The upper troposphere/lower stratosphere in a changing climate – A global model study

Kurzfassung

The upper troposphere/lower stratosphere (UTLS) is the transition region around the tropopause. Due to the significant radiative effect of greenhouse gases (GHGs) in the UTLS on the surface climate, their representation is essential for climate studies. However, the UTLS is characterised by a complex interplay of transport barriers and atmospheric processes operating across a wide range of temporal and spatial scales, leading to steep gradients in abundances of chemical constituents. This complexity poses major challenges for observations and realistic model simulations of the UTLS. Therefore, our knowledge about the UTLS is subject to considerable uncertainty. Numerical model simulations are necessary to investigate the effect of climate change, but even with the same forcing scenario, the model results show a large spread. To identify the causes of this spread, sensitivity simulations with the ECHAM/MESSy Atmospheric Chemistry (EMAC) Chemistry Climate Model (CCM) are performed by varying a single aspect at a time. The influence of four model configurations on the simulated UTLS characteristics are examined: (1) Newtonian relaxation of the tropospheric dynamics, which is favourable for a direct comparison between CCM results and observations, (2) the grid resolution, which is crucial to resolve steep gradients, (3) the choice of the convection parameterisation, which is associated with large model uncertainty and the transport of tropospheric constituents into the UTLS, and (4) the effect of the convection parameterisation on the UTLS under climate change. Newtonian relaxation reduces temperature and wind biases in the troposphere and the lower stratosphere, but also significantly increases the moist bias in the lower part of the lowermost stratosphere (LMS) and alters the wave forcing. The enhanced stratospheric circulation thereby increases the ozone bias in the LMS. Therefore, the opposing effects complicate the drawing of conclusions about the model performance for the UTLS in a free-running simulation. Increasing the horizontal resolution for climate-scale CCM simulations substantially reduces biases caused by numerical diffusion in the lower part of the LMS. Especially the lower tropopause and the slower stratospheric circulation improve the model performance relative to observations. An additional finer vertical resolution in EMAC yields only minor improvements in model results. The choice of the convection parameterisation substantially affects the atmospheric transport, from the convective transport in the troposphere to the stratospheric circulation. In combination with its influence on chemical turnover rates, the choice affects the simulated composition of the UTLS, particularly the distribution of water vapour and ozone. Future projection simulations with different convection parameterisations show a robust response of the simulated UTLS to climate change. Significant differences in the response are due to the regeneration of ozone through decreased anthropogenic catalytic ozone depletion in the polar stratosphere.