To understand the role of atmospheric boundary layer (ABL) clouds in Arctic amplification, detailed studies of cloud–related processes influencing the ABL and the atmospheric energy budget are indispensable. The significance of these processes depends on various factors such as cloud macro- and microphysical properties, aerosol distributions, two–dimensional surface albedo, sea–ice characteristics, and synoptic regime. To overcome our incomplete knowledge of the influence of these factors, in (AC)3 we have so far collected extensive data during a series of three major aircraft campaigns using the AWI Polar 5/6 aircraft. The analyses revealed a clear impact of clouds on both radiation and turbulence in the ABL but also a large variability of flux profiles depending on various factors. Based on the airborne data we have quantified the cloud radiative forcing dependent on surface conditions. Our studies during phase I mainly concentrated on single-layer clouds while during phase II, we additionally studied multi-layer clouds and their impact on the ABL structure and energy budget. Based on data from the late summer aircraft campaign MOSAiC ACA we found that in conditions with multi-layer clouds the impact of the mid-level clouds on turbulence and radiative cooling effects is dominating, while the cloud layers below are of minor importance in this process. This work was supported by the DFG funded Transregio-project TR 172 “Arctic Amplification (AC)3“.