Abstract. The Earth's radiation budget is found to be sensitive to changes in the upper troposphere and lower stratosphere (UTLS) chemical composition. Stratosphere-troposphere exchange is the major process that influences the UTLS chemical composition with remaining uncertainties in current climate-chemistry models. 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. In this work, we examine the potential impact of vertical mixing by CAT on the UTLS chemical composition and its corresponding radiative impact by implementing a newly developed submodel parametrizing turbulent mixing in the free troposphere and stratosphere within the chemistry climate model EMAC. This submodel parametrizes the vertical mixing by CAT based on a newly introduced turbulence diagnostic Modified CAT Index (MoCATI). MoCATI shows a comparable performance with the well-established Ellrod-Knox index. Simulations are conducted with EMAC under the Quasi Chemistry transport Model (QCTM) mode to examine the sole impact of mixing, without taking the potential feedback into account. Results show that the radiatively active ozone in the UTLS is most sensitive to the vertical mixing of CAT and is significantly reduced by 10 % to 20 % by the CAT submodel. This modification is not a pure result of the physical mixing but also the chemical feedback of other tracer distributions modified by CAT. The tracer mixing through CAT also changes the atmospheric chemistry by shortening the CH4 lifetime and changing the O3 becoming more sensitive to NOx. It also leads to potential surface radiative heating and radiative cooling at the top of the atmosphere. The global average radiative effect is about −0.2 W m−2 without considering water vapour.
Chau et al. (2026) studied this question.