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- Author or Editor: Christina S. McCluskey x
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Abstract
Single-forcing large ensembles are a relatively new tool for quantifying the contributions of different anthropogenic and natural forcings to the historical and future projected evolution of the climate system. This study introduces a new single-forcing large ensemble with the Community Earth System Model, version 2 (CESM2), which can be used to separate the influences of greenhouse gases, anthropogenic aerosols, biomass burning aerosols, and all remaining forcings on the evolution of the Earth system from 1850 to 2050. Here, the forced responses of global near-surface temperature and associated drivers are examined in CESM2 and compared with those in a single-forcing large ensemble with CESM2’s predecessor, CESM1. The experimental design, the imposed forcing, and the model physics all differ between the CESM1 and CESM2 ensembles. In CESM1, an “all-but-one” approach was used whereby everything except the forcing of interest is time evolving, while in CESM2 an “only” approach is used, whereby only the forcing of interest is time evolving. This experimental design choice is shown to matter considerably for anthropogenic aerosol-forced change in CESM2, due to state dependence of cryospheric albedo feedbacks and nonlinearity in the Atlantic meridional overturning circulation (AMOC) response to forcing. This impact of experimental design is, however, strongly dependent on the model physics and/or the imposed forcing, as the same sensitivity to experimental design is not found in CESM1, which appears to be an inherently less nonlinear model in both its AMOC behavior and cryospheric feedbacks.
Abstract
Single-forcing large ensembles are a relatively new tool for quantifying the contributions of different anthropogenic and natural forcings to the historical and future projected evolution of the climate system. This study introduces a new single-forcing large ensemble with the Community Earth System Model, version 2 (CESM2), which can be used to separate the influences of greenhouse gases, anthropogenic aerosols, biomass burning aerosols, and all remaining forcings on the evolution of the Earth system from 1850 to 2050. Here, the forced responses of global near-surface temperature and associated drivers are examined in CESM2 and compared with those in a single-forcing large ensemble with CESM2’s predecessor, CESM1. The experimental design, the imposed forcing, and the model physics all differ between the CESM1 and CESM2 ensembles. In CESM1, an “all-but-one” approach was used whereby everything except the forcing of interest is time evolving, while in CESM2 an “only” approach is used, whereby only the forcing of interest is time evolving. This experimental design choice is shown to matter considerably for anthropogenic aerosol-forced change in CESM2, due to state dependence of cryospheric albedo feedbacks and nonlinearity in the Atlantic meridional overturning circulation (AMOC) response to forcing. This impact of experimental design is, however, strongly dependent on the model physics and/or the imposed forcing, as the same sensitivity to experimental design is not found in CESM1, which appears to be an inherently less nonlinear model in both its AMOC behavior and cryospheric feedbacks.
A Mesocosm Double Feature: Insights into the Chemical Makeup of Marine Ice Nucleating ParticlesAbstract
The abundance of atmospheric ice nucleating particles (INPs) is a source of uncertainty for numerical representation of ice-phase transitions in mixed-phase clouds. While sea spray aerosol (SSA) exhibits less ice nucleating (IN) ability than terrestrial aerosol, marine INP emissions are linked to oceanic biological activity and are potentially an important source of INPs over remote oceans. Inadequate knowledge of marine INP identity limits the ability to parameterize this complex INP source. A previous manuscript described abundances of marine INPs in relation to several aerosol composition and ocean biology observations during two laboratory mesocosm experiments. In this study, the abundances and chemical and physical properties of INPs found during the same mesocosm experiments were directly probed in SSA, seawater, and surface microlayer samples. Two unique marine INP populations were found: 1) dissolved organic carbon INPs are suggested to be composed of IN-active molecules, and 2) particulate organic carbon INPs are attributed as intact cells or IN-active microbe fragments. Both marine INP types are likely to be emitted into SSA following decay of phytoplankton biomass when 1) the surface microlayer is significantly enriched with exudates and cellular detritus and SSA particles are preferentially coated with IN-active molecules or 2) diatom fragments and bacteria are relatively abundant in seawater and therefore more likely transferred into SSA. These findings inform future efforts for incorporating marine INP emissions into numerical models and motivate future studies to quantify specific marine molecules and isolate phytoplankton, bacteria, and other species that contribute to these marine INP types.
Abstract
The abundance of atmospheric ice nucleating particles (INPs) is a source of uncertainty for numerical representation of ice-phase transitions in mixed-phase clouds. While sea spray aerosol (SSA) exhibits less ice nucleating (IN) ability than terrestrial aerosol, marine INP emissions are linked to oceanic biological activity and are potentially an important source of INPs over remote oceans. Inadequate knowledge of marine INP identity limits the ability to parameterize this complex INP source. A previous manuscript described abundances of marine INPs in relation to several aerosol composition and ocean biology observations during two laboratory mesocosm experiments. In this study, the abundances and chemical and physical properties of INPs found during the same mesocosm experiments were directly probed in SSA, seawater, and surface microlayer samples. Two unique marine INP populations were found: 1) dissolved organic carbon INPs are suggested to be composed of IN-active molecules, and 2) particulate organic carbon INPs are attributed as intact cells or IN-active microbe fragments. Both marine INP types are likely to be emitted into SSA following decay of phytoplankton biomass when 1) the surface microlayer is significantly enriched with exudates and cellular detritus and SSA particles are preferentially coated with IN-active molecules or 2) diatom fragments and bacteria are relatively abundant in seawater and therefore more likely transferred into SSA. These findings inform future efforts for incorporating marine INP emissions into numerical models and motivate future studies to quantify specific marine molecules and isolate phytoplankton, bacteria, and other species that contribute to these marine INP types.
A Dynamic Link between Ice Nucleating Particles Released in Nascent Sea Spray Aerosol and Oceanic Biological Activity during Two Mesocosm ExperimentsAbstract
Emission rates and properties of ice nucleating particles (INPs) are required for proper representation of aerosol–cloud interactions in atmospheric models. Few investigations have quantified marine INP emissions, a potentially important INP source for remote oceanic regions. Previous studies have suggested INPs in sea spray aerosol (SSA) are linked to oceanic biological activity. This proposed link was explored in this study by measuring INP emissions from nascent SSA during phytoplankton blooms during two mesocosm experiments. In a Marine Aerosol Reference Tank (MART) experiment, a phytoplankton bloom was produced with chlorophyll-a (Chl a) concentrations reaching 39 μg L−1, while Chl a concentrations more representative of natural ocean conditions were obtained during the Investigation into Marine Particle Chemistry and Transfer Science (IMPACTS; peak Chl a of 5 μg L−1) campaign, conducted in the University of California, San Diego, wave flume. Dynamic trends in INP emissions occurred for INPs active at temperatures > −30°C. Increases in INPs active between −25° and −15°C lagged the peak in Chl a in both studies, suggesting a consistent population of INPs associated with the collapse of phytoplankton blooms. Trends in INP emissions were also compared to aerosol composition, abundances of microbes, and enzyme activity. In general, increases in INP concentrations corresponded to increases in organic species in SSA and the emissions of heterotrophic bacteria, suggesting that both microbes and biomolecules contribute to marine INP populations. INP trends were not directly correlated with a single biological marker in either study. Direct measurements of INP chemistry are needed to accurately identify particles types contributing to marine INP populations.
Abstract
Emission rates and properties of ice nucleating particles (INPs) are required for proper representation of aerosol–cloud interactions in atmospheric models. Few investigations have quantified marine INP emissions, a potentially important INP source for remote oceanic regions. Previous studies have suggested INPs in sea spray aerosol (SSA) are linked to oceanic biological activity. This proposed link was explored in this study by measuring INP emissions from nascent SSA during phytoplankton blooms during two mesocosm experiments. In a Marine Aerosol Reference Tank (MART) experiment, a phytoplankton bloom was produced with chlorophyll-a (Chl a) concentrations reaching 39 μg L−1, while Chl a concentrations more representative of natural ocean conditions were obtained during the Investigation into Marine Particle Chemistry and Transfer Science (IMPACTS; peak Chl a of 5 μg L−1) campaign, conducted in the University of California, San Diego, wave flume. Dynamic trends in INP emissions occurred for INPs active at temperatures > −30°C. Increases in INPs active between −25° and −15°C lagged the peak in Chl a in both studies, suggesting a consistent population of INPs associated with the collapse of phytoplankton blooms. Trends in INP emissions were also compared to aerosol composition, abundances of microbes, and enzyme activity. In general, increases in INP concentrations corresponded to increases in organic species in SSA and the emissions of heterotrophic bacteria, suggesting that both microbes and biomolecules contribute to marine INP populations. INP trends were not directly correlated with a single biological marker in either study. Direct measurements of INP chemistry are needed to accurately identify particles types contributing to marine INP populations.
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 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 R/V 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 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, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals 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.
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 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 R/V 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 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, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals 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.
