The cloud properties and governing processes in Southern Ocean marine boundary layer clouds have emerged as a central issue in understanding the Earth's climate sensitivity. While the simulated cloud feedbacks in Southern Ocean clouds have evolved in the most recent climate model intercomparison, the background properties of simulated summertime clouds in the Southern Ocean are not consistent with measurements due to known biases in simulating cloud condensation nuclei concentrations. This paper presents several case studies collected during the Capricorn 2 and Marcus campaigns held aboard Australian research vessels in the Austral Summer of 2018. Combining the surface-observed cases with MODIS data along forward and backward air mass trajectories, we demonstrate the evolution of cloud properties with time. These cases are consistent with multi-year statistics showing that long trajectories of air masses over the Antarctic ice sheet are critical to creating high droplet number clouds in the high latitude summer Southern Ocean. We speculate that secondary aerosol production via the oxidation of biogenically derived aerosol precursor gasses over the high actinic flux region of the high latitude ice sheets is fundamental to maintaining relatively high droplet numbers in Southern Ocean clouds during Summer.

Shallow cloud decks residing in or near the boundary layer cover a large fraction of the Southern Ocean (SO) and play a major role in determining the amount of shortwave radiation reflected back to space from this region. In this article, we examine the macrophysical characteristics and thermodynamic phase of low clouds (tops < 3 km) and precipitation using ground-based ceilometer, depolarization lidar and vertically-pointing W-band radar measurements collected during the Macquarie Island Cloud and Radiation Experiment (MICRE) from April 2016-March 2017. During MICRE, low clouds occurred ~65% of the time on average (slightly more often in austral winter than summer). About 2/3 of low clouds were cold-topped (temperatures < 0°C); these were thicker and had higher bases on average than warm-topped clouds. 83-88% of cold-topped low clouds were liquid phase at cloud base (depending on the season). The majority of low clouds had precipitation in the vertical range 150 to 250 meters below cloud base, a significant fraction of which did not reach the surface. Phase characterization is limited to the period between April 2016 and November 2016. Small-particle (low-radar-reflectivity) precipitation (which dominates precipitation occurrence) was mostly liquid below-cloud, while large-particle precipitation (which dominates total accumulation) was predominantly mixed/ambiguous or ice phase. Approximately 40% of cold-topped clouds had mixed/ambiguous or ice phase precipitation below (with predominantly liquid phase cloud droplets at cloud base). Below-cloud precipitation with radar reflectivity factors below about -10 dBZ were predominantly liquid, while reflectivity factors above about 0 dBZ were predominantly ice.

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.

The properties of Southern Ocean (SO) liquid phase non precipitating clouds (hereafter clouds) are examined using shipborne data collected during the Measurements of Aerosols, Radiation and Clouds over the Southern Ocean (MARCUS) and the Clouds Aerosols Precipitation Radiation and atmospheric Composition Over the SoutheRN ocean (CAPRICORN) I and II campaigns that took place in the Southern Ocean south of Australia during 2016 and late 2017 into early 2018. The cloud properties are derived using W-band radar, lidar, and microwave radiances using an optimal estimation algorithm. The SO clouds tended to have larger liquid water paths (LWP, 115 ±117 g m-2), smaller effective radii (, 8.7 ±3um), and higher number concentrations (, 90 ±107 cm), than typical values of eastern ocean basin stratocumulus. The clouds demonstrated a tendency for the LWP to increase with presumably due to precipitation suppression up to of approximately 100 cm when mean LWP decreased with increasing . Due to higher optical depth, cloud albedos were less susceptible to changes in compared to subtropical stratocumulus. The high latitude clouds observed along and near the Antarctic coast presented a distinctly bimodal character. One mode had the properties of marine clouds further north. The other mode occurred in an aerosol environment characterized by high cloud condensation nuclei concentrations and elevated sulfate aerosol without any obvious continental aerosol markers that had much higher , smaller and overall higher LWP suggesting distinct sensitivity of the clouds to seasonal biogenic aerosol production in the high latitude regions.

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.

We report on observations of corona discharges at the uppermost region of clouds characterized by emissions in a blue band of nitrogen molecules at 337 nm, with little activity in a red band of lightning leaders at 777.4 nm. Past work suggests they are generated in cloud tops reaching the tropopause and above. Here we explore their occurrence in two convection environments of the same storm: one is developing with clouds reaching above the tropopause, and one is collapsing with lower clouds. We focus on those that form a distinct category with fast risetimes below 20 µs, signifying they are at the very top of the clouds. The discharges are observed in both environments. In the collapsing cells they are related to substructures of convection. The observations suggest that a range of storm environments may generate corona discharges, and that they may be quite common in convective surges.

Intense snowfall sublimation was observed during a precipitation event over Davis in the Vestfold Hills, East Antarctica, from 08 to 10 January 2019. Radar observations and simulations from the Weather Research and Forecasting model revealed that orographic gravity waves (OGWs), generated by a north-easterly flow impinging on the ice ridge upstream of Davis, were responsible for snowfall sublimation through a Foehn effect. Despite the strong meridional moisture advection associated with an atmospheric river (AR) during this event, almost no precipitation reached the ground at Davis. We found that the direction of the synoptic flow with respect to the orography determined the intensity of OGWs over Davis, which in turn directly influenced the snowfall microphysics. Turbulence induced by the OGWs likely enhanced the aggregation process, as revealed by dual-polarization and dual-frequency radar observations. This study suggests that despite the intense AR, the precipitation distribution was determined by local processes tied to the orography. The mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main Antarctic oasis.

This study investigates the occurrence of mixed-phase clouds (MPC) over the Southern Ocean (SO) using space- and surface-based lidar and radar observations. The occurrence of supercooled clouds is dominated by geometrically thin (< 1km) layers that are rarely MPC. We diagnose layers that are geometrically thicker than 1 km to be MPC approximately 65%, and 4% of the time from below by surface remote sensors and from above by orbiting remote sensors, respectively. We examine the discrepancy in MPC as diagnosed from the below and above. From above, we find that MPC occurrence has a gradient associated with the Antarctic Polar Front near 55°S with the rare occurrence of satellite-derived MPC south of that latitude. In contrast, surface sensors find MPC in 33% of supercooled layers. We infer that space-based lidar cannot identify the occurrence of MPC except when secondary ice-forming processes operate in convection that is sufficiently strong to loft ice crystals to cloud tops. We conclude that the CALIPSO phase statistics of MPC have a severe low bias in MPC occurrence. Based on surface-based statistics, we present a parameterization of the frequency of MPC as a function of cloud top temperature that differs substantially from that used in recent climate model simulations.

This study focuses on methods to estimate dry marine aerosol surface area (SA) from bulk optical measurements. Aerosol SA is used in many models’ ice nucleating particle (INP) parameterizations, as well as influencing particle light scattering, hygroscopic growth, and reactivity, but direct observations are scarce in the Southern Ocean (SO). Two campaigns jointly conducted in austral summer 2018 provided co-located measurements of aerosol surface area from particle size distributions and lidar to evaluate SA estimation methods in this region. Mie theory calculations based on measured size distributions were used to test a proposed approximation for dry aerosol SA, which relies on estimating effective scattering efficiency (Q) as a function of Ångström exponent (å). For distributions with dry å<1, Q=2 was found to be a good approximation within ±50%, but for distributions with dry å>1, an assumption of Q=3 as in some prior studies underestimates dry aerosol surface area by a factor of 2 or more. We propose a new relationship between dry å and Q, which can be used for -0.2