Career Development office and guest lectures by the CAESAR team that will also be virtual and recorded to extend their use. Several graduate students are also part of the team and will have the opportunity to extend their learning far beyond textbooks.

“Field campaigns are important for educating the next generation of scientists. It’s good for students to see scientists struggling with the research questions of the day and even to realize that adults don’t always know everything. I hope the experience helps the students become the scientists our society will need for  the future,” said Zuidema. 

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