Article Type:
Research Article

Incremental Analysis Update Implementation into a Sequential Ocean Data Assimilation System

Y. Ourmières LEGI, Grenoble, France

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J-M. Brankart LEGI, Grenoble, France

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L. Berline LEGI, Grenoble, France

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P. Brasseur LEGI, Grenoble, France

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J. Verron LEGI, Grenoble, France

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Print Publication:
01 Dec 2006
DOI:
https://doi.org/10.1175/JTECH1947.1
Page(s):
1729–1744
Restricted access

Abstract

This study deals with the enhancement of a sequential assimilation method applied to an ocean general circulation model (OGCM). A major drawback of sequential assimilation methods is the time discontinuity of the solution resulting from intermittent corrections of the model state. The data analysis step can induce shocks in the model restart phase, causing spurious high-frequency oscillations and data rejection. A method called Incremental Analysis Update (IAU) is now recognized to efficiently tackle these problems.

In the present work, an IAU-type method is implemented into an intermittent data assimilation system using a low-rank Kalman filter [Singular Evolutive Extended Kalman (SEEK)] in the case of an OGCM with a 1/3° North Atlantic grid. A 1-yr (1993) experiment has been conducted for different setups in order to evaluate the impact of the IAU scheme. Results from all of the different tests are compared with a specific interest in high-frequency output behaviors and solution consistency. The improvements brought up by the IAU implementation, such as the disappearance of spurious high-frequency oscillations and the time continuity of the solution, are shown. An overall assessment of the impact of this new approach on the assimilated runs is discussed. Advantages and drawbacks of the IAU method are pointed out.

Corresponding author address: Dr. Yann Ourmières, LEGI, UMR 5519 CNRS, BP53X, F-38041 Grenoble CEDEX, France. Email: yann.ourmieres@hmg.inpg.fr

Abstract

This study deals with the enhancement of a sequential assimilation method applied to an ocean general circulation model (OGCM). A major drawback of sequential assimilation methods is the time discontinuity of the solution resulting from intermittent corrections of the model state. The data analysis step can induce shocks in the model restart phase, causing spurious high-frequency oscillations and data rejection. A method called Incremental Analysis Update (IAU) is now recognized to efficiently tackle these problems.

In the present work, an IAU-type method is implemented into an intermittent data assimilation system using a low-rank Kalman filter [Singular Evolutive Extended Kalman (SEEK)] in the case of an OGCM with a 1/3° North Atlantic grid. A 1-yr (1993) experiment has been conducted for different setups in order to evaluate the impact of the IAU scheme. Results from all of the different tests are compared with a specific interest in high-frequency output behaviors and solution consistency. The improvements brought up by the IAU implementation, such as the disappearance of spurious high-frequency oscillations and the time continuity of the solution, are shown. An overall assessment of the impact of this new approach on the assimilated runs is discussed. Advantages and drawbacks of the IAU method are pointed out.

Corresponding author address: Dr. Yann Ourmières, LEGI, UMR 5519 CNRS, BP53X, F-38041 Grenoble CEDEX, France. Email: yann.ourmieres@hmg.inpg.fr

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Cover Journal of Atmospheric and Oceanic Technology
Journal of Atmospheric and Oceanic Technology

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