The terminator "toy" chemistry test: A simple tool to assess errors in transport schemes

The test consists of transporting two interacting chemical species in the Nair and Lauritzen 2-D idealized flow field. The sources and sinks for these two species are given by a simple, but non-linear, "toy" chemistry that represents combination (X + X → X₂) and dissociation (X₂ → X + X).

This chemistry mimics photolysis-driven conditions near the solar terminator, where strong gradients in the spatial distribution of the species develop near its edge. Despite the large spatial variations in each species, the weighted sum XT = X + 2X₂ should always be preserved at spatial scales at which molecular diffusion is excluded. The terminator test demonstrates how well the advection-transport scheme preserves linear correlations.