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Discussion papers
https://doi.org/10.5194/acp-2018-1178
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1178
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Nov 2018

Research article | 23 Nov 2018

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

Understanding Aerosol-Cloud-Radiation Interactions Using Local Meteorology and Cloud State Constraints

Alyson Douglas and Tristan L'Ecuyer Alyson Douglas and Tristan L'Ecuyer
  • University of Wisconsin-Madison, USA

Abstract. While many studies have tried to quantify the sign and the magnitude of the warm cloud response to aerosol loading, both remain uncertain owing to the multitude of factors that modulate microphysical and thermodynamic processes within the cloud. Constraining aerosol-cloud interactions using the local meteorology and cloud liquid water may offer a way to account for covarying influences, potentially increasing our confidence in observational estimates of warm cloud indirect effects. Four years of collocated satellite observations from the NASA A-Train constellation, combined with reanalaysis from MERRA-2, are used to partition warm clouds into regimes based on stability, the free atmospheric relative humidity, and liquid water path. Organizing the sizable number of satellite observations into regimes is shown to minimize the covariance between the environment or liquid water path and the indirect effect. Controlling for local meteorology and cloud state mitigates artificial signals and reveals substantial variance in both the sign and magnitude of the cloud radiative response, including regions where clouds become systematically darker with increased aerosol concentration in dry, unstable environments. The reverse Twomey effect, as it has been called, is evident even under the most stringent of constraints, confirming it is not an artificial signal or an isolated phenomenon. These results suggest it is not meaningful to report a single global sensitivity of cloud radiative effect to aerosol. To the contrary, we find the sensitivity can range from −.46 to .11 Wm−2ln(AI) regionally.

Alyson Douglas and Tristan L'Ecuyer
Interactive discussion
Status: open (until 18 Jan 2019)
Status: open (until 18 Jan 2019)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Alyson Douglas and Tristan L'Ecuyer
Data sets

NASA A-Train Data NASA https://doi.org/10.17616/R3106C

MERRA-2 Data NASA Goddard Earth Sciences Data and Information Services Center https://doi.org/10.17616/R3X599

Alyson Douglas and Tristan L'Ecuyer
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Latest update: 13 Dec 2018
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Short summary
Aerosols are released by natural and human activities. When aerosols encounter clouds they interact in what is known as the indirect effect. Brighter clouds are expected due to the microphysical response, however certain environments can trigger a modified response. Limits on the stability, humidity, and cloud thickness are applied regionally to investigate local cloud responses to aerosol, resulting in a range of indirect effects that would result in significant cooling to slight warming.
Aerosols are released by natural and human activities. When aerosols encounter clouds they...
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