Publication Abstracts

McCoy et al. 2019, submitted

McCoy, D., P. Field, H. Gordon, G. Elsaesser, and D. Grosvenor, 2019: Untangling causality in midlatitude aerosol-cloud adjustments. Atmos. Chem. Phys., submitted, doi:10.5194/acp-2019-665.

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 considerably in the last decade, but the sign and strength of forcing associated with changes in cloud macrophysics in response to aerosol (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 cloud macrophysical state. Once variability associated with meteorology has been removed, covariance between LWP (here, characterizing the macrophysical state) and Nd (characterizing the microphysical) is the sum of 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. Here, we restrict our analysis to the regime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP, and MODIS are compared to simulations in the MetOffice Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 and UKESM1). The meteorological predictors of cloud liquid water path (LWP) are found to be similar between the model and observations. There is also agreement with previous literature on cloud-controlling factors finding that stability, moisture, and sensible heat flux enhance LWP, while subsidence, and SST decrease it. A simulation where cloud microphysics are insensitive to changes in Nd is used to characterize scavenging-induced covariance between Nd and LWP. By removing variability associated with meteorology and scavenging we infer the sensitivity of LWP to changes in Nd. The sensitivity of LWP to Nd mediated by adjustments arrived at by this method is able to predict the change in LWP between pre-industrial and present day simulations in the NH midlatitudes and reproduces the sensitivity in simulations where causality is forced to flow from Nd to LWP. Observational constraints developed using modeled scavenging 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.

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BibTeX Citation

  author={McCoy, D. and Field, P. and Gordon, H. and Elsaesser, G. and Grosvenor, D.},
  title={Untangling causality in midlatitude aerosol-cloud adjustments},
  journal={Atmos. Chem. Phys.},
  note={Manuscript submitted for publication}

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RIS Citation

ID  - mc06100q
AU  - McCoy, D.
AU  - Field, P.
AU  - Gordon, H.
AU  - Elsaesser, G.
AU  - Grosvenor, D.
PY  - 2019
TI  - Untangling causality in midlatitude aerosol-cloud adjustments
JA  - Atmos. Chem. Phys.
DO  - 10.5194/acp-2019-665
ER  -

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