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Assessment of an Ensemble System That Assimilates Jason-1/Envisat Altimeter Data in a Probabilistic Model of the North Atlantic Ocean Circulation : Volume 11, Issue 3 (04/06/2015)

By Candille, G.

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Book Id: WPLBN0004020264
Format Type: PDF Article :
File Size: Pages 14
Reproduction Date: 2015

Title: Assessment of an Ensemble System That Assimilates Jason-1/Envisat Altimeter Data in a Probabilistic Model of the North Atlantic Ocean Circulation : Volume 11, Issue 3 (04/06/2015)  
Author: Candille, G.
Volume: Vol. 11, Issue 3
Language: English
Subject: Science, Ocean, Science
Collections: Periodicals: Journal and Magazine Collection, Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Brankart, J., Brasseur, P., & Candille, G. (2015). Assessment of an Ensemble System That Assimilates Jason-1/Envisat Altimeter Data in a Probabilistic Model of the North Atlantic Ocean Circulation : Volume 11, Issue 3 (04/06/2015). Retrieved from

Description: CNRS, LGGE, 38041 Grenoble, France. A realistic circulation model of the North Atlantic ocean at 0.25° resolution (NATL025 NEMO configuration) has been adapted to explicitly simulate model uncertainties. This is achieved by introducing stochastic perturbations in the equation of state to represent the effect of unresolved scales on the model dynamics. The main motivation for this work is to develop ensemble data assimilation methods, assimilating altimetric data from past missions Jason-1 and Envisat. The assimilation experiment is designed to provide a description of the uncertainty associated with the Gulf Stream circulation for years 2005/2006, focusing on frontal regions which are predominantly affected by unresolved dynamical scales. An ensemble based on such stochastic perturbations is first produced and evaluated using along-track altimetry observations. Then each ensemble member is updated by a square root algorithm based on the SEEK (singular evolutive extended Kalman) filter (Brasseur and Verron, 2006). These three elements – stochastic parameterization, ensemble simulation and 4-D observation operator – are then used together to perform a 4-D analysis of along-track altimetry over 10-day windows. Finally, the results of this experiment are objectively evaluated using the standard probabilistic approach developed for meteorological applications (Toth et al., 2003; Candille et al., 2007).

The results show that the free ensemble – before starting the assimilation process – correctly reproduces the statistical variability over the Gulf Stream area: the system is then pretty reliable but not informative (null probabilistic resolution). Updating the free ensemble with altimetric data leads to a better reliability with an information gain of around 30% (for 10-day forecasts of the SSH variable). Diagnoses on fully independent data (i.e. data that are not assimilated, like temperature and salinity profiles) provide more contrasted results when the free and updated ensembles are compared.

Assessment of an ensemble system that assimilates Jason-1/Envisat altimeter data in a probabilistic model of the North Atlantic ocean circulation

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