Initial ensemble generation and validation for ocean data assimilation using HYCOM in the Pacific |
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| Authors: | Liying Wan Jiang Zhu Laurent Bertino Hui Wang |
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| Institution: | (1) Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China;(2) Graduate University of Chinese Academy of Sciences, Beijing, China;(3) National Marine Environmental Forecasting Center, State Oceanic Administration, Beijing, China;(4) Mohn–Sverdrup Center, Nansen Environmental and Remote Sensing Center, Bergen, Norway;(5) Chinese Academy of Meteorological Sciences, Chinese Meteorological Administration, Beijing, China |
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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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| Keywords: | Ensemble Kalman filter Initial ensemble HYCOM model Model uncertainty Ensemble spread Correlation pattern |
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