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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 Stratocumulus cloud top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0‐D parcel model simulations, initialized with below‐cloud in situ measurements, and validated with in situ measurements of cloud properties, the shortwave cloud radiative forcing (SWCF) was reduced by up to 100 W m⁻²by cloud top entrainment in the Southern Ocean. The impact of entrainment‐corrected SWCF is between 2 and 20 times that of changes in the aerosol particle concentration or updraft at cloud base. The variability in entrainment‐corrected SWCF accounts for up to 50 W m⁻²uncertainty in estimating cloud forcing. Measurements necessary for estimating the impact of entrainment on cloud properties can be constrained from existing airborne platforms and provide a first‐order approximation for cloud radiative properties of nonprecipitating stratocumulus clouds. These measurement‐derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models.