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Single- and multi-layer clouds are commonly observed over the Southern Ocean in varying synoptic settings, yet few studies have characterized and contrasted their properties. This study provides a statistical analysis of the microphysical properties of single- and multi-layer clouds using in-situ observations acquired during the Southern Ocean Cloud-Radiation Aerosol Transport Experimental Study. The relative frequencies of ice-containing samples (i.e., mixed and ice phase) for multi-layer clouds are 0.05–0.25 greater than for single-layer clouds, depending on cloud layer height. In multi-layer clouds, the lowest cloud layers have the highest ice-containing sample frequencies, which decrease with increasing cloud layer height up to the third highest cloud layer. This suggests a prominent seeder-feeder mechanism over the region. Ice nucleating particle (cloud condensation nuclei) concentrations are positively (negatively) correlated with ice-containing sample frequencies in select cases. Differences in microphysical properties are observed for single- and multi-layer clouds. Drop concentrations (size distributions) are greater (narrower) for single-layer clouds compared with the lowest multi-layer clouds. When differentiating cloud layers by top (single- and highest multi-layer clouds) and non-top layers (underlying multi-layer clouds), total particle size distributions (including liquid and ice) are similarly broader for non-top cloud layers. Additionally, drop concentrations in coupled environments are approximately double those in decoupled environments. 

Abstract Southern Ocean (S. Ocean) clouds are important for climate prediction. Yet previous global climate models failed to accurately represent cloud phase distributions in this observation‐sparse region. In this study, data from the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES) experiment is compared to constrained simulations from a global climate model (the Community Atmosphere Model, CAM). Nudged versions of CAM are found to reproduce many of the features of detailed in situ observations, such as cloud location, cloud phase, and boundary layer structure. The simulation in CAM6 has improved its representation of S. Ocean clouds with adjustments to the ice nucleation and cloud microphysics schemes that permit more supercooled liquid. Comparisons between modeled and observed hydrometeor size distributions suggest that the modeled hydrometeor size distributions represent the dual peaked shape and form of observed distributions, which is remarkable given the scale difference between model and observations. Comparison to satellite observations of cloud physics is difficult due to model assumptions that do not match retrieval assumptions. Some biases in the model's representation of S. Ocean clouds and aerosols remain, but the detailed cloud physical parameterization provides a basis for process level improvement and direct comparisons to observations. This is crucial because cloud feedbacks and climate sensitivity are sensitive to the representation of S. Ocean clouds.