Clear air turbulence induced tracer mixing in the UTLS using Chemistry-Climate Model EMAC

The Earth's radiation budget could be affected by the distribution of the chemical composition in the upper troposphere/lower stratosphere (UTLS). Bi-directional stratosphere-troposphere exchange is one of the processes affecting the UTLS chemistry. This exchange could be e.g., facilitated by clear air turbulence (CAT), as it leads to diabatic mixing of chemical tracers between stratosphere and troposphere. Chemistry climate models usually ignore such small scale processes. In order to examine its importance, we developed a new submodel CAT for the climate chemistry model EMAC to parameterize the turbulent tracer mixing in the UTLS. The mixing scheme uses a 2-layer mixing algorithm based on turbulence diagnostics including the widely used Ellrod index and a newly introduced MoCATI. The model results show that the new mixing scheme could lead to a significant difference of the UTLS radiative active tracer gases in a climatological time scale. We also compared the results of the CAT submodel in EMAC with the turbulent mixing scheme of the non-hydrostatic regional model COSMO to investigate the impact of the turbulent mixing on a smaller spatial and temporal scale. The result shows that the new scheme could mix tracer significantly near the tropopause as the COSMO mixing scheme. The implementation of the new CAT submodel could allow further research on the long term climatic impact of turbulent mixing in the UTLS such as radiative budget or ozone destructive substances.