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| Titel |
Technical Note: On the use of nudging for aerosol–climate model intercomparison studies |
| VerfasserIn |
K. Zhang, H. Wan, X. Liu, S. J. Ghan, G. J. Kooperman, P.-L. Ma, P. J. Rasch, D. Neubauer, U. Lohmann |
| 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 ; 14, no. 16 ; Nr. 14, no. 16 (2014-08-26), S.8631-8645 |
| Datensatznummer |
250118975
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| Publikation (Nr.) |
 copernicus.org/acp-14-8631-2014.pdf |
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| Zusammenfassung |
Nudging as an assimilation technique
has seen increased use in recent years in the development
and evaluation of climate models. Constraining the simulated wind
and temperature fields using global weather reanalysis facilitates
more straightforward comparison between simulation and observation,
and reduces uncertainties associated with natural variabilities of
the large-scale circulation. On the other hand, the forcing
introduced by nudging can be strong enough to change the basic
characteristics of the model climate. In the paper we show that for
the Community Atmosphere Model version 5 (CAM5), due to the systematic
temperature bias in the standard model and the sensitivity of
simulated ice formation to anthropogenic aerosol concentration,
nudging towards reanalysis results in substantial reductions in the
ice cloud amount and the impact of anthropogenic aerosols on
long-wave cloud forcing.
In order to reduce discrepancies between the nudged and
unconstrained simulations, and meanwhile take the advantages of
nudging, two alternative experimentation methods are evaluated. The
first one constrains only the horizontal winds. The second method
nudges both winds and temperature, but replaces the long-term
climatology of the reanalysis by that of the model. Results show
that both methods lead to substantially improved agreement with the
free-running model in terms of the top-of-atmosphere radiation
budget and cloud ice amount. The wind-only nudging is more
convenient to apply, and provides higher correlations of the wind
fields, geopotential height and specific humidity between simulation
and reanalysis.
Results from both CAM5 and a second aerosol–climate model ECHAM6-HAM2
also indicate that compared to the wind-and-temperature nudging,
constraining only winds leads to better agreement with
the free-running model in terms of the estimated shortwave cloud forcing
and the simulated convective activities.
This suggests nudging the horizontal winds but not
temperature is a good strategy for the investigation of aerosol
indirect effects since it provides
well-constrained meteorology without strongly perturbing the model's
mean climate. |
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