Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation and radiative processes, and their interactions. Projects between 2016 and 2018 used in-situ probes, radar, lidar and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN) and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase cloudsnucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF/NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the RV Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons.Results show a largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multi-layered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

[1] A climatology of the thermodynamic phase of the clouds over the Southern Ocean (40-65 S,100-160 E) has been constructed with the A-Train merged data product DARDAR-MASK for the four-year period 2006-2009 during Austral winter and summer. Low-elevation clouds with little seasonal cycle dominate this climatology, with the cloud tops commonly found at heights less than 1 km. Such clouds are problematic for the DARDAR-MASK in that the Cloud Profiling Radar (CPR) of CloudSat is unable to distinguish returns from the lowest four bins (heights up to 720-960 m), and the CALIOP lidar of CALIPSO may suffer from heavy extinction. The CPR is further limited for all of the low-altitude clouds (tops below 3 km) as they are predominantly in the temperature range from 0 C to À20 C, where understanding the CPR reflectivity becomes difficult due to the unknown thermodynamic phase. These shortcomings are seen to flow through to the merged CloudSat-CALIPSO product. A cloud top phase climatology comparison has been made between CALIPSO, the DARDAR-MASK and MODIS. All three products highlight the extensive presence of supercooled liquid water over the Southern Ocean, particularly during summer. The DARDAR-MASK recorded substantially more ice at cloud tops as well as mixed-phase in the low-elevation cloud tops in comparison to CALIPSO and MODIS. Below the cloud top through the body of the cloud, the DARDAR-MASK finds ice to be dominant at heights greater than 1 km, especially once the lidar signal is attenuated. The limitations demonstrated in this study highlight the continuing challenge that remains in better defining the energy and water budget over the Southern Ocean.

Macquarie Island (54.50°S, 158.94°E) is an isolated island with modest orography in the midst of the Southern Ocean with precipitation records dating back to 1948. These records (referred to as MAC) are of particular interest because of the relatively large biases in the energy and water budgets commonly found in climate simulations and reanalysis products over the region. A basic climatology of the surface precipitation P is presented and compared with the ERA-Interim (ERA-I) reanalysis. The annual ERA-I precipitation (953 mm) is found to underestimate the annual MAC precipitation (1023 mm) by 6.8% from 1979 to 2011. The frequency of 3-h surface precipitation at MAC is 36.4% from 2003 to 2011. Light precipitation (0.066 ≤ P < 0.5 mm h−1) dominates this dataset (29.7%), and heavy precipitation (P ≥ 1.5 mm h−1) is rare (1.1%). Drizzle (0 < P < 0.066 mm h−1) is commonly produced by ERA-I (43.9%) but is weaker than the detectable threshold of MAC. Warm rain intensity and frequency from CloudSat products were compared with those from MAC. These CloudSat products also recorded considerable drizzle (16%–30%) but were not significantly different from MAC when P ≥ 0.5 mm h−1. Heavy precipitation events were, in general, more commonly associated with fronts and cyclonic lows. Some heavy precipitation events were found to arise from weaker fronts and lows that were not adequately represented in the reanalysis products. Yet other heavy precipitation events were observed at points/times not associated with either fronts or cyclonic lows. Two case studies are employed to further examine this finding.

Ice particles present at temperatures warmer than −9 °C were encountered in unexpectedly high number concentrations (up to 54 L−1) by an instrumented aircraft over the Southern Ocean (SO), off the southwest coast of Tasmania, Australia, on 7 September 2013. The sampled clouds were precipitating, characterized by mixed‐phase, open‐cellular shallow convection. These clouds were present within a large‐scale environment characterized by cold air advection, in a pristine air mass for over 72 h. Using a Cloud and Aerosol Spectrometer, aerosol particles (diameters > 0.6 µm) size and number concentrations were measured and ice nucleating particle (INP) number concentrations were estimated with a recognized ice nuclei parametrization scheme. The estimated INP number concentrations were in the range of 10−5–10−1 L−1 at temperature above −9 °C, which is up to three orders of magnitude less than the ice number concentrations typically observed. The high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. The evidence suggests that secondary ice processes (likely the Hallett–Mossop mechanism) were playing a key role in generating the high ice number concentrations observed. Satellite observations from an A‐Train overpass in the neighbourhood during the flight period reveal a qualitatively consistent story, with patchy, mixed‐phase (but predominantly supercooled liquid water) clouds observed at cloud‐top temperatures around −6 °C. Using back trajectory calculations, these clouds are tracked over 23 and 46 h with A‐Train observations. The presence of these clouds is found to be common over the SO during this period of time. This suggests that the ice particles present in a relatively warm temperature range could potentially be commonplace, within the widespread (up to thousands of kilometres) shallow convective cloud fields over the SO. These clouds may have important implication for the energy budget and precipitation production over this climatically important region.

A climatology of the structure of the low-altitude cloud field (tops below 4 km) over the Southern Ocean (40°–65°S) in the vicinity of Australia (100°–160°E) has been constructed with CloudSat products for liquid water and ice water clouds. Averaging over longitude and time, CloudSat produces a roughly uniform cloud field between heights of approximately 750 and 2250 m across the extent of the domain for both winter and summer. This cloud field makes a transition from consisting primarily of liquid water at the lower latitudes to ice water at the higher latitudes. This transition is primarily driven by the gradient in the temperature, which is commonly between 0° and −20°C, rather than by direct physical observation. The uniform lower boundary is a consequence of the CloudSat cloud detection algorithm being unable to reliably separate radar returns because of the bright surface versus returns due to clouds, in the lowest four range bins above the surface. This is potentially very problematic over the Southern Ocean where the depth of the boundary layer has been observed to be as shallow as 500 m. Cloud fields inferred from upper-air soundings at Macquarie Island (54.62°S, 158.85°E) similarly suggest that the peak frequency lies between 260 and 500 m for both summer and winter. No immediate explanation is available for the uniformity of the cloud-top boundary. This lack of a strong seasonal cycle is, perhaps, remarkable given the large seasonal cycles in both the shortwave (SW) radiative forcing experienced and the cloud condensation nuclei (CCN) concentration over the Southern Ocean.

Cloud droplet concentration (Nd), effective radius (reff) and liquid water content (LWC) measured by a DMT CAPS and an SEA WCM‐2000 of wintertime low‐altitude clouds over the Southern Ocean (SO) are presented for 20 flights taken over 3 years (June–October, 2013–2015). Such clouds have been reported to have the lowest Nd on record (10–40 cm−3) from the Southern Ocean Cloud Experiment (SOCEX I) field campaign in 1993. Of the total 20 357 one‐second records spent in cloud, 38.5% were found to contain ice crystals, primarily in mixed‐phase clouds (36.7%). Ice was observed at some point during 19 of the 20 missions. The droplet spectra and temperature range suggest these clouds were often ideal for the Hallett–Mossop ice multiplication process. The average Nd and reff for liquid clouds were 28 (±30) cm−3 and 12.5 (±2.9) µm, which are consistent with those from SOCEX I. Forty‐nine percent of all liquid cloud samples were observed to be drizzling with an average drizzle rate of 0.733 mm h−1. As drizzle samples were commonly in the neighbourhood of mixed‐phase or non‐drizzling clouds, it was rare to observe solid patches of drizzle of greater than 10 s. On average, drizzling clouds had lower Nd and greater reff and LWC than those of non‐drizzling clouds. Distinct observations of non‐drizzling clouds with relatively high Nd (∼89 cm−3), small reff (∼8.5 µm) and low LWC (∼0.173 g kg−1) were noted for two flights. An initial examination of the local environment and synoptic meteorology for these flights failed to identify any particular forcing that may have led to these unique microphysical properties, although these were the only observations of closed mesoscale cellular convection. This research highlights that greater variability exists in the microphysics of wintertime clouds over the SO, when a wider range of synoptic meteorology is investigated.

The characteristics of the marine atmospheric boundary layer (MABL) in relation to synoptic meteorology over the Southern Ocean are examined using upper-air soundings and surface precipitation at Macquarie Island (54.62 ∘ S, 158.85 ∘ E), with a primary focus on the post-cold-frontal environment where large cloud and radiative biases are presented in a multitude of climate models. Thermodynamic profiles from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalyses are compared with the observations to evaluate their representation of the MABL characteristics. Observations confirm that boundary layer clouds over the Southern Ocean commonly reside within a shallow MABL under the influence of frequent midlatitude cyclones and fronts. The evaluation of MABL height shows that, for both observations and reanalysis, the inversion is higher northward of the low center and under postcold front conditions. Under cold frontal passages, however, the main inversions are not well represented by ERA-Interim, which is featured by an underestimate of the MABL height by 22%. Significant differences are found in the moisture profiles within the MABL between the observations and ERA-Interim soundings within the context of cold frontal passages. The moisture in the ERA-Interim is found to be too confined to the surface layer, which is consistent with the shallower MABL represented by the ERA-Interim. Analysis of the surface precipitation shows that ERA-Interim overestimates the amount of precipitation over Macquarie Island in the vicinity of cyclone cores but underestimates the precipitation not immediately associated with cold fronts, leading to an overall underestimate of the annual precipitation by 11%.

Cloud and precipitation properties of the midlatitude storm-track regions over the Southern Ocean (SO) and North Atlantic (NA) are explored using reanalysis datasets and A-Train observations from 2007 to 2011. In addition to the high-level retrieval products, lower-level observed variables-CloudSat radar reflectivity and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar attenuated backscatter-are directly examined using both contoured frequency by altitude diagrams (CFADs) and contoured frequency by temperature diagrams (CFTDs) to provide direct insight into thermodynamic phase properties. While the wintertime temperature profiles are similar over the two regions, the summertime environment is warmer over the NA. The NA atmosphere is generally moister than the SO, while the SO boundary layer is moister during winter. The results herein suggest that although the two regions exhibit many similarities in the prevalence of boundary layer clouds (BLCs) and frontal systems, notable differences exist. The NA environment exhibits stronger seasonality in thermodynamic structure, cloud, and precipitation properties than the SO. The regional differences of cloud properties are dominated by microphysics in winter and thermodynamics in summer. Glaciated clouds with higher reflectivities are found at warmer temperatures over the NA. BLCs (primarily below 1.5 km) are a predominant component over the SO. The wintertime boundary layer is shallower over the SO. Midlevel clouds consisting of smaller hydrometeors in higher concentration (potentially supercooled liquid water) are more frequently observed over the SO. Cirrus clouds are more prevalent over the NA. Notable differences exist in both the frequencies of thermodynamic phases of precipitation and intensity of warm rain over the two regions.