The realistic representation of air mass transformations during warm air intrusions (WAIs) and marine cold air outbreaks (mCAOs) into and out of the Arctic in numerical weather prediction and climate models is a major challenge. It is known from previous studies that the evolution of thermodynamic profiles in the Arctic atmospheric boundary layer and the life cycle of low-level Arctic clouds during WAIs and mCAOs are not well captured in models. This is particularly unfortunate because WAIs and mCAOs can contribute to Arctic amplification and are suspected of causing important links between Arctic and midlatitude weather and climate. Therefore, observations and simulations of these transformation processes during meridional transport of air masses need to be opposed. This will help to reveal inadequacies in both models and measurements, and aims to mitigate the problems identified. Without reliable descriptions of the transformation processes, projections of future climate in the Arctic and midlatitudes remain highly uncertain. 

Based on experiences from the previous HALO-(AC)3 mission and using the proven remote sensing payload of HALO, we aim at comparing air mass transformations in winter without sunlight during the polar night with data collected in late spring/early summer including sunlight during the polar day.

With the measurements, we aim to answer the following questions:

1. How do thermodynamic, cloud, and aerosol profile properties change during WAIs and mCAOs in the polar night and polar day?
2. How do the radiative effects of clouds above and below low-level clouds depend on environmental conditions, e.g., atmospheric stability, horizontal temperature gradient, and surface albedo during polar night and polar day?
3. What are the timescales of air mass transformations in terms of heating/cooling, drying/moistening, and cloud development during WAIs and mCAOs, and how do they differ between polar night and polar day? (iv) How well does ICON reproduce the observed transformations and effects?