This study investigates the representation of stratocumulus (Sc) clouds, cloud variability, and precipitation statistics over the Southern Ocean (SO) to understand the dominant ice processes within the Icosahedral Nonhydrostatic (ICON) model at the kilometer scale using real case simulations. The simulations are evaluated using the shipborne observations as open-cell stratocumuli were continuously observed during two days (26th -27th of March 2016), south of Tasmania. The radar retrievals are used to effectively analyze the forward- simulated radar signatures from Passive and Active Microwave TRAnsfer (PAMTRA). We contrast cloud-precipitation statistics, and microphysical process rates between simulations performed with one-moment (1M) and two-moment (2M) microphysics schemes. We further analyze their sensitivity to primary and secondary ice-phase processes (Hallett–Mossop and collisional breakup). Both processes have previously been shown to improve the ice properties of simulated shallow mixed-phase clouds over the SO in other models. We find that only simulations with continuous formation, growth, and subsequent melting of graupel, and the effective riming of in-cloud rain by graupel, capture the observed cloud-precipitation vertical structure. In particular, the 2M microphysics scheme requires additional tuning for graupel processes in SO stratocumuli. Lowering the assumed graupel density and terminal velocity, in combination with secondary ice processes, enhances graupel formation in 2M microphysics ICON simulations. Overall, all simulations capture the observed intermittency of precipitation irrespective of the microphysics scheme used, and most of them sparsely distribute intense precipitation (>1mm h-1 ) events. Furthermore, the simulated clouds are too reflective as they are optically thick and/or have high cloud cover.

Thermal coral bleaching events (CBEs) over the Pacific, including those over the Great Barrier Reef (GBR), have commonly been linked to the El Niño–Southern Oscillation (ENSO), with bleaching reported to be a direct result of sea surface temperature (SST) anomalies driven by El Niño. However, such a relationship cannot explain CBEs that occurred during La Niña or the neutral phase of the ENSO. Here we show that the GBR is characterized by a significant negative correlation between total cloud cover anomaly (TCCA) and lagged SST anomaly (SSTA) whose magnitude and spatial extent are greater than the SSTA-ENSO correlation. This significant negative TCCA-SSTA (lagged) correlation prevails over two-thirds of the study domain even after the ENSO signal is removed, which suggests that local-scale reduced cloud cover is a key component of the regional warm shallow water formation over the GBR and the occurrence of thermal CBEs.

The persistent Southern Ocean (SO) shortwave radiation biases in climate models and reanalyses have been associated with the poor representation of clouds, precipitation, aerosols, the atmospheric boundary layer, and their intrinsic interactions. Capitalizing on shipborne observations collected during the Clouds Aerosols Precipitation Radiation and atmospheric Composition Over the Southern Ocean (CAPRICORN) 2016 and 2018 field campaigns, this research investigates and characterizes cloud and precipitation processes from synoptic to micro scales. Distinct cloud and precipitation regimes are found to correspond to the seven thermodynamic clusters established using a K-means clustering technique, while less distinctions are evident using the cyclone and (cold) front compositing methods. Cloud radar and disdrometer data reveal that light precipitation is common over the SO with higher intensities associated with cyclonic and warm frontal regions. While multiple microphysical processes and properties are present in several cloud regimes, ice aggregation appears to be dominant in deep precipitating clouds. Mixed phase, and in some cases, riming was detected in shallow convective clouds away from the frontal conditions. Two unique clusters with contrasting cloud and precipitation properties are observed over the high-latitude SO and coastal Antarctica, suggesting distinct physical processes therein. Through a single case study, in-situ and remote-sensing data collected by an overflight of the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) were also evaluated and complement the ship-based analysis.

In-situ observations made over twenty flights during three Austral winters (Jun–Oct, 2013–15) 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 to -31ºC). 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℃, 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 towards improving the representation of SO clouds and their radiative properties in climate models.