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

Abstract

We examine the sensitivity of two‐dimensional model simulations of stratospheric tracers to uncertainties in the model transport and explore how such uncertainties impact the simulation of a lower stratospheric perturbation due to high‐speed civil transport (HSCT) aircraft emissions. To define the transport uncertainty, we vary the model transport fields so that the resulting tracer simulations roughly bracket the observations. This provides an estimate of the upper and lower limits on realistic transport rates in our two‐dimensional (2‐D) model. Increasing the advective residual circulation strength or the lower stratospheric vertical diffusion (Kzz) decreases the mean age and residence time of the HSCT emissions and diminishes the negative response in total column ozone globally. Increasing the stratospheric horizontal diffusion (Kyy) either globally or in the tropics only has the opposite effect of increasing the age and emission residence time and enhancing the negative total ozone response. Uncertainties in the mechanical eddy forcing derivation affect both Kyy and the residual circulation simultaneously, resulting in some cancellation of effects. This produces a smaller range of uncertainty in the tracer and perturbation simulations than given by uncertainties in the circulation or Kyy components separately. The model simulations in the lower and middle stratosphere are relatively insensitive to the strength of the mesospheric gravity wave effects and the magnitude of the horizontal diffusive transport across the tropopause. The base model transport compares most favorably with tracer data and gives a global and annual 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‐particle conversion of the SO2 emission. For the range of transport uncertainty examined in this study, the global total ozone perturbation response ranges from −0.34% to −0.74%, with a mainly strong correlation between the total ozone response and mean age.