Tangent Linear and Adjoint Versions of NASA/GMAO’s Fortran 90 Global Weather Forecast Model

Abstract

The NASA finite-volume General Circulation Model (fvGCM) is a three-dimensional Navier-Stokes solver being used for quasi-operational weather forecasting at NASA/GMAO. We use the automatic differentiation tool TAF to generate eficient tangent linear and adjoint versions from the Fortran 90 source code of fvGCM’s dynamical core. fvGCM’s parallelisation capabilities based on OpenMP and MPI have been transferred to the tangent linear and adjoint codes. For OpenMP, TAF automatically inserts corresponding OpenMP directives in the derivative code. For MPI, TAF generates interfaces to hand-written tangent linear and adjoint wrapper routines. TAF also generates a scheme that allows the tangent linear and adjoint models to linearise around an external trajectory of the model state. The generation procedure is set up in an automated way, allowing quick updates of the derivative codes after modifications of fvGCM.