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

The impact of mesoscale vertical atmospheric motion on the thermodynamic, microphysical, and convective transformations of air masses during marine cold air outbreaks (MCAOs) is largely unknown, partly due to scarcity of suitable observations in Arctic regions. To help close this gap, this study investigates the effects of mesoscale subsidence on the evolution of atmospheric boundary-layer (ABL) properties, cloud phase, and precipitation characteristics for a case study of a shallow MCAO observed in the Fram Strait during the HALO–(𝒜𝒞) 3 campaign in March 2022. Quasi-Lagrangian Large-Eddy Simulations (LES) are conducted with observational initialisation and larger-scale forcing, based on airborne in-situ and remote-sensing measurements. The LES control simulation accurately reproduces the measured thermodynamic ABL structure and the temporal evolution of the observed air mass moving over the Arctic sea ice onto the open ocean. Specifically, the measured ABL height, integrated water vapour, and cloud water paths are well represented by the LES control run. Sensitivity experiments using the LES assuming prescribed subsidence reveal that weaker subsidence substantially deepens the ABL and causes an earlier onset of cloud glaciation. Decoupling of the cloud layer occurs sooner under weaker mesoscale subsidence, triggering convective graupel formation. This link between glaciation and decoupling explains the typical evolution of the cloud liquid water path observed in many MCAOs.