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Abstract Stratocumulus cloud top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0‐D parcel model simulations, initialized with below‐cloud in situ measurements, and validated with in situ measurements of cloud properties, the shortwave cloud radiative forcing (SWCF) was reduced by up to 100 W m⁻²by cloud top entrainment in the Southern Ocean. The impact of entrainment‐corrected SWCF is between 2 and 20 times that of changes in the aerosol particle concentration or updraft at cloud base. The variability in entrainment‐corrected SWCF accounts for up to 50 W m⁻²uncertainty in estimating cloud forcing. Measurements necessary for estimating the impact of entrainment on cloud properties can be constrained from existing airborne platforms and provide a first‐order approximation for cloud radiative properties of nonprecipitating stratocumulus clouds. These measurement‐derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models. 

Abstract Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One likely reason is that this region has few continental and anthropogenic sources of cloud‐nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur‐based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea‐spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region's expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean‐cloud‐climate feedbacks in the region. 

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. 

Marine boundary layer (MBL) clouds are an important, though uncertain, part of Earth’s radiative budget. Previous studies have shown sources of aerosol particles in the remote MBL consist of primary sea spray, the oxidation of organic and inorganic vapors derived from the ocean, entrainment from the free troposphere, and anthropogenic pollution. The potential for these particles to become cloud condensation nuclei (CCN) varies largely dependent on their hygroscopic properties. Furthermore, when clouds form, physical processes can alter the optical properties of the cloud. This dissertation aims to identify variations in aerosol sources that affect MBL CCN concentrations and physical processes throughout the cloud lifetime that influence cloud optical properties. Ambient measurements of marine particles and clouds were made throughout two campaigns in the north Pacific and four campaigns in the north Atlantic. Both clean marine and polluted clouds were sampled. In addition, dry MBL particles were measured to identify their chemical composition and size distribution, which is necessary to identify their potential to be CCN active. The organic hygroscopicity influenced CCN concentrations and cloud optical properties significantly for particles that were mostly organic, such as ship stack and generated smoke particles. For a typical range of organic hygroscopicity the amount of reflected solar radiation varied by 2-7% for polluted conditions and less than 1% for clean conditions. Simulated droplet spectral width was shown to be more representative of observations when using a weighted distribution of cloud base heights and updraft velocities, and increased the cloud reflectivity up to 2%. Cloud top entrainment and decoupling of the MBL were found to account for a decrease in cloud radiative forcing. Cloud top entrainment was corrected for homogeneous entrainment and accounted for a decrease in radiative forcing of up to 50 Wm-2. Clustering of individual marine aerosol particles resulted in the identification of particle types derived from dimethyl-sulfide (DMS) oxidation. Two particle types were identified to come from DMS oxidation products and accounted for approximately 25% and 65% of CCN at 0.1% supersaturation during the winter and summer, respectively. One of the particle types was found to be entrained from the free troposphere.