Cloud Properties and Boundary Layer Stability Above Southern Ocean Sea Ice and Coastal Antarctica

Significant variability in climate predictions originates from the simulated cloud cover over the Southern Ocean. Historically, Southern Ocean cloud and aerosol properties have been less studied than their northern hemisphere counterparts, and cloud-sea-ice interactions over the Southern Ocean also remain largely unexamined. We used data from combined radar, lidar, radiometer, radiosonde, and ERA5 reanalysis profiles to investigate cloud property relationships to cloud temperature, sea-ice concentration, and boundary layer stability. Our findings show correlations between both cloud macrophysical properties and radiative effects and sea-ice concentration, and that the marine atmospheric boundary layer is more stable over higher sea-ice concentrations. Mixed-phase cloud frequency of occurrence was highest over the sea-ice zone at 15%, three times higher than over cold water south of the Antarctic Polar Front. For temperatures greater than -15 degrees C, low-level, single-layer clouds were more likely to precipitate ice if they were coupled to cold-water or sea-ice surfaces than if they were decoupled from these surfaces, with the highest percentage of clouds precipitating ice observed over sea ice. These findings suggest a surface source of ice-nucleating particles at high southern latitudes that increases cloud glaciation probability. We discuss the implications of our results for future studies into the relationship between cloud properties, aerosols, sea ice, and boundary layer stability at high latitudes over the Southern Ocean. The atmosphere above the Southern Ocean remains difficult for most modern climate models to simulate well. Clouds containing liquid water below 0 degrees C, also known as supercooled liquid water, are likely responsible for much of the simulation difficulties. In this study we investigated observations of the atmosphere made by several different instruments during a shipborne campaign from 2017 to 2018, as well as satellite observations of sea ice. We found that clouds containing supercooled liquid water were more likely to occur above sea ice than above open ocean at high latitudes, and that the lowest layer of the atmosphere is more stable above sea ice than above open ocean. We also found that, over sea ice, clouds containing supercooled liquid water were less likely to contain and precipitate ice when these clouds were separated from the lowest layer of the atmosphere. Colder, drier air over Antarctic sea ice is associated with increased inversion strength and stability of the atmospheric boundary layer The prevalence of mixed-phase clouds at high southerly latitudes is greatest over sea ice For temperatures greater than -10 degrees C, clouds coupled to the surface are more likely to contain ice