Séminaire du CEREA - 28 avril 2006
Adaptive observations and assimilation in the unstable subspace (AUS) by reeding on the data-assimilation system (BDAS), Alberto Carrassi
Results of targeting and assimilation experiments in a quasi-geostrophic
model will be presented and discussed. The main idea is to exploit the
estimated flow-dependent unstable subspace of the assimilation system to
estimate and reduce the background error. The estimate of the unstable
directions, consistent with the stability analysis of the data assimilation
system, is obtained by a modified breeding technique, (BDAS), that naturally
incorporates the information on the observational network, the assimilation
system and its dynamical instabilities. The analysis update is obtained
assimilating the observations in the unstable subspace (AUS); as a consequence
the analysis increment has the same structure as the orced bred modes. The
comparison with a 3DVar algorithm, in the context of adaptive observations
experiments under perfect model assumption, demonstrates the ability of the
BDAS technique to estimate the actual unstable modes of the assimilation
system and the e±ciency of the dynamically based assimilation AUS. Use of
adaptive observations, taken at locations where bred vectors have maximum
amplitude, enhances the efficiency of the procedure and allows the use of a
very limited number of observations and modes.
The key to this success is in the mutual enhancement of two beneficial effects obtained by:
- the strategy of observing the most unstable components of the data-assimilation system (BDAS);
- the effective reduction of the error in the unstable subspace achieved by the assimilation (AUS).
Statistics accumulated over two-years analysis cycle experiments show that a drastic reduction of the RMS analysis error is obtained when assimilation is performed by AUS. Moreover, as predicted by the theory, the observational forcing reduces the dimension of the unstable subspace of the assimilation system. Results show that the number of observations needed to stabilize the system is related to the instability properties of the free system, that is to the number and growth rate of the unstable modes.
Le séminaire aura lieu dans la salle de réunion du CEREA B220 à 14h00.
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