Introduction of tropospheric chemistry in the CNRM Earth System Model (ESM): towards methane emission-driven capability.

Methane (CH₄) is the second most important greenhouse gas after carbon dioxide (CO₂), and improving the representation of its cycle in climate models is a key step to reduce uncertainties in climate projections. Its main sink is chemical removal through oxidation with hydroxyl radicals (OH) in the troposphere, which are produced during the photolysis of ozone (O₃) in presence of water vapour. This process is an example of the complex interactions between methane and climate system, highlighting the necessity to have an explicit and interactive representation of atmospheric composition in Earth System Models. Here we present the introduction of two tropospheric/stratospheric chemical schemes of various complexities in ARPEGE-Climat 7.0, the future version of the atmospheric component of CNRM-ESM, and the impact on methane representation. This work includes the addition and changes of multiple processes at stakes in the troposphere, for instance emissions, deposition or production of NO_x by lightning strikes. We first present an evaluation of tropospheric air composition in our model including all the aforementioned developments. Diagnostics from both chemical schemes in AMIP-type simulations are compared to observations and to state of the art atmospheric composition reanalyses such as the CAMS reanalysis. We highlight, in particular, the performance of both chemical schemes in terms of biases and seasonal cycles of major tropospheric tracers like O₃, CO or NO₂. We also compute from RFMIP-type simulations the ozone ERF, and compare it to previous estimates. Secondly, we present an evaluation of the behaviour of pre-industrial simulations in a methane “emission-driven” mode. These simulations are compared to more classical “concentration-driven” simulations in terms of global methane budget and methane chemical lifetime.