Observing local and Remote Controls on Arctic Air mass evolution (ORCA2)

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The Arctic is currently warming at unprecedented rates, accompanied by dramatic loss in sea ice coverage. This accelerated warming is referred to as Arctic Amplification. While a variety of climate feedback processes is known to play a role, recent research has emphasized the similar importance of air mass exchanges between the Arctic climate system and the mid-latitudes. This involves southbound extrusions of cold air as well as northbound intrusions of warm and moist air into the high Arctic. Strong moisture intrusion events in particular can contribute significantly, through the associated enhanced meridional transport of heat and moisture. In addition, low level liquid clouds embedded in moisture intrusions enhance the downwelling long-wave radiative flux at the surface, which dramatically increases the surface heat budget and thus enhances the melt of the sea ice. Elucidating the exact nature of Arctic air mass exchanges, in particular moisture intrusions, has therefore been intensely researched in recent years. What has become clear is that the evolution of moist air masses is a net effect of a multitude of processes that covers an incredibly wide range of scales, stretching from micro-scale turbulence and cloud physics all the way to global Rossby waves. Smaller scale processes act like local controls on the air mass, while larger-scale processes like subsidence act like remote controls. Recent LES studies of cloud-bearing turbulent mixed-layers in the Arctic have highlighted two aspects of moisture intrusions; i) the strong control of large-scale subsidence on the air mass evolution, and ii) the potentially important role of humidity inversions in this process. While these new insights represent important progress, observational support for these modeling results is still lacking. What is needed to this purpose are reliable observations of large-scale subsidence during moisture intrusions in the Arctic. The aim of this project is to fill this data gap by using data collected by the High Altitude and LOng-range research aircraft (HALO) during the HALO-(AC)3 campaign, scheduled to take place in March-April 2021. A clear benefit of HALO in this respect are its long rage and high air speed, allowing the Lagrangian sampling of moving air masses in remote areas in the Arctic effectively and frequently. Second, use is made of a new technique for measuring large-scale divergence that relies on dropsondes released during large circular flight patterns, as explored during the recent NARVAL2 campaign. These observations, in combination with additional indepdendent measurements of mixed-phase clouds, radiation and humidity inversions, will be combined with Lagrangian high-resolution Large-Eddy Simulations that follow the air mass. This synergy of high-resolution Lagrangian simulation and observation of moisture intrusions is a novelty, and will thus create new opportunities for increasing our insight into air mass transformations.