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Preprints
https://doi.org/10.5194/acp-2020-1030
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-1030
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Oct 2020

08 Oct 2020

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This preprint is currently under review for the journal ACP.

Observing the timescales of aerosol-cloud interactions in snapshot satellite images

Edward Gryspeerdt1, Tom Goren2, and Tristan W. P. Smith3 Edward Gryspeerdt et al.
  • 1Space and Atmospheric Physics Group, Imperial College London, UK
  • 2Institute for Meteorology, Universität Leipzig, Germany
  • 3UCL Energy Institute, University College London, UK

Abstract. The response of cloud processes to an aerosol perturbation is one of the largest uncertainties in the anthropogenic forcing of the climate. It occurs at a variety of timescales, from the near-instantaneous Twomey effect, to the longer timescales required for cloud adjustments. Understanding the temporal evolution of cloud properties following an aerosol perturbation is necessary to interpret the results of so-called "natural experiments" from a known aerosol source, such as a ship or industrial site. This work uses reanalysis windfields and ship emission information matched to observations of shiptracks to measure the timescales of cloud responses to aerosol in instantaneous (or "snapshot") images taken by polar-orbiting satellites.

As found in previous studies, the local meteorological environment is shown to have a strong impact on the occurrence and properties of shiptracks, but there is a strong time dependence in their properties. The largest droplet number concentration (Nd) responses are found within three hours of emission, while cloud adjustments continue to evolve over periods of ten hours or more. Cloud fraction is increased within the early life of shiptracks, with the formation of shiptracks in otherwise clear skies indicating that around 5–10 % of clear-sky cases in this region may be aerosol-limited.

The liquid water path (LWP) enhancement and the Nd-LWP sensitivity are also time dependent and strong functions of the background cloud and meteorological state. The near-instant response of the LWP within shiptracks may be evidence of a retrieval bias in previous estimates of the LWP response to aerosol derived from natural experiments. These results highlight the importance of temporal development and the background cloud field for quantifying the aerosol impact on clouds, even in situations where the aerosol perturbation is clear.

Edward Gryspeerdt et al.

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Edward Gryspeerdt et al.

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Short summary
Cloud responses to aerosol are time-sensitive, but this development is rarely observed. This study uses isolated aerosol perturbations from ships to measure this development, showing the macrophysical (width, cloud fraction, detectability) and microphysical (droplet number) properties of shiptracks vary strongly with time since emission and the background cloud and meteorological state. This temporal development should be accounted when constraining aerosol-cloud interactions with observations.
Cloud responses to aerosol are time-sensitive, but this development is rarely observed. This...
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