Transport of SF6 and 14CO2 in the atmospheric general circulation model ECHAM4

Two tracers, SF₆ with sources at the earth’s surface and 14CO₂ with a source in the lower stratosphere, are used to investigate the simulation of global scale transport in an atmospheric general circulation model. The simulated mixing ratios of SF₆ in the troposphere are generally in close agreement to observations revealing a realistic description of the large scale tropospheric transport. The interhemispheric exchange time for SF₆ is calculated to be 0.9 years, indicating a slightly too strong interhemispheric exchange. In the lowermost stratosphere the simulated vertical gradient of SF₆ is in good agreement with observations within the 1st 4 to 5 km above the tropopause indicating that the flux from the troposphere to the lowermost stratosphere is captured by the model. On the other hand, downward transport of 14CO₂ from the stratosphere into the troposphere is found to be overestimated. From a comparison with observations it is concluded that it is the downward transport in the subtropics that is overestimated, at high latitudes the vertical gradients in the tropopause region are close to observations. Finally, the tracer tests show that the transport into the uppermost two levels, above 20 km, is underestimated as these levels serve as sponge layers and not as layers with a reasonable transport. Consequently, the tracer concentrations in that altitude interval are underestimated, by up to a factor 2.