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| Titel |
The global aerosol-climate model ECHAM-HAM, version 2: sensitivity to improvements in process representations |
| VerfasserIn |
K. Zhang, D. O'Donnell, J. Kazil, P. Stier, S. Kinne, U. Lohmann, S. Ferrachat, B. Croft, J. Quaas, H. Wan, S. Rast, J. Feichter |
| 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 ; 12, no. 19 ; Nr. 12, no. 19 (2012-10-01), S.8911-8949 |
| Datensatznummer |
250011486
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| Publikation (Nr.) |
 copernicus.org/acp-12-8911-2012.pdf |
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| Zusammenfassung |
This paper introduces and evaluates the second version of the global
aerosol-climate model ECHAM-HAM. Major changes have been brought into the
model, including new parameterizations for aerosol nucleation and water
uptake, an explicit treatment of secondary organic aerosols, modified
emission calculations for sea salt and mineral dust, the coupling of aerosol
microphysics to a two-moment stratiform cloud microphysics scheme, and
alternative wet scavenging parameterizations. These revisions extend the
model's capability to represent details of the aerosol lifecycle and its
interaction with climate. Nudged simulations of the year 2000 are carried out
to compare the aerosol properties and global distribution in HAM1 and HAM2,
and to evaluate them against various observations. Sensitivity experiments
are performed to help identify the impact of each individual update in model
formulation.
Results indicate that from HAM1 to HAM2 there is a marked weakening of
aerosol water uptake in the lower troposphere, reducing the total aerosol
water burden from 75 Tg to 51 Tg. The main reason is the newly introduced
κ-Köhler-theory-based water uptake scheme uses a lower value for the
maximum relative humidity cutoff. Particulate organic matter loading in HAM2
is considerably higher in the upper troposphere, because the explicit
treatment of secondary organic aerosols allows highly volatile oxidation
products of the precursors to be vertically transported to regions of very
low temperature and to form aerosols there. Sulfate, black carbon,
particulate organic matter and mineral dust in HAM2 have longer lifetimes
than in HAM1 because of weaker in-cloud scavenging, which is in turn related
to lower autoconversion efficiency in the newly introduced two-moment cloud
microphysics scheme. Modification in the sea salt emission scheme causes a
significant increase in the ratio (from 1.6 to 7.7) between accumulation mode
and coarse mode emission fluxes of aerosol number concentration. This leads
to a general increase in the number concentration of smaller particles over
the oceans in HAM2, as reflected by the higher Ångström parameters.
Evaluation against observation reveals that in terms of model performance,
main improvements in HAM2 include a marked decrease of the systematic
negative bias in the absorption aerosol optical depth, as well as smaller
biases over the oceans in Ångström parameter and in the accumulation
mode number concentration. The simulated geographical distribution of aerosol
optical depth (AOD) is better correlated with the MODIS data, while the
surface aerosol mass concentrations are very similar to those in the old
version. The total aerosol water content in HAM2 is considerably closer to
the multi-model average from Phase I of the AeroCom intercomparison project.
Model deficiencies that require further efforts in the future include (i)
positive biases in AOD over the ocean, (ii) negative biases in AOD and
aerosol mass concentration in high-latitude regions, and (iii) negative
biases in particle number concentration, especially that of the Aitken mode,
in the lower troposphere in heavily polluted regions. |
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