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Insights from very Large Ensemble Data Assimilation Experiments with a High Resolution General circulation model of the Red Sea
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  • sivareddy sanikommu,
  • Naila Raboudi,
  • Mohamad Gharamti,
  • Peng Zhan,
  • Bilel Hadri,
  • Ibrahim Hoteit
sivareddy sanikommu
King Abdullah University of Science and Technology
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Naila Raboudi
King Abdullah University of Science and Technology
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Mohamad Gharamti
Earth Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
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Peng Zhan
southern university of science and technology
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Bilel Hadri
King Abdullah University of Science and Technology
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Ibrahim Hoteit
King Abdullah Univerity of Science and Technology

Corresponding Author:[email protected]

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Ensemble Kalman Filters (EnKFs), which assimilate observations based on statistics derived from samples of ocean states called ensemble, have become the norm for ocean data assimilation (DA) and forecasting. These schemes are commonly implemented with inflation and localization techniques to increase their ensemble spread and to filter out spurious long-range correlations resulting from the limited-size ensembles imposed by computational burden constraints. Such ad hoc methods were found not necessary in ensemble DA experiments with simplified ocean/atmospheric models and large ensembles. Here, we conduct a series of 1-year-long ensemble experiments with a fully realistic EnKF-DA system in the Red Sea using tens-to-thousands of ensemble members. The system assimilates satellite and in-situ observations and accounts for model uncertainties by integrating a 4km-resolution ocean model with ECMWF atmospheric ensemble fields, perturbed internal physics and initial conditions for forecasting.
Our results indicate that accounting for model uncertainties is more beneficial than simply increasing the ensemble size, with the improvements due to large ensemble leveling off at about 250 members. Besides, and in contrast to what is commonly observed with simplified models, the investigated ensemble DA system still required localization even when implemented with thousands of members. These findings are explained by (i) amplified spurious long-range correlations produced by the low-rank nature of the ECMWF atmospheric forcing ensemble, and (ii) non-Gaussianity generated by the perturbed internal physical parameterization schemes. Large ensemble forcing fields and non-Gaussian DA methods might be needed to take full benefits from large ensembles in ocean DA.
07 Sep 2023Submitted to ESS Open Archive
11 Sep 2023Published in ESS Open Archive