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1. Cloud‐Nucleating Particles Over the Southern Ocean in a Changing Climate

   https://doi.org/10.1029/2020EF001673  

   Twohy, Cynthia H. ; DeMott, Paul J. ; Russell, Lynn M. ; Toohey, Darin W. ; Rainwater, Bryan ; Geiss, Roy ; Sanchez, Kevin J. ; Lewis, Savannah ; Roberts, Gregory C. ; Humphries, Ruhi S. ; et al ( March 2021 , Earth's Future)

   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.

    

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2. Observations and Modeling of Rime Splintering in Southern Ocean Cumuli

   https://doi.org/10.1029/2021JD035479  

   Lasher‐Trapp, Sonia ; Scott, Emma L. ; Järvinen, Emma ; Schnaiter, Martin ; Waitz, Fritz ; DeMott, Paul J. ; McCluskey, Christina S. ; Hill, Thomas C. J. ( December 2021 , Journal of Geophysical Research: Atmospheres)

   Recent studies have suggested a correct representation of cloud phase in the Southern Ocean region is important in climate models for an accurate representation of the energy balance. Satellite retrievals indicate many of the clouds are predominantly liquid, despite their low temperatures. However, clouds containing high numbers of ice crystals have sometimes been observed in this region and implicated the secondary ice production process called rime splintering. This study re‐examines rime splintering in Southern Ocean cumuli using both a new data set and high‐resolution numerical modeling. Measurements acquired during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) provide an evaluation of the amount of ice in shallow cumuli sampled over two days in this region. The measurements sometimes exhibit seven orders of magnitude or more ice particles compared to amounts expected from measurements of ice‐nucleating particles (INP) on the same days. Cumuli containing multiple updrafts had the greatest tendency to contain high ice concentrations and meet the expected conditions for rime splintering. Idealized numerical modeling, constrained by the observations, suggests that the multiple updrafts produce more frozen raindrops/graupel, and allow them to travel through the rime‐splintering zone over an extended period of time, increasing the number of ice particles by many orders of magnitude. The extremely low number of INP in the Southern Ocean thus appears to require special conditions like multiple updrafts to help glaciate the cumuli in this region, potentially explaining the predominance of supercooled cumuli observed there.

    

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3. Important Ice Processes Are Missed by the Community Earth System Model in Southern Ocean Mixed‐Phase Clouds: Bridging SOCRATES Observations to Model Developments

   https://doi.org/10.1029/2022JD037513  

   Zhao, Xi ; Liu, Xiaohong ; Burrows, Susannah ; DeMott, Paul J. ; Diao, Minghui ; McFarquhar, Greg M. ; Patade, Sachin ; Phillips, Vaughan ; Roberts, Greg C. ; Sanchez, Kevin J. ; et al ( February 2023 , Journal of Geophysical Research: Atmospheres)

   Global climate models (GCMs) are challenged by difficulties in simulating cloud phase and cloud radiative effect over the Southern Ocean (SO). Some of the new‐generation GCMs predict too much liquid and too little ice in mixed‐phase clouds. This misrepresentation of cloud phase in GCMs results in weaker negative cloud feedback over the SO and a higher climate sensitivity. Based on a model comparison with observational data obtained during the Southern Ocean Cloud Radiation and Aerosol Transport Experimental Study, this study addresses a key uncertainty in the Community Earth System Model version 2 (CESM2) related to cloud phase, namely ice formation in pristine remote SO clouds. It is found that sea spray organic aerosols (SSOAs) are the most important type of ice nucleating particles (INPs) over the SO with concentrations 1 order of magnitude higher than those of dust INPs based on measurements and CESM2 simulations. Secondary ice production (SIP) which includes riming splintering, rain droplet shattering, and ice‐ice collisional fragmentation as implemented in CESM2 is the dominant ice production process in moderately cold clouds with cloud temperatures greater than −20°C. SIP enhances the in‐cloud ice number concentrations (Ni) by 1–3 orders of magnitude and predicts more mixed‐phase (with percentage occurrence increased from 15% to 21%), in better agreement with the observations. This study highlights the importance of accurately representing the cloud phase over the pristine remote SO by considering the ice nucleation of SSOA and SIP processes, which are currently missing in most GCM cloud microphysics parameterizations.

    

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4. Numerical Representations of Marine Ice‐Nucleating Particles in Remote Marine Environments Evaluated Against Observations

   https://doi.org/10.1029/2018GL081861  

   McCluskey, C. S. ; DeMott, P. J. ; Ma, P. ‐L. ; Burrows, S. M. ( July 2019 , Geophysical Research Letters)

   The abundance and sources of ice‐nucleating particles, particles required for heterogeneous ice nucleation, are long‐standing sources of uncertainty in quantifying aerosol‐cloud interactions. In this study, we demonstrate near closure between immersion freezing ice‐nucleating particle number concentration (n_INPs) observations andn_INPscalculated from simulated sea spray aerosol and dust. The Community Atmospheric Model with constrained meteorology was used to simulate aerosol concentrations at the Mace Head Research Station (North Atlantic) and over the Southern Ocean to the south of Tasmania (Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRN ocean campaign). Model‐predictedn_INPswere within a factor of 10 ofn_INPsobserved with an off‐line ice spectrometer at Mace Head Research Station and Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRN ocean campaign, for 93% and 69% of observations, respectively. Simulated vertical profiles ofn_INPsreveal that transported dust may be critical ton_INPsin remote regions and that sea spray aerosol may be the dominate contributor to primary ice nucleation in Southern Ocean low‐level mixed‐phase clouds.

    

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5. Measurement report: Cloud processes and the transport of biological emissions affect southern ocean particle and cloud condensation nuclei concentrations

   https://doi.org/10.5194/acp-21-3427-2021  

   Sanchez, Kevin J. ; Roberts, Gregory C. ; Saliba, Georges ; Russell, Lynn M. ; Twohy, Cynthia ; Reeves, Michael J. ; Humphries, Ruhi S. ; Keywood, Melita D. ; Ward, Jason P. ; McRobert, Ian M. ( January 2021 , Atmospheric Chemistry and Physics)

   null (Ed.)

   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. 

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