Impact of vertical resolution on the transport of passive tracers in the ECHAM4 model

The transport of the passive tracers ¹⁴CO₂ and SF₆ has been modelled with two versions of the general circulation model ECHAM4 with different vertical resolutions: the standard model with 19 model layers (L19) and a higher-resolution version with 39 layers (L39). We study the impact of vertical resolution on the modelled transport characteristics. Both models are able to capture the observed SF6concentrations in the troposphere, but in the stratosphere the SF₆ mixing ratios are overestimated. L39 generally calculates higher stratospheric SF₆ mixing ratios than L19. This deviation increases with altitude. The difference between the modelled profiles is partly attributed to the residual mean meridional circulation, which is stronger in L39 than in L19, and partly to the initial globally constant SF₆ concentration. The comparison of modelled ¹⁴CO₂ surface concentrations and vertical profiles with observations has shown that an increased vertical resolution in the climate model ECHAM4 reduces the strength of stratosphere–troposphere exchange. L39 allows a better representation of sharp gradients at the tropopause than L19. This results in a weaker downward transport across the tropopause. However, compared to observations the downward transport is still too strong even in the L39 simulation.