Wintertime In Situ Cloud Microphysical Properties of Mixed-Phase Clouds Over the Southern Ocean

In situ observations made over 20 flights during three Austral winters (June to October 2013–2015) were analyzed to characterize the cloud microphysical properties and natural variability of mid-latitude shallow convective clouds over the Southern Ocean (SO), with a focus on pristine conditions and the mixed-phase temperature range (MPTR, 0°C to −31°C). Liquid, mixed-phase, and ice cloud fractions were observed 39%, 44%, and 17% of the time, respectively, under various meteorological settings. Liquid phase clouds were typically characterized by low droplet number concentrations and the common presence of drizzle. Supercooled liquid water was prevalent in the MPTR, while freezing of supercooled raindrops likely formed the primary ice nucleation mechanism in these shallow clouds. Ice particles of various habits were present in the mature/maturing convective cloud cells, suggesting the operation of multiple particle growth regimes. Increased ice particle concentrations (exceeding 100 L⁻¹), well in excess of the expected ice nuclei concentrations, were measured at temperature warmer than approximately −12°C, signaling the operation of secondary ice production mechanisms. However, these cloud segments were spatiotemporally inhomogeneous, suggesting the chaotic and turbulent nature of the secondary ice-forming processes. Accurately representing these processes in global models, while necessary, is likely a challenge. Our analysis also found marked inconsistencies between several satellite-based cloud phase products that have underpinned recent developments of model parameterization frameworks. Understanding and addressing these inconsistencies are critical toward improving the representation of SO clouds and their radiative properties in climate models.