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Abstract Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even to temperatures below −20°C, and comprise a large fraction of the marine boundary layer (MBL) clouds. Earth system models and reanalysis products have struggled to reproduce the observed cloud phase distribution and occurrence of cloud ice in the region. Recent simulations found the microphysical representation of ice nucleation and growth has a large impact on these properties, however, measurements of SO ice nucleating particles (INPs) to validate simulations are sparse. This study presents measurements of INPs from simultaneous aircraft and ship campaigns conducted over the SO in austral summer 2018, which include the first in situ observations in and above cloud in the region. Our results confirm recent observations that INP concentrations are uniformly lower than measurements made in the late 1960s. While INP concentrations below and above cloud are similar, higher ice nucleation efficiency above cloud supports model simulations that the dominant INP composition varies with height. Model parameterizations based solely on aerosol properties capture the mean relationship between INP concentration and temperature but not the observed variability, which is likely related to the only modest correlations observed between INPs and environmental or aerosol metrics. Including wind speed in addition to activation temperature in a marine INP parameterization reduces bias but does not explain the large range of observed INP concentrations. Direct and indirect inference of marine INP size suggests MBL INPs, at least during Austral summer, are dominated by particles with diameters smaller than 500 nm. 

Abstract. Long-range transport of biogenic emissions from the coastof Antarctica, precipitation scavenging, and cloud processing are the mainprocesses that influence the observed variability in Southern Ocean (SO)marine boundary layer (MBL) condensation nuclei (CN) and cloud condensationnuclei (CCN) concentrations during the austral summer. Airborne particlemeasurements on the HIAPER GV from north–south transects between Hobart,Tasmania, and 62∘ S during the Southern Ocean Clouds, RadiationAerosol Transport Experimental Study (SOCRATES) were separated into fourregimes comprising combinations of high and low concentrations of CCN andCN. In 5 d HYSPLIT back trajectories, air parcels with elevated CCNconcentrations were almost always shown to have crossed the Antarctic coast,a location with elevated phytoplankton emissions relative to the rest of theSO in the region south of Australia. The presence of high CCN concentrationswas also consistent with high cloud fractions over their trajectory,suggesting there was substantial growth of biogenically formed particlesthrough cloud processing. Cases with low cloud fraction, due to the presenceof cumulus clouds, had high CN concentrations, consistent with previouslyreported new particle formation in cumulus outflow regions. Measurementsassociated with elevated precipitation during the previous 1.5 d of theirtrajectory had low CCN concentrations indicating CCN were effectivelyscavenged by precipitation. A coarse-mode fitting algorithm was used todetermine the primary marine aerosol (PMA) contribution, which accounted for<20 % of CCN (at 0.3 % supersaturation) and cloud dropletnumber concentrations. Vertical profiles of CN and large particleconcentrations (Dp>0.07 µm) indicated that particleformation occurs more frequently above the MBL; however, the growth ofrecently formed particles typically occurs in the MBL, consistent with cloudprocessing and the condensation of volatile compound oxidation products. CCN measurements on the R/V Investigator as part of the second Clouds, Aerosols,Precipitation, Radiation and atmospheric Composition Over the southeRn Ocean(CAPRICORN-2) campaign were also conducted during the same period as theSOCRATES study. The R/V Investigator observed elevated CCN concentrations near Australia,likely due to continental and coastal biogenic emissions. The Antarcticcoastal source of CCN from the south, CCN sources from the midlatitudes, andenhanced precipitation sink in the cyclonic circulation between the Ferreland polar cells (around 60∘ S) create opposing latitudinalgradients in the CCN concentration with an observed minimum in the SObetween 55 and 60∘ S. The SOCRATES airbornemeasurements are not influenced by Australian continental emissions butstill show evidence of elevated CCN concentrations to the south of60∘ S, consistent with biogenic coastal emissions. In addition, alatitudinal gradient in the particle composition, south of the Australianand Tasmanian coasts, is apparent in aerosol hygroscopicity derived from CCNspectra and aerosol particle size distribution. The particles are morehygroscopic to the north, consistent with a greater fraction of sea saltfrom PMA, and less hygroscopic to the south as there is more sulfate andorganic particles originating from biogenic sources in coastal Antarctica. 

Abstract 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.