Abstract/Summary

Assessing global models in the upper troposphere (UT) and in the lowermost stratosphere (LS) is an important step towards a better understanding of the chemical composition near the tropopause. For this purpose, the current study focuses on an evaluation of long-term simulations from four chemistry-climate/transport models, based on the In-service Aircraft for a Global Observing System (IAGOS) measurements. Most simulations span over the 1995–2017 period, and follow a common protocol among the models. The assessment focuses on climatological averages of ozone (O3), water vapour (H2O), carbon monoxide (CO), and reactive nitrogen (NOy). In the extra-tropics, the models reproduce the seasonality of ozone, H2O, and NOy in both UT and LS, but none of them reproduces CO springtime maximum in the UT. Tropospheric tracers (CO, H2O) tend to be underestimated, consistently with an overestimation of cross-tropopause exchanges. Most models systematically overestimate ozone in the UT, and nitrogen oxides (NOx) background appears as the main contributor to ozone variability across the models. The partitioning between NOy species changes drastically across the models, and acts as a source of uncertainty on NOx mixing ratio and on its impacts on atmospheric composition. However, we highlight some well-reproduced geographical variations, as the ITCZ seasonal shifts above Africa, or extratropical ozone (H2O) in the LS (UT) correlated with the observations. These features are encouraging regarding the simulated dynamics in both layers. The current study confirms the importance of separating the UT and the LS with a dynamical tracer for model results evaluation, and for model intercomparisons.