Abstract
Abstract. A new methodology for the formulation of an adjoint to the transport component of the chemistry transport model TOMCAT is described and implemented in a new model, RETRO-TOM. The Eulerian backtracking method is used, allowing the forward advection scheme (Prather's second-order moments) to be efficiently exploited in the backward adjoint calculations. Prather's scheme is shown to be time symmetric, suggesting the possibility of high accuracy. To attain this accuracy, however, it is necessary to make a careful treatment of the "density inconsistency" problem inherent to offline transport models. The results are verified using a series of test experiments. These demonstrate the high accuracy of RETRO-TOM when compared with direct forward sensitivity calculations, at least for problems in which flux limiters in the advection scheme are not required. RETRO-TOM therefore combines the flexibility and stability of a "finite difference of adjoint" formulation with the accuracy of an "adjoint of finite difference" formulation.
Highlights
The past 20 years or so have seen an explosion in the development of adjoint models for chemistry transport models (CTMs)
The numerical accuracy as well as the reliability of adjoint models are key to the above applications
The aim of the present work is to describe the development of an adjoint RETRO-TOM to the dynamical core of the TOMCAT CTM (Chipperfield, 2006) that combines the desirable numerical and conceptual properties of the finite difference of adjoint” (FDA) approach with the accuracy of an adjoint of finite difference” (AFD) model
Summary
The past 20 years or so have seen an explosion in the development of adjoint models for chemistry transport models (CTMs). The aim of the present work is to describe the development of an adjoint RETRO-TOM to the dynamical core of the TOMCAT CTM (Chipperfield, 2006) that combines the desirable numerical and conceptual properties of the FDA approach with the accuracy of an AFD model. Our position is that a CTM adjoint should be built upon a numerical scheme for the “dynamical core” that is both highly accurate and numerically well formulated, at least in the absence of advection-scheme-related nonlinearities due to e.g. flux limiters (Thuburn and Haine, 2001; Vukicevic et al, 2001; Hourdin et al, 2006) which raise various separate issues Such a model provides as solid as possible a foundation for the applications listed above, and in particular allows numerical errors to be excluded, as far as possible, when assessing results.
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