PNNL

The Science

Accurate computer modeling is essential to predicting climate fluctuation. Human emissions of aerosol particles lead to an increase in global cloud droplet number concentrations, which strongly affects Earth's energy balance. Large eddy simulations and Earth system models are used to quantify aerosol effects on clouds, but these models do not resolve turbulence at the smallest scales. In this study, we show that small-scale turbulent fluctuations lead to larger concentrations of cloud droplets than would be possible in conventional models of atmospheric clouds.

The Impact

Interactions between aerosol particles and cloud properties are a large source of uncertainty in predicting human impacts on Earth’s energy balance. Accurate simulation of aerosol effects on clouds requires accurate predictions of changes in cloud droplet number. Our findings suggest that conventional cloud models, which neglect turbulent fluctuations in supersaturation, will underestimate cloud condensation nuclei activity under specific supersaturation regimes. This misrepresentation may lead to error in modeled cloud properties.

Summary

Increases in cloud droplet number concentrations from human emissions of aerosol particles modify cloud properties, which strongly affects Earth's energy balance. Large eddy simulations and Earth system models are used to quantify these aerosol–cloud interactions, but the spatial and temporal resolution of these models is too coarse to represent the impact of turbulence at the smallest scales. In this study, we show that small-scale turbulent fluctuations lead to cloud droplet formation even when air is, on average, subsaturated, which would be impossible in conventional models of cloud microphysics. The effect of aerosols on the properties of clouds is a large source of uncertainty in predictions of weather and climate. These aerosol–cloud interactions depend critically on the ability of aerosol particles to form cloud droplets. A challenge in modeling aerosol–cloud interactions is representing interactions between turbulence and cloud microphysics. Our findings suggest that models that neglect turbulent fluctuations in supersaturation will underestimate cloud condensation nuclei activity under specific supersaturation regimes, which may lead to error in modeled cloud properties.

PNNL Contact

Laura Fierce, Pacific Northwest National Laboratory, laura.fierce@pnnl.gov, corresponding author

Funding

L. Fierce and M. Ovchinnikov were supported by the U.S. Department of Energy's (DOE’s) Atmospheric System Research program, an Office of Science, Biological and Environmental Research program. Pacific Northwest National Laboratory (PNNL) is operated for DOE by Battelle Memorial Institute. J. Anderson and P. Beeler were supported by the DOE, Office of Science, Office of Workforce Development for Teachers and Scientists, Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU). All opinions expressed in this paper are the author's and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE. The Michigan Technological University coauthors were supported by the U.S. National Science Foundation. S. Krueger was supported by the National Science Foundation (managed by Michigan Technology University). F. Yang was supported by the DOE Office of Science, Biological and Environmental Research program through Brookhaven National Laboratory. The research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility located at Lawrence Berkeley National Laboratory.