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| Titel |
Ensemble initialization of the oceanic component of a coupled model through bred vectors at seasonal-to-interannual timescales |
| VerfasserIn |
J. Baehr, R. Piontek |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 7, no. 1 ; Nr. 7, no. 1 (2014-02-28), S.453-461 |
| Datensatznummer |
250115552
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| Publikation (Nr.) |
 copernicus.org/gmd-7-453-2014.pdf |
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| Zusammenfassung |
We evaluate the ensemble spread at seasonal-to-interannual timescales for two
perturbation techniques implemented in the ocean component of a coupled
model: (1) lagged initial conditions as commonly used for decadal
predictions; (2) bred vectors as commonly used for weather and seasonal
forecasting. We show that relative to an uninitialized reference simulation
the implementation for bred vectors can improve the ensemble spread compared
to lagged initialization at timescales from one month up to three years.
As bred vectors have so far mostly been used at short timescales, we
initially focus on the implementation of the bred vectors in the ocean
component. We introduce a depth-dependent vertical rescaling norm, accounting
for the vertical dependence of the variability, and extending the commonly
used upper-ocean rescaling norm to the full water column. We further show
that it is sufficient for the (sub-surface) ocean to breed temperature and
salinity (i.e., scalar quantities), and rely on the governing physics to
carry the temperature and salinity perturbations to the flow field.
Using these bred vectors with a rescaling interval of 12 months, we
initialize hindcast simulations and compare them to hindcast simulations
initialized with lagged initial conditions. We quantify the ensemble spread
by analyzing Talagrand diagrams and spread–error ratios. For both
temperature and salinity, the lagged initialized ensemble is particularly
under-dispersive for the first few months of predictable lead time. The
ensemble initialized with bred vectors improves the spread for temperature
and salinity for the 0–700 m and 1000–3500 m means, compared to the
lagged ensemble at lead times of several months to one year. As the lead time
increases to years, the differences between the two ensemble initialization
techniques become more difficult to discern. While the results need to be
confirmed in an initialized framework, the present analysis represents a
first step towards improved ensemble generation at the transition from
seasonal to interannual timescales, in particular at lead times up to one
year. |
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