Sensitivity of tracers and a stratospheric aircraft perturbation to two-dimensional model transport variations

We examine the sensitivity of two-dimensional model simulations of stratosp heric tracers to uncertainties in the model transport and explore how such uncertainties impact the simulation of a lower stratospheric perturbation d ue to highspeed civil transport (HSCT) aircraft emissions. To define the tr ansport uncertainty, we vary the model transport fields so that the resulti ng tracer simulations roughly bracket the observations. This provides an es timate of the upper and lower limits on realistic transport rates in our tw o-dimensional (2-D) model, Increasing the advective residual circulation st rength or the lower stratospheric vertical diffusion (K-zz) decreases the m ean age and residence time of the HSCT emissions and diminishes the negativ e response in total column ozone globally. Increasing the stratospheric hor izontal diffusion (K-yy) either globally or in the tropics only has the opp osite effect of increasing the age and emission residence time and enhancin g the negative total ozone response. Uncertainties in the mechanical eddy f orcing derivation affect both K-yy and the residual circulation simultaneou sly, resulting in some cancellation of effects. This produces a smaller ran ge of uncertainty in the tracer and perturbation simulations than given by uncertainties in the circulation or Kyy components separately. The model si mulations in the lower and middle stratosphere are relatively insensitive t o the strength of the mesospheric gravity wave effects and the magnitude of the horizontal diffusive transport across the tropopause. The base model t ransport compares most favorably with tracer data and gives a global and an nual mean steady state HSCT perturbation response in total ozone of -0.62%, assuming a NOx emission index of 5 g/kg, 500 airplanes, and a 10% gas-to-p article conversion of the SO2 emission. For the range of transport uncertai nty examined in this study, the global total ozone perturbation response ra nges from -0.34% to -0.74%, with a mainly strong correlation between the to tal ozone response and mean age.