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1. 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)

   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, whichmore »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.« less
2. Impact of Giant Sea Salt Aerosol Particles on Precipitation in Marine Cumuli and Stratocumuli: Lagrangian Cloud Model Simulations

   https://doi.org/10.1175/JAS-D-21-0041.1  

   ( December 2021 , Journal of the Atmospheric Sciences)

   The impact of giant sea salt aerosols released from breaking waves on rain formation in marine boundary layer clouds is studied using large-eddy simulations (LES). We perform simulations of marine cumuli and stratocumuli for various concentrations of cloud condensation nuclei (CCN) and giant CCN (GCCN). Cloud microphysics are modeled with a Lagrangian method that provides key improvements in comparison to previous LES of GCCN that used Eulerian bin microphysics. We find that GCCN significantly increase precipitation in stratocumuli. This effect is strongest for low and moderate CCN concentrations. GCCN are found to have a smaller impact on precipitation formation in cumuli. These conclusions are in agreement with field measurements. We develop a simple parameterization of the effect of GCCN on precipitation, accretion, and autoconversion rates in marine stratocumuli.

   Breaking sea waves release salt particles into the atmosphere. Cloud droplets formed on these salt particles can grow larger than droplets formed on other smaller particles. Therefore, sea salt particles can be important for rain formation over oceans. To investigate this effect, we performed idealized computer simulations of stratocumulus and cumulus clouds. Sea salt particles were modeled with an unprecedented precision thanks to the use of an emerging modeling method.more »In our simulations sea salt particles significantly enhance rain formation in stratocumuli, but not in cumuli. Our study has implications for climate models, because stratocumuli are important for Earth’s energy budget and for rain enhancement experiments.

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3. Observations of clouds, aerosols, precipitation, and surface radiation over the Southern Ocean: An overview of CAPRICORN, MARCUS, MICRE and SOCRATES

   https://doi.org/10.1175/BAMS-D-20-0132.1  

   McFarquhar, Greg M. ; Bretherton, Chris ; Marchand, Roger ; Protat, Alain ; DeMott, Paul J. ; Alexander, Simon P. ; Roberts, Greg C. ; Twohy, Cynthia H. ; Toohey, Darin ; Siems, Steve ; et al ( November 2020 , Bulletin of the American Meteorological Society)

   null (Ed.)

   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,more »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.« less
4. Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA)

   Wang, Jian ; Dong, Xiquan et ( January 2021 , Bulletin of the American Meteorological Society)

   With their extensive coverage, marine low clouds greatly impact global climate. Presently, marine low clouds are poorly represented in global climate models, and the response of marine low clouds to changes in atmospheric greenhouse gases and aerosols remains the major source of uncertainty in climate simulations. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In addition, the ENA is periodically impacted by continental aerosols, making it an excellent location to study the cloud condensation nuclei (CCN) budget in a remote marine region periodically perturbed by anthropogenic emissions, and to investigate the impacts of long-range transport of aerosols on remote marine clouds. The Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign was motivated by the need of comprehensive in-situ measurements for improving the understanding of marine boundary layer CCN budget, cloud and drizzle microphysics, and the impact of aerosol on marine low cloud and precipitation. The airborne deployments took place from June 21 to July 20, 2017 and January 15 to February 18, 2018 in the Azores. The flights were designed to maximize the synergy between in-situ airbornemore »measurements and ongoing long-term observations at a ground site. Here we present measurements, observation strategy, meteorological conditions during the campaign, and preliminary findings. Finally, we discuss future analyses and modeling studies that improve the understanding and representation of marine boundary layer aerosols, clouds, precipitation, and the interactions among them.« less
5. Cloud Processes and the Transport of Biological Emissions Regulate Southern Ocean Particle and Cloud Condensation Nuclei Concentrations

   Sanchez, K. J. ; Roberts, G. C. ; Saliba, G. ; Russell, L. M. ; Twohy, C. ; Reeves, M. ; Humphries, R. S. ; Keywood, M. D. ; Ward, J. P. ; McRobert, I. M. ( January 2021 , Atmospheric chemistry and physics)

   Long-range transport of biogenic emissions from the coast of Antarctica, precipitation scavenging, and cloud processing are the main processes that influence the observed variability in Southern Ocean (SO) marine boundary layer (MBL) condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations during the austral summer. Airborne particle measurements on the HIAPER GV from north-south transects between Hobart, Tasmania and 62°S during the Southern Ocean Clouds, Radiation Aerosol Transport Experimental Study (SOCRATES) were separated into four regimes comprising combinations of high and low concentrations of CCN and CN. In 5-day HYSPLIT back trajectories, air parcels with elevated CCN concentrations were almost always shown to have crossed the Antarctic coast, a location with elevated phytoplankton emissions relative to the rest of the SO in the region south of Australia. The presence of high CCN concentrations was also consistent with high cloud fractions over their trajectory, suggesting there was substantial growth of biogenically formed particles through cloud processing. Cases with low cloud fraction, due to the presence of cumulus clouds, had high CN concentrations, consistent with previously reported new particle formation in cumulus outflow regions. Measurements associated with elevated precipitation during the previous 1.5-days of their trajectory had low CCN concentrations indicating CCNmore »were effectively scavenged by precipitation. A coarse-mode fitting algorithm was used to determine the primary marine aerosol (PMA) contribution which accounted for < 20% of CCN (at 0.3% supersaturation) and cloud droplet number concentrations. Vertical profiles of CN and large particle concentrations (Dp > 0.07µm) indicated that particle formation occurs more frequently above the MBL; however, the growth of recently formed particles typically occurs in the MBL, consistent with cloud processing and the condensation of volatile compound oxidation products.« less