The influence of the convection parameterisation on simulated present and future UTLS greenhouse gas distributions

State-of-the-art Chemistry Climate Models (CCMs) exhibit a large model spread in their simulated chemical composition of the Upper Troposphere / Lower Stratosphere (UTLS). As the surface climate is highly sensitive to differences in greenhouse gas composition in the UTLS region, understanding and reducing this model spread is required for more confidence in climate projections. One large source of uncertainty in atmospheric simulations is the representation of convection. While heavily parameterised, convection is crucial for the water distribution in the air, the formation of clouds, and their radiative effect, as well as for the fast vertical transport in the troposphere. In addition to its direct effect on the UTLS composition, it may also have an indirect impact on transport into the stratosphere. By affecting the wind and temperature fields, convection influences the creation, propagation, and dissipation of Rossby waves that drive the Brewer-Dobson circulation in the stratosphere.

To investigate the effect of the convection parameterisation on the simulated UTLS composition, we performed sensitivity simulations with the CCM ECHAM/MESSy Atmospheric Chemistry (EMAC). Two simulations were performed under present climate conditions. The simulations are identical except for the applied convection parameterisation. The simulations were repeated, but with projected future climate boundary conditions, to investigate the effect of different convection parameterisations on the simulated UTLS composition under climate change.

Our results show that the simulated UTLS composition is highly sensitive to the applied convection parameterisation. The convective transport strength and outflow altitude vary strongly between different parameterisations, affecting the distribution of short-lived tracers in the upper troposphere as well as their transport into the tropical lower stratosphere. Significant differences in cloud and water distribution lead to changes in chemical reaction rates, particularly in the polar lower stratospheric ozone chemistry. Despite these differences, the effects of climate change on convective transport are in close agreement between the sensitivity simulations. Nevertheless, the strong coupling between temperature, water, and ozone creates large differences in the projected changes of the UTLS composition.

We found that the choice of the convection parameterisation influences the composition and the transport in the entire atmosphere, far beyond its direct effect in the troposphere.