气候变化领域集成服务门户(CCPortal): Untangling causality in midlatitude aerosol-cloud adjustments

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DOI	10.5194/acp-20-4085-2020
	Untangling causality in midlatitude aerosol-cloud adjustments
	McCoy D.T.; Field P.; Gordon H.; Elsaesser G.S.; Grosvenor D.P.
发表日期	2020
ISSN	1680-7316
起始页码	4085
结束页码	4103
卷号	20 期号:7
英文摘要	Aerosol-cloud interactions represent the leading uncertainty in our ability to infer climate sensitivity from the observational record. The forcing from changes in cloud albedo driven by increases in cloud droplet number (Nd) (the first indirect effect) is confidently negative and has narrowed its probable range in the last decade, but the sign and strength of forcing associated with changes in cloud macrophysics in response to aerosol (aerosol-cloud adjustments) remain uncertain. This uncertainty reflects our inability to accurately quantify variability not associated with a causal link flowing from the cloud microphysical state to the cloud macrophysical state. Once variability associated with meteorology has been removed, covariance between the liquid water path (LWP) averaged across cloudy and clear regions (here characterizing the macrophysical state) and Nd (characterizing the microphysical) is the sum of two causal pathways linking Nd to LWP: Nd altering LWP (adjustments) and precipitation scavenging aerosol and thus depleting Nd. Only the former term is relevant to constraining adjustments, but disentangling these terms in observations is challenging. We hypothesize that the diversity of constraints on aerosol-cloud adjustments in the literature may be partly due to not explicitly characterizing covariance flowing from cloud to aerosol and aerosol to cloud. Here, we restrict our analysis to the regime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP (Multisensor Advanced Climatology of Liquid Water Path) and MODIS are compared to simulations in the Met Office Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 and UKESM1). The meteorological predictors of LWP are found to be similar between the model and observations. There is also agreement with previous literature on cloud-controlling factors finding that increasing stability, moisture, and sensible heat flux enhance LWP, while increasing subsidence and sea surface temperature decrease it. A simulation where cloud microphysics are insensitive to changes in Nd is used to characterize covariance between Nd and LWP that is induced by factors other than aerosol-cloud adjustments. By removing variability associated with meteorology and scavenging, we infer the sensitivity of LWP to changes in Nd. Application of this technique to UM GA7.1 simulations reproduces the true model adjustment strength. Observational constraints developed using simulated covariability not induced by adjustments and observed covariability between Nd and LWP predict a 25 %-30% overestimate by the UM GA7.1 in LWP change and a 30 %-35% overestimate in associated radiative forcing. © 2020 Copernicus GmbH. All rights reserved.
语种	英语
scopus关键词	aerosol; cloud cover; cloud droplet; cloud microphysics; cyclone; heat flux; low pressure system; radiative forcing; uncertainty analysis
来源期刊	Atmospheric Chemistry and Physics
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/141428
作者单位	Institute for Climate and Atmospheric Science, University of Leeds, Leeds, United Kingdom; Met Office, Fitzroy Rd, Exeter, United Kingdom; Engineering Research Accelerator, Carnegie Mellon University, Forbes Avenue, Pittsburgh, PA, United States; Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, United States; NASA Goddard Institute for Space Studies, New York, NY, United States; National Centre for Atmospheric Science, Leeds, United Kingdom; Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071, United States
推荐引用方式 GB/T 7714	McCoy D.T.,Field P.,Gordon H.,et al. Untangling causality in midlatitude aerosol-cloud adjustments[J],2020,20(7).
APA	McCoy D.T.,Field P.,Gordon H.,Elsaesser G.S.,&Grosvenor D.P..(2020).Untangling causality in midlatitude aerosol-cloud adjustments.Atmospheric Chemistry and Physics,20(7).
MLA	McCoy D.T.,et al."Untangling causality in midlatitude aerosol-cloud adjustments".Atmospheric Chemistry and Physics 20.7(2020).