Parametrizing the mixing by clear air turbulence in the chemistry climate model EMAC and its respective radiative impact

Abstract. The Earth’s radiation budget is found to be sensitive to changes in the upper troposphere/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 sensitivity 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 climate chemistry model EMAC. This submodel parametrizes the vertical mixing by CAT based on a newly introduced turbulence diagnostic MoCATI. MoCATI shows a comparable performance with the well-established Ellrod-Knox index. Simulations are conducted with EMAC-QCTM 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 modified tracers. The tracer mixing through CAT also changes the atmospheric chemistry by shortening the CH₄ lifetime and changing the O₃ becoming relatively sensitive to NO_x. 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/m².