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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-811
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
26 Sep 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Predicting decadal trends in cloud droplet number concentration using reanalysis and satellite data
Daniel T. McCoy1, Frida A.-M. Bender2, Daniel P. Grosvenor1, Johannes K. Mohrmann3, Dennis L. Hartmann3, Robert Wood3, and Paul R. Field1,4 1University of Leeds, Leeds, LS2 9JT, UK
2Stockholm University, Stockholm, 114 18, Sweden
3University of Washington, Seattle, 98195, USA
4MetOffice, Exeter, EX1 3PB, UK
Abstract. Cloud droplet number concentration (CDNC) is the key state variable that moderates the relationship between aerosol and the radiative forcing arising from aerosol-cloud interactions. Uncertainty related to the effect of anthropogenic aerosol on cloud properties represents the largest uncertainty in total anthropogenic radiative forcing. Here we show that regionally-averaged time series of Moderate-Resolution Imaging Spectroradiometer (MODIS) observed CDNC are well-predicted by MERRA2 reanalysis near-surface sulfate mass concentration over decadel timescales. A multiple linear regression between MERRA2 reanalysis masses of sulfate (SO4), black carbon (BC), organic carbon (OC), sea salt (SS), and dust (DU) shows that CDNC across many different regimes can be reproduced by a simple power law fit to near-surface SO4, with smaller contributions from BC, OC, SS, and DU. This confirms previous work using a less-sophisticated retrieval of CDNC at monthly time scales. The analysis is supported by examination of remotely-sensed sulfur dioxide (SO2) over maritime volcanoes and the east coasts of North America and Asia, revealing that maritime CDNC responds to changes in SO2 as observed by the Ozone Monitoring Instrument (OMI). This investigation of aerosol reanalysis and top-down remote sensing observations reveals that emission controls in Asia and North America have decreased CDNC in their maritime outflow on a decadal time scale.

Citation: McCoy, D. T., Bender, F. A.-M., Grosvenor, D. P., Mohrmann, J. K., Hartmann, D. L., Wood, R., and Field, P. R.: Predicting decadal trends in cloud droplet number concentration using reanalysis and satellite data, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-811, in review, 2017.
Daniel T. McCoy et al.
Daniel T. McCoy et al.
Daniel T. McCoy et al.

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Short summary
The interaction between clouds and aerosols represents the largest source of uncertainty in the anthropogenic radiative forcing. Cloud droplet number concentration (CDNC) is the state variable that moderates the interaction between aerosol and clouds. Here we show that CDNC decreases off the coasts of East Asia and North America due to controls on emissions. We support this analysis by examination of volcanism in Hawaii and Vanuatu.
The interaction between clouds and aerosols represents the largest source of uncertainty in the...
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