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| Titel |
Understanding the contributions of aerosol properties and parameterization discrepancies to droplet number variability in a global climate model |
| VerfasserIn |
R. Morales Betancourt, A. Nenes |
| 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. 9 ; Nr. 14, no. 9 (2014-05-14), S.4809-4826 |
| Datensatznummer |
250118702
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| Publikation (Nr.) |
 copernicus.org/acp-14-4809-2014.pdf |
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| Zusammenfassung |
| Aerosol indirect effects in climate models strongly depend on the
representation of the aerosol activation process. In this study, we assess
the process-level differences across activation parameterizations that
contribute to droplet number uncertainty by using the adjoints of the
Abdul-Razzak and Ghan (2000) and Fountoukis and Nenes (2005) droplet activation parameterizations
in the framework of the Community Atmospheric Model version 5.1 (CAM5.1). The
adjoint sensitivities of Nd to relevant input parameters are used
to (i) unravel the spatially resolved contribution of aerosol number, mass,
and chemical composition to changes in Nd between present-day and
pre-industrial simulations and (ii) identify the key variables responsible for
the differences in Nd fields and aerosol indirect effect
estimates when different activation schemes are used within the same modeling
framework. The sensitivities are computed online at minimal computational
cost. Changes in aerosol number and aerosol mass concentrations were found to
contribute to Nd differences much more strongly than chemical
composition effects. The main sources of discrepancy between the activation
parameterizations considered were the treatment of the water uptake by coarse
mode particles, and the sensitivity of the parameterized Nd
accumulation mode aerosol geometric mean diameter. These two factors explain
the different predictions of Nd over land and over oceans when
these parameterizations are employed. Discrepancies in the sensitivity to
aerosol size are responsible for an exaggerated response to aerosol volume
changes over heavily polluted regions. Because these regions are collocated
with areas of deep clouds, their impact on shortwave cloud forcing is
amplified through liquid water path changes. The same framework is also
utilized to efficiently explore droplet number uncertainty attributable to
hygroscopicity parameter of organic aerosol (primary and secondary).
Comparisons between the parameterization-derived sensitivities of droplet
number against predictions with detailed numerical simulations of the
activation process were performed to validate the physical consistency of the
adjoint sensitivities. |
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