Ses travaux ont porté sur la modélisation numérique des écoulements atmosphériques et océaniques. Il a contribué à la théorie de l’assimilation des observations météorologiques, particulièrement par le développement de méthodes variationnelles appropriées (‘4D-Var’). Il s’est aussi intéressé aux méthodes de prévision probabiliste, et à l’étude de la circulation des atmosphères des planètes telluriques (Mars, Venus, Titan). His interests are in numerical modelling of the atmospheres of the Earth and the terrestrial planets. He has worked particularly on assimilation of meteorological observations, to the theory of which he has contributed in many respects, in particular through the development of variational methods for assimilation (‘4D-Var’). He has also done work on numerical weather prediction, predictability of the atmosphere and methods for statistical prediction. And he has contributed to the development and validation of numerical models of the atmospheric circulation of terrestrial planets (Mars, Venus and Titan).