HALO-(AC)3: Airborne Observations of Arctic Clouds in Airmass Transformations

Clear indications of the phenomenon of Arctic Amplification include the above-average increase of the near-surface air temperature and the related dramatic retreat of sea ice observed in the last decades. The mechanisms behind these features are widely discussed. Especially the role of clouds and of air mass transports into and out of the Arctic associated with related transformation processes are still poorly understood. Therefore, the HALO-(AC)³ campaign was performed to provide observations of meridional air mass transports and corresponding transformations in a quasi-Lagrangian approach. Three research aircraft equipped with state-of-the-art instrumentation performed measurements over the Arctic ocean and sea ice in March/April 2022. The German High Altitude and Long Range Research Aircraft (HALO), equipped with a comprehensive suite of active and passive remote sensing instruments and dropsondes, was operated from Kiruna, Sweden. The flight pattern covered long distances at high altitudes up to the North Pole probing air masses multiple times on their way into and out of the Arctic. The Polar 5 (remote sensing) and Polar 6 (in-situ) aircraft from the Alfred Wegener Institute operated in the lower troposphere out of Longyearbyen in the lower troposphere over Fram Strait West of Svalbard. Several coordinated flights between the three aircraft were conducted with Polar 6 sampling in-situ aerosol, cloud, and precipitation particles within the boundary layer, Polar 5 observing clouds and precipitation from above roughly at 3 km altitude, and HALO providing the large scale view on the scene following air masses.
The observations cover a major warm air intrusion event with atmospheric river embedded bringing warm and moist air far into the Arctic. Multiple cold air outbreaks were characterized in their initial stage close to the sea ice edge with Polar 5 and 6 and in a quasi-Lagrangian perspective with HALO, which allowed to quantify the air mass transformation by changes of thermodynamic profiles, large scale subsidence, and cloud properties over a period of 24 hours. Single events of high latitude Arctic cirrus and the formation of a polar low are included in the data set. The presentation reports on first results of the campaign by illustrating the capabilities of the multi-aircraft operation.