Estimates of stratosphere-troposphere exchange: Sensitivity to model formulation and horizontal resolution

The vertical transport between the stratosphere and the troposphere is studied by evaluating a matrix that gives the amount of air transferred between the different model layers of a three-dimensional off-line transport model coupled to European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The latitudinal structure of vertical exchange reflects the Hadley circulation with upward transport in the tropics and with downward transport at mid-latitudes. Maxima in downward transport occur near the subtropical and polar jet streams. The seasonal cycle of downward transport and the asymmetry between the two hemispheres are at least qualitatively represented by the model. However, it is found that the evaluated total amount of air transferred from the stratosphere to the troposphere per month is sensitive to the chosen horizontal model resolution. Refinement of this resolution from 8 degrees x 10 degrees to 4 degrees x 5 degrees leads to almost a doubling of the computed downward transport. Also, the vertical transport is found to depend strongly on changes in the formulation of the parent model (ECMWF). It is therefore important to use internally consistent meteorological input data when performing multiyear simulations with off-line chemistry-transport models.