Impacts of mesoscale atmospheric subsidence on cloud glaciation and decoupling in Arctic marine cold air outbreaks

When cold, dry air from the high Arctic is advected southward over the open ocean, strong sensible and latent heat fluxes can cause rapid boundary-layer growth and clouds that are predominantly mixed-phase. These clouds in marine cold air outbreaks (MCAOs) are strongly controlled by ice nucleation processes and interactions between cloud ice and supercooled liquid droplets. The role of large-scale vertical motion in shaping the thermodynamic, microphysical, and convective evolution of MCAOs remains poorly constrained. This uncertainty largely reflects the scarcity of high-resolution observations in Arctic source regions. To address this gap, we investigate how mesoscale subsidence influences atmospheric boundary-layer (ABL) development, cloud phase transitions, and mixed-phase precipitation characteristics during a shallow MCAO observed over the Fram Strait in March 2022 as part of the HALO–(AC)³ campaign. During the campaign, mesoscale flight circles with regularly spaced dropsonde releases were conducted, allowing the estimation of subsidence following a method previously applied in the sub-tropics during the NARVAL2 and EUREC⁴A campaigns. Our analysis is based on quasi-Lagrangian large-eddy simulations (LES) that are initialised and forced exclusively with airborne in-situ and remote-sensing observations. The control LES realistically reproduces both the thermodynamic structure of the ABL and the temporal evolution of the air mass as it is advected from Arctic sea ice toward the open ocean. In particular, the simulated ABL depth, integrated water vapour, and cloud liquid and ice water paths agree well with observations. A set of sensitivity simulations with prescribed subsidence rates demonstrates that reduced mesoscale subsidence substantially alters cloud-phase evolution, resulting in a deeper boundary layer and a more rapid transition toward fully glaciated clouds. This response is closely linked to earlier development of internal ABL decoupling under weaker subsidence conditions. The earlier onset of decoupling promotes convective graupel production, thereby accelerating the conversion of liquid cloud droplets. The strong link between boundary-layer decoupling and cloud glaciation provides a plausible explanation for the frequently observed evolution of cloud liquid water path in MCAOs, and establishes a mechanistic understanding of how mesoscale subsidence governs Arctic air-mass transformation.