Summary

One of the most intense air mass transformations on Earth happens when cold Arctic air flows out over the much warmer open oceans in so-called Cold-Air Outbreaks (CAOs). The surface heat fluxes are amongst the highest observed on Earth, supporting highly convective clouds capable of producing heavy snowfall and occasionally spawning intense “polar lows”. Surprisingly little is known about their Lagrangian evolution, 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. Thus, we propose a dedicated field campaign, CAESAR, to examine the structure of marine boundary layer clouds during CAOs. CAESAR will deploy the NSF/NCAR C-130 aircraft, with in situ and remote sensors sampling Arctic airmasses 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 Norway and the Arctic ice edge.