Allen, RJ, Amiri-Farahani, A, Lamarque, J-F et al. (4 more authors) (2019) Observationally constrained aerosol–cloud semi-direct effects. npj Climate and Atmospheric Science, 2. 16. ISSN 2397-3722
Abstract
Absorbing aerosols, like black carbon (BC), give rise to rapid adjustments, and the associated perturbation to the atmospheric temperature structure alters the cloud distribution. The level of scientific understanding of these rapid cloud adjustments—otherwise known as semi-direct effects (SDEs)—is considered low, with models indicating a likely negative (−0.44 to +0.1 Wm−2) forcing. Recent studies suggest this negative SDE is primarily driven by decreases in high-level clouds and enhanced longwave cooling. Here, we investigate the SDE using multiple models driven by observationally constrained fine-mode aerosol forcing without dust and sea salt. Unlike aerosol simulations, which yield a relatively vertically uniform aerosol atmospheric heating profile with significant upper-tropospheric heating, observation-based heating peaks in the lower-troposphere and then decays to zero in the mid-troposphere. We find a significant global annual mean decrease in low- and mid-level clouds, and weaker decreases in high-level clouds, which leads to a positive SDE dominated by shortwave radiation. Thus, in contrast to most studies, we find a robust positive SDE, implying cloud adjustments act to warm the climate system. Sensitivity tests with identical average, but vertically uniform observationally constrained aerosol atmospheric heating result in a negative SDE, due to enhanced longwave cooling as a result of large reductions in high-level clouds. Our results therefore suggest that model simulations lead to a negatively biased SDE, due to an aerosol atmospheric heating profile that is too vertically uniform.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © The Author(s) 2019. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. |
Keywords: | black carbon; clouds; semi-direct effect; rapid adjustments; radiative forcing |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Inst for Climate & Atmos Science (ICAS) (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 31 May 2019 13:07 |
Last Modified: | 31 May 2019 13:07 |
Status: | Published |
Publisher: | Nature Research |
Identification Number: | 10.1038/s41612-019-0073-9 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:146648 |