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Initial ensemble generation and validation for ocean data assimilation using HYCOM in the Pacific

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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

  1. 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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Authors and Affiliations

  1. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

    Liying Wan & Jiang Zhu

  2. Graduate University of Chinese Academy of Sciences, Beijing, China

    Liying Wan

  3. National Marine Environmental Forecasting Center, State Oceanic Administration, Beijing, China

    Liying Wan

  4. Mohn–Sverdrup Center, Nansen Environmental and Remote Sensing Center, Bergen, Norway

    Laurent Bertino

  5. Chinese Academy of Meteorological Sciences, Chinese Meteorological Administration, Beijing, China

    Hui Wang

Authors
  1. Liying Wan
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  2. Jiang Zhu
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  3. Laurent Bertino
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  4. Hui Wang
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Correspondence to Liying Wan.

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Responsible editor: Jean-Marie Beckers

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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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