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Discussion papers
https://doi.org/10.5194/acp-2018-1342
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1342
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Jan 2019

Research article | 21 Jan 2019

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

An emulator approach to stratocumulus susceptibility

Franziska Glassmeier1,4, Fabian Hoffmann1,2, Jill S. Johnson3, Takanobu Yamaguchi1,2, Ken S. Carslaw3, and Graham Feingold1 Franziska Glassmeier et al.
  • 1NOAA Earth Systems Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80302, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
  • 3University of Leeds, School of Earth and Environment, Woodhouse Lane Leeds, LS2 9JT, UK
  • 4Wageningen University, Department of Environmental Sciences, Droevendaalsesteeg 4, 6708PB Wageningen, The Netherlands

Abstract. The climatic relevance of aerosol-cloud interactions depends on the sensitivity of the radiative effect of clouds to cloud droplet number N and liquid water path LWP. We derive the dependence of cloud fraction CF, cloud albedo AC and the relative cloud radiative effect rCRE = CF · AC on N and LWP from 159 large-eddy simulations of nocturnal stratocumulus. These simulations vary in their initial conditions for temperature, moisture, boundary-layer height and aerosol concentration but share boundary conditions for surface fluxes and subsidence. Our approach is based on Gaussian process emulation, a statistical technique related to machine learning. We succeed in building emulators that accurately predict simulated values of CF, AC and rCRE for given values of N and LWP. Emulator-derived susceptibilities ∂ ln rCRE/∂ ln N and ∂ ln rCRE/∂ ln LWP cover the non-drizzling, fully-overcast regime as well as the drizzling regime with broken cloud cover. Theoretical results, which are limited to the non-drizzling regime, are reproduced. The susceptibility ∂ ln rCRE/∂ ln N captures the strong sensitivity of the cloud radiative effect to cloud fraction, while the susceptibility ∂ ln rCRE/∂ ln LWP describes the influence of cloud amount on cloud albedo irrespective of cloud fraction. Our emulation-based approach provides a powerful tool for summarizing complex data in a simple framework that captures the sensitivities of cloud field properties over a wide range of states.

Franziska Glassmeier et al.
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Franziska Glassmeier et al.
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Short summary
The climatic relevance of aerosol-cloud interactions depends on the sensitivity of the radiative effect of clouds to certain cloud properties. We derive the dependence of cloud fraction, cloud albedo and the relative cloud radiative effect on the number of cloud droplets and on liquid water path from a large set of detailed simulations of stratocumulus clouds.
The climatic relevance of aerosol-cloud interactions depends on the sensitivity of the radiative...
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