Section: New Results
Assimilation of Image Data
At the present time the observation of Earth from space is done by more than thirty satellites. These platforms provide information two kinds of observations:
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Eulerian information as radiance measurements: the radiative properties of the earth and it's fluid envelopps. These data can be plugged into numerical models by solving some inverse problems.
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Lagrangian information: the movement of fronts and vortices give information on the dynamics of the fluid. Presently this information is scarcely used in meteorology by following small cumulus clouds and using then as Lagrangian tracers, but the selection of these clouds must be done by hand and the altitude of the selected clouds must be known, this is done by using the temperature of the top of the cloud.
MOISE is the leader of the ADDISA project selected and funded by Agence Nationale de la Recherche dedicated to the assimilation of images. The member of the ADDISA group are the INRIA project CLIME, Laboratoire des Ecoulements Géophysique et Industriels (CNRS,Grenoble), Institut de Mathématiques de Toulouse and MétéoFrance The principle is to link images and numerical models in order to retrieve the initial condition at best. Two basic techniques are tested:
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from images deduce pseudo-observation as the velocity of the flow, then assimilate these data as pseudo observations using a regular variational data assimilation scheme.
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Consider “objects" in the images (fronts, vortices) then compare with the same objects created by the model and inject them into a scheme of assimilation which take them into account.
The method is already used by Météo France to detect precursors of severe storms.
Assimilation of Images: the ADDISA project
Participants : François-Xavier Le Dimet, Didier Auroux, Innocent Souopgui, Olivier Titaud, Arthur Vidard, Laurent Debreu, Emilie Neveu.
ADDISA is a project coordinated by F.-X. Le Dimet and supported for three years (2007-2009) by the ANR : http://addisa.gforge.inria.fr
F.-X. Le Dimet, A. Vidard, O. Titaud and I. Souopgui implemented a shallow-water model coupled with an advection-diffusion model which simulates the drift of a vortex submitted to the Coriolis force. This simulation corresponds to some experiments performed by LEGI with the Coriolis platform (see figures 2 and 3 ) where the motion of the vortex is focused by a passive tracer (cf. [43] ). D. Auroux works on the extraction of velocity fields from sequences of images, providing pseudo-observations of the fluid velocity [40] .
Application of Variational Methods to the processing of space imagery
Participants : François-Xavier Le Dimet, Gennady Korotaev.
In cooperation with the Institute of Oceanography ( G. Korotiaev Ukrainian Academy of Sciences) and CLIME ( I. Herlin, E. Huot, Rocquencourt).
From the observation of sea surface imagery, the surface current velocity, at the mesoscale level, is extracted by using optimal control methods. It is assumed that the imagery contrast could be described by a transport diffusion equation. The method permits to retrieve an initial field of passive tracer together with surface current velocity from the sequence of images. Examples of processing of AVHRR observations and validations of results have been carried out.
