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| Titel |
Wind extraction potential from 4D-Var assimilation of stratospheric O3, N2O, and H2O using a global shallow water model |
| VerfasserIn |
D. R. Allen, K. W. Hoppel, D. D. Kuhl |
| 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. 7 ; Nr. 14, no. 7 (2014-04-04), S.3347-3360 |
| Datensatznummer |
250118562
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| Publikation (Nr.) |
 copernicus.org/acp-14-3347-2014.pdf |
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| Zusammenfassung |
| The wind extraction due to assimilation of stratospheric trace gas (tracer)
data is examined using a 4D-Var (four-dimensional variational) data assimilation system based on the
shallow water equations coupled to the tracer continuity equation. The
procedure is outlined as follows. First, a nature run is created, simulating
middle stratospheric winter conditions. Second, ozone (O3), nitrous
oxide (N2O), and water vapor (H2O) (treated in this study as
passive tracers) are initialized using Aura Microwave Limb Sounder (MLS) mixing
ratios at 850 K potential temperature and are advected by the nature run
winds. Third, the initial dynamical conditions are perturbed by using a 6 h
offset. Fourth, simulated hourly tracer observations on the full model grid
are assimilated with a 4D-Var system in which tracer and winds are coupled
via the adjoint of the tracer continuity equation. Multiple assimilation
experiments are performed by varying the amount of random observation error
added to the simulated measurements. Finally, the wind extraction potential
(WEP) is calculated as the reduction of the vector wind root mean square error
(RMSE) relative to the maximum possible reduction. For a single 6 h
assimilation cycle with the smallest observation error, WEP values are
~60% for all three tracers, while 10-day multi-cycle
simulations result in WEP of ~90%, wind errors of
~0.3 m s−1, and height errors of ~13 m.
There is therefore sufficient information in the tracer fields to almost
completely constrain the dynamics, even without direct assimilation of
dynamical information. When realistic observation error is added (based on
MLS precisions at 10 hPa), the WEP after 10 days is 90% for O3,
87% for N2O, and 72% for H2O. O3 and N2O provide
more wind information than H2O due to stronger background gradients
relative to the MLS precisions. The RMSE for wind reach a minimum level of
~0.3–0.9 m s−1 for the MLS precisions, suggesting a
limit to which realistic tracers could constrain the winds, given complete
global cover age. With higher observation noise levels, the WEP values
decrease, but the impact on the winds is still positive up to noise levels
of 100% (relative to the global mean value) when compared to the case of
no data assimilation. |
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