Currently, the Arctic climate is changing at an unprecedented pace and intensity. As an example, a remarkable increase of the Arctic near-surface air temperature has been observed starting about 25 years ago, which exceeds the global warming at least by a factor of two. This phenomenon is commonly referred to as Arctic amplification. The excessive Arctic warming, which is projected to progress during the 21st century, is both a consequence and a driver of feedback processes specific to the Arctic. These interlinked feedback mechanisms cause further partly dramatic climate changes in the Arctic, such as the stronger-than-expected decline of the Arctic sea ice observed since about 1970 and other phenomena. The feedback processes are driven by Arctic-specific and unique conditions, such as pronounced temperature inversions close to the ground, heterogeneous surface properties (open ocean, sea ice), and persisting low-level clouds (often mixed–phase). In addition, there are coupled local feedback mechanisms (surface albedo, water vapour, cloud, lapse rate, and Planck feedback processes), which appear particularly effective in the Arctic. Furthermore, remote feedback processes seem to link the Arctic and the mid-latitudes by meridional transport of heat, humidity, and momentum.

Arctic amplification might cause a weaker jet stream, which would result in an increase of undulations (wavier jet), an amplification of Rossby wave amplitudes, and more blocking situations. Thus, meridional transports of air-masses would occur more frequently, connecting the Arctic and mid-latitudes more closely. There are two major types of meridional air-mass transport: Poleward warm/moist air intrusions into, and southward cold/dry air outbreaks out of the Arctic. An increased occurrence of warm air intrusions and cold air outbreaks would further amplify the Arctic warming (positive feedback). However, these seemingly comprehensible chains of processes are debated. The only way to verify such feedback mechanisms is by using models. Unfortunately, climate models suffer from specific deficiencies, in particular with regard to air-mass transformation processes along the meridional transport pathways, which may trigger additional local feedback mechanisms, such as cloud formation and boundary layer development. For example, show that most climate models lack a realistic representation of the cloudy state and that the misrepresentation of mixed-phase clouds causes climate model biases in Arctic wintertime temperature inversions.

So far, observations of air-mass transformations in the Arctic have mostly been conducted from a fixed location. Only few aircraft-based samplings of air-mass properties over a limited regional area have been reported. This Eulerian point of view does not permit the observations of air-mass modifying processes along their meridional pathway, which are required for model validations. Therefore, we are proposing a quasi-Lagrange approach following air-masses to and from the Arctic to observe the air-mass transformation processes during warm air intrusions and cold air outbreaks, whereby we focus on warm air intrusions, which are less extensively documented in literature. This quasi-Lagrange approach requires a long-endurance airborne facility, which may carry the necessary equipment for the observations. HALO with its exceptional endurance and high lifting capacity is most suited for these observations.

The HALO campaign studying air-mass transformations in the Arctic is combined with the Transregional Collaborative research Centre SFB/TR 172 (http://www.ac3-tr.de/), called (AC)³, which investigates and evaluates major atmospheric and surface processes and feedback mechanisms causing Arctic amplification. Therefore we refer to this HALO campaign as to HALO–(AC)³ in the following.

HALO will be based in Kiruna (northern Sweden), and the campaign will take place between 12 March and 15 April 2022. The instrumentation will be installed in Oberpfaffenhofen (December 2021 to March 2022). After obtaining the necessary permissions, HALO will be transferred to Kiruna and perform measurements over a period of five weeks with altogether 142 flight hours (including test flight, EMV certification flight, and transfer to Kiruna). HALO will return to Oberpfaffenhofen in April 2022.

The general objectives of the HALO–(AC)³ mission are:

(i) To perform quasi-Lagrange observations of air-mass transformation processes during meridional transports with a particular focus on pronounced warm air intrusions and marine cold air outbreaks, and

(ii) To test the ability of numerical atmospheric models to reproduce the measurements, which then can be applied to investigate the linkages between Arctic amplification and mid-latitude weather.