Post-Doctoral Research Themes
Forewords
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MERCATOR Research
Engineer Subject
Incremental Analysis Update
implementation into an intermittent data
assimilation system for an ocean general circulation model.
Abstract
This study deals with the enhancement
of an intermittent assimilation method applied to an Ocean General Circulation
Model (OGCM). A major drawback of sequential assimilation methods is the
discontinuity between the model forecast and the data assimilated state
estimates due to the suboptimal processing of the intermittent approach. The
data assimilation step, named the analysis step, is known to 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. The IAU methodology is detailed and the
different schemes are discussed. An IAU scheme is chosen and implemented to an
intermittent data assimilation system, using a low rank Kalman filter (SEEK),
in the case of the OGCM OPA 8.1 set for ocean state prediction in the North
Atlantic basin with a 1/3° grid resolution. A one year (1993) experiment has
been conducted for three different set-ups: the first test is a free run of the
model without data assimilation, the second test is a run with intermittent
data assimilation using the SEEK filter and the third test is an IAU
implemented run. Results from all the runs are then compared, with a specific
interest on high frequency output behaviours 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 scheme on the assimilated runs is discussed. Advantages and
drawbacks of the IAU method are pointed out.
Keywords
Kalman Filter, SEEK filter,
intermittent data assimilation, Incremental Analysis Update, OPA8.1, OGCM.
>>> Download Technical Report <<<
Also
refer to :
(2006) Yann Ourmières, J.M. Brankart, L. Berline, P. Brasseur and J. Verron, Incremental
Analysis Update implementation into a sequential ocean data assimilation
system. Journal
of Atmospheric and Oceanic Technologies, in press.
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MERSEA Post-doctoral Research Subject
Implementation of a data assimilation system into a coupled physical-biochemical Ocean General Circulation Model for the North Atlantic basin.
Abstract
Specific task :
Study the assimilation impact on biogeochemical variables
Objective
To evaluate the performance of the coupled system
with a focus on the response of the biogeochemical component, and to setup a
feedback loop for improving the modelling and
assimilation components.
Description of work
A coupled physical-biogeochemical model (CPBM)
is set for the north Atlantic basin with a ¼ degree resolution. A original assimilation system using the SEEK filter is
adapted to this CPBM in order to control the physical and biogeochemical
fields. The assimilated data are both physical and biogeochemical. The assimilated
physical data are the altimetry, the SST and the SSS while the assimilated
biogeochemical data are the Nitrate fields. The CPBM performances are assessed
and comparisons are made between the free run , the assimilated
solution and the available data.
Work Progress and Results pages
Additional information on MERSEA (general website)
Keywords
SEEK filter, intermittent data assimilation, Incremental Analysis Update, OGCM, coupled physical-biochemical model, OPA9, NATL4, LOBSTER, physical and biogeochemical data assimilation.
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