CAESAR

Why It Matters - Cold-air outbreaks can lead to severe weather at both high and mid-latitudes, impact global weather patterns, and cause direct consequences for human populations in affected areas.   

One of Earth's most intense air mass transformations occurs when cold Arctic air spills over much warmer open oceans in Cold-Air Outbreaks (CAOs). The surface heat fluxes are among the strongest observed on Earth, driving vigorous cloud development, heavy snowfall and occasionally spawning intense "polar lows." Surprisingly little is known about their Lagrangian evolution, the relationship between up- and downstream conditions, and between the surface fluxes, boundary-layer structure, cloud and precipitation properties, and mesoscale circulations. These clouds provide a powerful modeling test bed for improving the representation of mixed-phase cloud processes in large eddy simulations, numerical weather prediction, and global climate models. 

The Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR) campaign examined the structure of marine boundary layer clouds during CAOs. CAESAR deployed the NSF/NCAR C-130 aircraft, with in situ and remote sensors sampling Arctic air masses from the CAO origin at the ice edge throughout their transformation downstream. A rich array of airborne radars and lidars, aerosol, cloud, precipitation, and trace gas probes, deployed during CAO events over the open waters between northern Sweden and the Arctic ice edge for 45 days in early 2024, provides a detailed characterization that will form the backbone of modeling studies across a range of scales and form a long-lasting legacy dataset.