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Discussion papers
https://doi.org/10.5194/acp-2019-474
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-474
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Jul 2019

Submitted as: research article | 18 Jul 2019

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

The challenge of simulating the sensitivity of the Amazonian clouds microstructure to cloud condensation nuclei number concentrations

Pascal Polonik1,a, Christoph Knote1, Tobias Zinner1, Florian Ewald2, Tobias Kölling1, Bernhard Mayer1, Meinrat O. Andreae3,4, Tina Jurkat-Witschas4, Thomas Klimach4, Christoph Mahnke5,6, Sergej Molleker5, Christopher Pöhlker4, Mira L. Pöhlker4, Ulrich Pöschl4, Daniel Rosenfeld7, Christiane Voigt2,6, Ralf Weigel6, and Manfred Wendisch8 Pascal Polonik et al.
  • 1Meteorologisches Institut, Ludwig Maximilians-Universität München, Munich, Germany
  • 2Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
  • 3Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA
  • 4Multiphase Chemistry and Biogeochemistry Departments, Max Planck Institute for Chemistry, Mainz, Germany
  • 5Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 6Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany
  • 7Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel
  • 8Leipziger Institut für Meteorologie, Universität Leipzig, Leipzig, Germany
  • anow at: Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA

Abstract. The realistic representation of cloud-aerosol interactions is of primary importance for accurate climate model projections. The investigation of these interactions in strongly contrasting clean and polluted atmospheric conditions in the Amazon area has been one of the motivations for several field observations, including the airborne Aerosol, Cloud, Precipitation, and Radiation Interactions and DynamIcs of CONvective cloud systems – Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON-CHUVA) campaign based in Manaus, Brazil in September 2014. In this work we combine in situ and remotely sensed aerosol, cloud, and atmospheric radiation data collected during ACRIDICON-CHUVA with regional, online-coupled chemistry-transport simulations to evaluate the model’s ability to represent the indirect effects of biomass burning aerosol on cloud microphysical properties (droplet number concentration and effective radius).

We found agreement between modeled and observed median cloud droplet number concentrations (CDNC) for low values of CDNC, i.e., low levels of pollution. In general, a linear relationship between modeled and observed CDNC with a slope of two was found, which means a systematic underestimation of modeled CDNC as compared to measurements. Variability in cloud condensation nuclei (CCN) number concentrations and cloud droplet effective radii (reff) was also underestimated by the model.

Modeled effective radius profiles began to saturate around 500 CCN per cm3 at cloud base, indicating an upper limit for the model sensitivity well below CCN concentrations reached during the burning season in the Amazon Basin. Regional background aerosol concentrations were sufficiently high such that the additional CCN emitted from local fires did not cause a notable change in modelled cloud microphysical properties.

In addition, we evaluate a parameterization of CDNC at cloud base using more readily available cloud microphysical properties, aimed at in situ observations and satellite retrievals. Our study casts doubt on the validity of regional scale modeling studies of the cloud albedo effect in convective situations for polluted situations where the number concentration of CCN is greater than 500 cm−3.

Pascal Polonik et al.
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Short summary
A realistic representation of cloud-aerosol interactions is central for accurate climate projections. Here we combine observations collected during the ACRIDICON-CHUVA campaign with chemistry-transport simulations to evaluate the model’s ability to represent the indirect effects of biomass burning aerosol on cloud microphysics. We find an upper limit for the model sensitivity on cloud condensation nuclei concentrations well below the levels reached during the burning season in the Amazon Basin.
A realistic representation of cloud-aerosol interactions is central for accurate climate...
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