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CSIRO Marine and Atmospheric Research
Past Seminars

Seminar Abstract

Ensemble-based data assimilation and Bluelink: a blinking challenging test bed

One of the main objectives of Bluelink is the development of a forecast system for the mesoscale ocean circulation around Australia. Central to this objective is the Bluelink ocean data assimilation system (BODAS).

The role of BODAS is to generate initial conditions for a model, based on a now-cast, and all available observations. The initial conditions are used to initialise a model, to produce a useful forecast of the ocean circulation for up to a week into the future. At present, BODAS assimilates sea-level anomalies from altimetry; and temperature and salinity profiles from field surveys and/or ARGO floats. The assimilation method used by BODAS is an ensemble-based form of optimal interpolation (EnOI): a “poor mans” Ensemble Kalman Filter (EnKF). It uses anomalies of modeled fields to determine how the observations correct the model.

In this talk, ensemble-based data assimilation methods will be explained, with reference to a simple two-variable, linear advection example. Using this example, the performance of EnOI and EnKF are compared. It is demonstrated that EnOI produces results that are comparable to EnKF. Both methods suffer from sampling error when a small ensemble size is used - a necessary evil for most realistic applications. This problem is remedied by localisation (artificially limiting the spatial region of influence of each observation). However, localisation compromises the dynamical consistency of the solution. The implications of these results for BODAS will be discussed. Preliminary results using the OFAM grid will also be presented and discussed.

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CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
Peter Oke, (Oceanographic seminars) CSIRO Marine Research (03) 6232 5387
Keith Hayes, (Biological seminars) CSIRO Marine Research (03) 6232 5298
Katrina Nitschke, Antarctic Climate and Ecosystems CRC (03) 6226 2265 & IASOS, University of Tasmania (03) 6226 2509