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Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean

Zheng Lu, Xiaohong Liu, Zhibo Zhang, Chun Zhao, Kerry Meyer, Chamara Rajapakshe, Chenglai Wu, Zhifeng Yang and Joyce E. Penner
PNAS March 5, 2018. 201713703; published ahead of print March 5, 2018. https://doi.org/10.1073/pnas.1713703115
Zheng Lu
aDepartment of Atmospheric Science, University of Wyoming, Laramie, WY 82071;
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Xiaohong Liu
aDepartment of Atmospheric Science, University of Wyoming, Laramie, WY 82071;bInternational Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;
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  • For correspondence: xliu6@uwyo.edu
Zhibo Zhang
cPhysics Department, University of Maryland Baltimore County, Baltimore, MD 21250;dJoint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD 21250;
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Chun Zhao
eSchool of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;
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Kerry Meyer
fClimate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771;
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  • ORCID record for Kerry Meyer
Chamara Rajapakshe
cPhysics Department, University of Maryland Baltimore County, Baltimore, MD 21250;
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  • ORCID record for Chamara Rajapakshe
Chenglai Wu
aDepartment of Atmospheric Science, University of Wyoming, Laramie, WY 82071;bInternational Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;
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Zhifeng Yang
cPhysics Department, University of Maryland Baltimore County, Baltimore, MD 21250;dJoint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD 21250;
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Joyce E. Penner
gDepartment of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109
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  1. Edited by James Haywood, University of Exeter, Exeter, United Kingdom, and accepted by Editorial Board Member A. R. Ravishankara February 6, 2018 (received for review August 6, 2017)

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Significance

Each year biomass burning (BB) aerosols emitted in southern Africa during the fire season (July–October) transport westward over the southeast Atlantic Ocean and interact with underlying stratocumulus decks. Previous studies showed that BB aerosols greatly perturb the top of atmosphere radiation balance by scattering and absorbing solar radiation and by altering cloud properties via changing the lower tropospheric stability (direct and semidirect effects). Using a state-of-the-art model in combination with satellite observations, we found that BB aerosols that are entrained into the clouds function as cloud condensation nuclei, and increase the brightness of stratocumulus clouds (indirect effect). In these models, this indirect effect dominates the direct and semidirect effects of BB aerosols over this region.

Abstract

Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance. The semipermanent marine stratocumulus deck over the southeastern Atlantic Ocean is of particular interest, because of its interactions with seasonal biomass burning aerosols that are emitted in southern Africa. Understanding the impacts of biomass burning aerosols on stratocumulus clouds and the implications for regional and global radiative balance is still very limited. Previous studies have focused on assessing the magnitude of the warming caused by solar scattering and absorption by biomass burning aerosols over stratocumulus (the direct radiative effect) or cloud adjustments to the direct radiative effect (the semidirect effect). Here, using a nested modeling approach in conjunction with observations from multiple satellites, we demonstrate that cloud condensation nuclei activated from biomass burning aerosols entrained into the stratocumulus (the microphysical effect) can play a dominant role in determining the total radiative forcing at the top of the atmosphere, compared with their direct and semidirect radiative effects. Biomass burning aerosols over the region and period with heavy loadings can cause a substantial cooling (daily mean −8.05 W m−2), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region.

  • stratocumulus clouds
  • biomass burning aerosols
  • aerosol–cloud interaction

Footnotes

  • ↵1To whom correspondence should be addressed. Email: xliu6{at}uwyo.edu.
  • Author contributions: Z.L., X.L., and Z.Z. designed research; Z.L., X.L., and Z.Z. performed research; Z.L., Z.Z., C.Z., K.M., C.R., C.W., and Z.Y. analyzed data; and Z.L., X.L., and J.E.P. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. J.H. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1713703115/-/DCSupplemental.

Published under the PNAS license.

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Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean
Zheng Lu, Xiaohong Liu, Zhibo Zhang, Chun Zhao, Kerry Meyer, Chamara Rajapakshe, Chenglai Wu, Zhifeng Yang, Joyce E. Penner
Proceedings of the National Academy of Sciences Mar 2018, 201713703; DOI: 10.1073/pnas.1713703115

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Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean
Zheng Lu, Xiaohong Liu, Zhibo Zhang, Chun Zhao, Kerry Meyer, Chamara Rajapakshe, Chenglai Wu, Zhifeng Yang, Joyce E. Penner
Proceedings of the National Academy of Sciences Mar 2018, 201713703; DOI: 10.1073/pnas.1713703115
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