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Discussion papers
https://doi.org/10.5194/acp-2018-885
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-885
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Sep 2018

Research article | 25 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Constraining the aerosol influence on cloud liquid water path

Edward Gryspeerdt1, Tom Goren2, Odran Sourdeval2, Johannes Quaas2, Johannes Mülmenstädt2, Sudhakar Dipu2, Claudia Unglaub2, Andrew Gettelman3, and Matthew Christensen4 Edward Gryspeerdt et al.
  • 1Space and Atmospheric Physics Group, Imperial College London, UK
  • 2Institute for Meteorology, Universität Leipzig, Germany
  • 3National Center for Atmospheric Research, Boulder, USA
  • 4Department of Physics, University of Oxford, UK

Abstract. The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic radiative forcing of the climate. In recent years, significant progress has been made in constraining this forcing using observations, but uncertainty still remains, particularly in the adjustments of cloud properties to aerosol perturbations. Cloud liquid water path (LWP) is the leading control on liquid-cloud albedo, making it important to observationally constrain the aerosol impact LWP.

Previous modelling and observational studies have shown that multiple processes play a role in determining the LWP response to aerosol perturbations, but that the aerosol effect can be difficult to isolate. Following previous studies using mediating variables, this work investigates use of the relationship between cloud droplet number concentration (Nd) and LWP for constraining the role of aerosols. Using joint probability histograms to account for the non-linear relationship, this work finds a relationship that is broadly consistent with previous studies. There is significant geographical variation in the relationship, partly due to role of meteorological factors (particularly relative humidity) in the relationship. However, the Nd-LWP relationship is negative in the majority of regions, suggesting that aerosol induced LWP reductions could offset a significant fraction of the radiative forcing from aerosol-cloud interactions (RFaci).

However, variations in the Nd-LWP relationship in response to volcanic and shipping aerosol perturbations indicate that the Nd-LWP relationship overestimates the Nd impact on LWP. As such, the estimate of LWP changes due to aerosol in this work provides an upper bound to the radiative forcing from aerosol-induced changes in the LWP.

Edward Gryspeerdt et al.
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Short summary
The liquid water path (LWP) is the strongest control on cloud albedo, such that a small change in LWP can have a large radiative impact. By changing the droplet number concentration (Nd), aerosols may be able to change the LWP, but the sign and magnitude of the effect is unclear. This work uses satellite data to investigate the relationship between Nd and LWP at a global scale and in response to large aerosol perturbations, suggesting that a strong decrease in LWP at high Nd may be overestimated.
The liquid water path (LWP) is the strongest control on cloud albedo, such that a small change...
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