Sensitivity study of cloud parameterizations with relative dispersion in CAM5.1: impacts on aerosol indirect effects
Aerosol-induced increase of relative dispersion of cloud droplet size distribution <i>ε</i> exerts a warming effect and partly offsets the cooling of aerosol indirect radiative forcing (AIF) associated with increased droplet concentration by increasing the cloud droplet effective radius...
Main Authors: | , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-05-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | http://www.atmos-chem-phys.net/17/5877/2017/acp-17-5877-2017.pdf |
Summary: | Aerosol-induced increase of relative dispersion of cloud droplet
size distribution <i>ε</i> exerts a warming effect and partly offsets
the cooling of aerosol indirect radiative forcing (AIF) associated with
increased droplet concentration by increasing the cloud droplet effective
radius (<i>R</i><sub>e</sub>) and enhancing the cloud-to-rain autoconversion rate
(Au) (labeled as the dispersion effect), which can help reconcile global climate
models (GCMs) with the satellite observations. However, the total dispersion
effects on both <i>R</i><sub>e</sub> and Au are not fully considered in most GCMs,
especially in different versions of the Community Atmospheric Model (CAM). In
order to accurately evaluate the dispersion effect on AIF, the new complete
cloud parameterizations of <i>R</i><sub>e</sub> and Au explicitly accounting for
<i>ε</i> are implemented into the CAM version 5.1 (CAM5.1), and a suite
of sensitivity experiments is conducted with different representations of
<i>ε</i> reported in the literature. It is shown that the shortwave cloud
radiative forcing is much better simulated with the new cloud
parameterizations as compared to the standard scheme in CAM5.1, whereas the
influences on longwave cloud radiative forcing and surface precipitation are
minimal. Additionally, consideration of the dispersion effect can significantly
reduce the changes induced by anthropogenic aerosols in the cloud-top
effective radius and the liquid water path, especially in the Northern
Hemisphere. The corresponding AIF with the dispersion effect considered can also
be reduced substantially by a range of 0.10 to 0.21 W m<sup>−2</sup> at the global scale
and by a much bigger margin of 0.25 to 0.39 W m<sup>−2</sup> for the
Northern Hemisphere in comparison with that of fixed relative dispersion, mainly
dependent on the change of relative dispersion and droplet concentrations
(Δ<i>ε</i>∕Δ<i>N</i><sub>c</sub>). |
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ISSN: | 1680-7316 1680-7324 |