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| Titel |
Aerosol indirect effects in a multi-scale aerosol-climate model PNNL-MMF |
| VerfasserIn |
M. Wang, S. Ghan, M. Ovchinnikov, X. Liu, R. Easter, E. Kassianov, Y. Qian, H. Morrison |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 11 ; Nr. 11, no. 11 (2011-06-09), S.5431-5455 |
| Datensatznummer |
250009811
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| Publikation (Nr.) |
 copernicus.org/acp-11-5431-2011.pdf |
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| Zusammenfassung |
| Much of the large uncertainty in estimates of anthropogenic aerosol effects
on climate arises from the multi-scale nature of the interactions between
aerosols, clouds and dynamics, which are difficult to represent in
conventional general circulation models (GCMs). In this study, we use a
multi-scale aerosol-climate model that treats aerosols and clouds across
multiple scales to study aerosol indirect effects. This multi-scale
aerosol-climate model is an extension of a multi-scale modeling framework
(MMF) model that embeds a cloud-resolving model (CRM) within each vertical
column of a GCM grid. The extension allows a more physically-based treatment
of aerosol-cloud interactions in both stratiform and convective clouds on
the global scale in a computationally feasible way. Simulated model fields,
including liquid water path (LWP), ice water path, cloud fraction, shortwave
and longwave cloud forcing, precipitation, water vapor, and cloud droplet
number concentration are in reasonable agreement with observations. The new
model performs quantitatively similar to the previous version of the MMF
model in terms of simulated cloud fraction and precipitation. The simulated
change in shortwave cloud forcing from anthropogenic aerosols is −0.77 W m−2,
which is less than half of that (−1.79 W m−2) calculated by
the host GCM (NCAR CAM5) with traditional cloud parameterizations and is
also at the low end of the estimates of other conventional global
aerosol-climate models. The smaller forcing in the MMF model is attributed
to a smaller (3.9 %) increase in LWP from preindustrial conditions (PI) to
present day (PD) compared with 15.6 % increase in LWP in stratiform clouds
in CAM5. The difference is caused by a much smaller response in LWP to a
given perturbation in cloud condensation nuclei (CCN) concentrations from PI
to PD in the MMF (about one-third of that in CAM5), and, to a lesser extent,
by a smaller relative increase in CCN concentrations from PI to PD in the
MMF (about 26 % smaller than that in CAM5). The smaller relative increase
in CCN concentrations in the MMF is caused in part by a smaller increase in
aerosol lifetime from PI to PD in the MMF, a positive feedback in aerosol
indirect effects induced by cloud lifetime effects from aerosols. The
smaller response in LWP to anthropogenic aerosols in the MMF model is
consistent with observations and with high resolution model studies, which
may indicate that aerosol indirect effects simulated in conventional global
climate models are overestimated and point to the need to use global high
resolution models, such as MMF models or global CRMs, to study aerosol
indirect effects. The simulated total anthropogenic aerosol effect in the
MMF is −1.05 W m−2, which is close to the Murphy et al. (2009) inverse
estimate of −1.1±0.4 W m−2 (1σ) based on the examination of the
Earth's energy balance. Further improvements in the representation of ice
nucleation and low clouds in MMF are needed to refine the aerosol indirect
effect estimate. |
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