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Abstract Supercooled liquid water (SLW) and mixed phase clouds containing SLW and ice over the Southern Ocean (SO) are poorly represented in global climate and numerical weather prediction models. Observed SLW exists at lower temperatures than threshold values used to characterize its detrainment from convection in model parameterizations, and processes controlling its formation and removal are poorly understood. High‐resolution observations are needed to better characterize SLW over the SO. This study characterizes the frequency and spatial distribution of different cloud phases (liquid, ice, and mixed) using in situ observations acquired during the Southern Ocean Clouds, Radiation, Aerosol Transport Experiment Study. Cloud particle phase is identified using multiple cloud probes. Results show occurrence frequencies of liquid phase samples up to 70% between −20°C and 0°C and of ice phase samples up to 10% between −5°C and 0°C. Cloud phase spatial heterogeneity is determined by relating the total number of 1 s samples from a given cloud to the number of segments whose neighboring samples are the same phase. Mixed phase conditions are the most spatially heterogeneous from −20°C to 0°C, whereas liquid phase conditions from −10°C to 0°C and ice phase conditions from −20°C to −10°C are the least spatially heterogeneous. Greater spatial heterogeneity is associated with broader distributions of vertical velocity. Decreasing droplet concentrations and increasing number‐weighted mean liquid diameters occur within mixed phase clouds as the liquid water fraction decreases, possibly suggesting preferential evaporation of smaller drops during the Wegener‐Bergeron‐Findeisen process. 

Abstract Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation and radiative processes, and their interactions. Projects between 2016 and 2018 used in-situ probes, radar, lidar and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN) and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase cloudsnucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF/NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the RV Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show a largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multi-layered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.