Assimilation de données satellitaires et inversion de sources pour l'étude de la composition chimique atmosphérique
Résumé
Human activities are at the origin of emissions of polluted gases that can perturb the chemical composition of the atmosphere and the climate. The measurements of the concentration of atmospheric constituents at global scale is needed to forecast long term consequences of these emissions. Several space missions embarking instruments dedicated to the observation of minor atmospheric species have been launched recently or will be launched in the near future. The measurements of chemical species are obtained at the global scale with vertical, horizontal and time sampling depending upon the instrument. Data assimilation techniques allow to reconstruct in an optimal way 3D global field of atmospheric parameters starting from a limited data set. They are widely used in weather forecast models and starts to develop in the field of atmospheric chemistry. This paper presents the work made within the French scientific community to develop and implement data assimilation methods in chemistry, dynamics and transport models in order to improve our knowledge of the composition of the troposphere and the stratosphere, using available measurements of atmospheric species (from satellites, balloons and aircrafts). The work presented here has been done in the frame of French National Program of Atmospheric Chemistry (PNCA) in a coordinated project " Satellite data assimilation and sources inversion for the study of the atmospheric chemistry ". It is made in interaction with other PNCA projects, " Chemistry at large scale and chemistry-climate interaction " and " Emissions of chemical compounds and aerosols ". The first part describes studies on the chemical composition of the troposphere. The major scientific issues concern the evolution of the oxydation potential of the troposphere, in particular: ■ the impact of changes in emission of trace gases (CO, CH 4 , hydrocarbons, …); ■ the influence of highly industrialized regions on the distribution of chemical species; ■ the effect of biomass burning in the tropics. MOPITT (Measurement Of Pollution In The Troposphere) satellite data have been assimilated in a tropospheric Chemistry Transport Model (CTM) to obtain global fields of CO. MOPITT data will be also used to invert sources of CO and CH 4 by data assimilation techniques. The second part presents development made to assimilate stratospheric ozone and minor species data in several CTM, using both variational and sequential assimilation techniques. The third part concerns methodologic studies for the use of variational methods in data assimilation in CTM.
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