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| Titel |
Radiative forcing and climate metrics for ozone precursor emissions: the impact of multi-model averaging |
| VerfasserIn |
C. R. MacIntosh, K. P. Shine, W. J. Collins |
| 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 ; 15, no. 7 ; Nr. 15, no. 7 (2015-04-15), S.3957-3969 |
| Datensatznummer |
250119628
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| Publikation (Nr.) |
 copernicus.org/acp-15-3957-2015.pdf |
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| Zusammenfassung |
Multi-model ensembles are frequently used to assess understanding of
the response of ozone and methane lifetime to changes in emissions
of ozone precursors such as NOx, VOCs (volatile organic compounds) and
CO. When these ozone changes are used to calculate radiative
forcing (RF) (and climate metrics such as the global warming
potential (GWP) and global temperature-change potential (GTP)) there is
a methodological choice, determined partly by the available
computing resources, as to whether the mean ozone (and methane) concentration changes are input to the radiation code, or whether each
model's ozone and methane changes are used as input, with the
average RF computed from the individual model RFs. We use data from
the Task Force on Hemispheric Transport of Air Pollution
source–receptor global chemical transport model ensemble to assess
the impact of this choice for emission changes in four regions (East
Asia, Europe, North America and South Asia).
We conclude that using the multi-model mean ozone and methane
responses is accurate for calculating the mean RF, with differences
up to 0.6% for CO, 0.7% for VOCs and 2%
for NOx. Differences of up to 60% for NOx
7% for VOCs and 3% for CO are introduced into
the 20 year GWP. The differences for the 20 year GTP are smaller than for the GWP for NOx, and similar for the other species.
However, estimates of the standard deviation calculated from the ensemble-mean input
fields (where the standard deviation at each point on the model grid is added to or
subtracted from the mean field) are almost always substantially
larger in RF, GWP and GTP metrics than the true standard deviation, and can be
larger than the model range for short-lived ozone RF, and for the
20 and 100 year GWP and 100 year GTP. The order of averaging has most impact on the metrics for NOx, as the net values for these quantities is the residual of the sum of terms of opposing signs. For
example, the standard deviation for the 20 year GWP is 2–3 times larger
using the ensemble-mean fields than using the individual models to
calculate the RF. The source of this effect is largely due to the construction of the input ozone fields, which overestimate the true ensemble spread.
Hence, while the average of multi-model fields are normally appropriate for
calculating mean RF, GWP and GTP, they are not a reliable method for
calculating the uncertainty in these fields, and in general
overestimate the uncertainty. |
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