Chemistry-transport model comparison with ozone observations in the midlatitude lowermost stratosphere

About 600 ozone sonde profiles and in situ ozone observations on 2300 aircr aft flights were compared with several three-dimensional global chemistry-t ransport models, in the midlatitude lowermost stratosphere during 1996. The models use the same top boundary conditions and parameterized ozone chemis try. The comparisons show that the models using general circulation model ( GCM) win's do not capture the seasonal ozone accumulation in the lowermost stratosphere. The models using winds from the European Centre for Medium-Ra nge Weather Forecasts (ECWMF) overestimate ozone above this maximum, especi ally during spring. This overestimation increases with increasing latitude. Close to the tropopause. the best agreement between models and observation s is found during winter and the worst occurs during summer. where most mod els underestimate the mean ozone concentration. This underestimation is par tly caused by inaccurate description of the relatively small-scale transpor t processes, mainly associated with convective activity! and partly by inco mplete ozone chemistry in the parameterized scheme. The models that use ass imilated winds show significant differences in calculated ozone, despite th eir common source (ECWMF) to calculate the transport. We illustrate that th e model performance significantly depends on the ECWMF processing method an d that interpolation of wind data should be avoided. In addition, the resul ts seem relatively insensitive to vertical resolution. To improve the model performance further in this region, the horizontal resolution should be hi gher than 2.5 degrees.