AUTOMATIC DIFFERENTIATION IN ODYSSEE

This paper describes the design of Odyssee, a system for FORTRAN progr ams manipulations and its application to automatic differentiation. Th e Odyssee system manipulates FORTRAN programs as symbolic objects. It is an open system built as a toolkit, written in a high-level programm ing language adapted to this purpose. The use of a variational method to perform data assimilation requires the computation of the gradient of a cost function represented by a large-size FORTRAN program. The us ual drawback in the reverse automatic differentiation method is the st orage requirement. The Odyssee system allows one to implement storage/ recomputation strategies in order to fit the needed compromizes. We pr esent the implementation of the strategy used in the weather forecasti ng ARPEGE/IFs project to produce the adjoint code from the code repres enting the numerical model. Odyssee produces the same code as the hand -written adjoint code for the ARPEGE/IFS project.