Coupling advection and chemical kinetics in a global atmospheric test model

In this paper we consider the numerical difficulties that arise when horizontal advection is coupled with chemistry on a sphere, using operator splitting. From a numerical point of view, these two processes are the most difficult parts of an atmospheric model for global studies. The advection process is solved on a uniform grid and on a so-called reduced grid, where less cells are used near the poles than at the equator. The chemical kinetics is solved with Twostep, a solver based on the two-step backward differentiation formula, with constant and variable time steps. A reproducible test to measure the accuracy and the mass conservation property is introduced. To reduce the splitting error we integrate the chemistry along characteristics.