Abstract
A method to initialize an ensemble, introduced by Evensen (Physica, D 77:108–129, 1994a; J Geophys Res 99(C5):10143–10162, 1994b; Ocean Dynamics 53:343–367, 2003), was applied to the Ocean General Circulation Model (OGCM) HYbrid Coordinate Ocean Model (HYCOM) for the Pacific Ocean. Taking advantage of the hybrid coordinates, an initial ensemble is created by first perturbing the layer interfaces and then running the model for a spin-up period of 1 month forced by randomly perturbed atmospheric forcing fields. In addition to the perturbations of layer interfaces, we implemented perturbations of the mixed layer temperatures. In this paper, we investigate the quality of the initial ensemble generated by this scheme and the influence of the horizontal decorrelation scale and vertical correlation on the statistics of the resulting ensemble. We performed six ensemble generation experiments with different combinations of horizontal decorrelation scales and with/without perturbations in the mixed layer. The resulting six sets of initial ensembles are then analyzed in terms of sustainability of the ensemble spread and realism of the correlation patterns. The ensemble spreads are validated against the difference between model and observations after 20 years of free run. The correlation patterns of six sets of ensemble are compared to each other. This study shows that the ensemble generation scheme can effectively generate an initial ensemble whose spread is consistent with the observed errors. The correlation pattern of the ensemble also exhibits realistic features. The addition of mixed layer perturbations improves both the spread and correlation. Some limitations of the ensemble generation scheme are also discussed. We found that the vertical shift of isopycnal coordinates provokes unrealistically large deviations in shallow layers near the islands of the West Pacific. A simple correction circumvents the problem.
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Notes
The covariance of a pair of points depends only on the distance that separates them.
Abbreviations
- EnKF:
-
Ensemble Kalman Filter
- KF:
-
Kalman filter
- KPZ:
-
Kardar–Parisi–Zhang
- HYCOM:
-
HYbrid Coordinate Ocean Model
- MICOM:
-
Miami Isopycnic Coordinate Ocean Model
- GDEM:
-
Generalized Digital Environmental Model
- GEBCO:
-
General Bathymetric Chart Of The Oceans Model
- ECMWF:
-
European Center for Medium-range Weather Forecasting
- SVD:
-
singular value decomposition
- OISST:
-
optimum interpolation sea surface temperature
- TOGA-COARE:
-
Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment
- TAO:
-
Tropical Atmosphere/Ocean
- SSH:
-
sea surface height
- CLS:
-
Collecte Localisation Satellites
- ECC:
-
equatorial countercurrent
- NEC:
-
north equatorial current
- SEC:
-
south equatorial current
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Acknowledgments
This work was supported by the “The Climate System Model Development and Application Studies” of the International Partnership Creative Group Program of the Chinese Academy of Sciences and Natural Sciences Foundation (contract nos. 40437017, 40221503, and 40225015). Hui Wang is supported by Natural Sciences Foundation (contract no. 40531006). We are very thankful to the Mohn–Sverdrup Center for Global Ocean Studies and Operational Oceanography for providing their version of the HYCOM model and the method for generating the initial ensemble. Thanks to Dr. Annette Samuelsen, Dr. Knut Arild Lisæter, Dr. Helge Drange, and Dr. Guangqing Zhou for their valuable suggestions.
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Wan, L., Zhu, J., Bertino, L. et al. Initial ensemble generation and validation for ocean data assimilation using HYCOM in the Pacific. Ocean Dynamics 58, 81–99 (2008). https://doi.org/10.1007/s10236-008-0133-x
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DOI: https://doi.org/10.1007/s10236-008-0133-x