Representation of model data and evaluation of diagnostic equations in pressure coordinates

This article presents a method for transforming atmospheric model data to isobaric coordinates in such a way that mass-weighted integrals are conserved and that the variable surface pressure is naturally treated. The transformation is based on integrating model variables over finite pressure intervals. Representative values at isobaric levels are defined in terms of these integrals. Evaluation of atmospheric budget equations in pressure coordinates using transformed data is discussed on the example of the thermodynamic energy equation. Layer-integrated budgets calculated in model coordinates may be satisfactorily reproduced if the layout of the target system is carefully designed. In the specific case treated, the most delicate component is the energy conversion term, which can be accurately evaluated in the isobaric frame only if the vertical resolution is comparable to that of the model system. Nevertheless, good quantitative estimates of vertical integrals and qualitatively reasonable three-dimensional distributions of individual terms are obtained even after a significant reduction of the vertical resolution. The transformation scheme is conceptually simple and easy to implement. The method is applicable to transformations between arbitrary coordinate systems.