Methane chemistry in a nutshell-the new submodels CH4 (v1.0) and TRSYNC (v1.0) in MESSy (v2.54.0)

Abstract

Climate projections including chemical feedbacks rely on state-of-the-art chemistry-climate models (CCMs). Of particular importance is the role of methane ( ) for the budget of stratospheric water vapour (SWV), which has an important climate impact. However, simulations with CCMs are, due to the large number of involved chemical species, computationally demanding, which limits the simulation of sensitivity studies. To allow for sensitivity studies and ensemble simulations with a reduced demand for computational resources, we introduce a simplified approach to simulate the core of methane chemistry in form of the new Modular Earth Submodel System (MESSy) submodel CH4. It involves an atmospheric chemistry mechanism reduced to the sink reactions of with predefined fields of the hydroxyl radical (OH), excited oxygen (O( D)), and chlorine (Cl), as well as photolysis and the reaction products limited to water vapour ( ). This chemical production of is optionally fed back onto the specific humidity ( q ) of the connected general circulation model (GCM), to account for the impact onto SWV and its effect on radiation and stratospheric dynamics. The submodel CH4 is further capable of simulating the four most prevalent isotopologues for carbon and hydrogen ( and , as well as span classCombining double low line"inline-formula"CH4 and span classCombining double low line"inline-formula"CH4). Furthermore, the production of deuterated water vapour (HDO) is, similar to the production of in the oxidation, optionally passed back to the isotopological hydrological cycle simulated by the submodel H2OISO, using the newly developed auxiliary submodel TRSYNC. Moreover, the simulation of a user-defined number of diagnostic age and emission classes is possible, the output of which can be used for offline inverse optimization techniques. The presented approach combines the most important chemical hydrological feedback including the isotopic signatures with the advantages concerning the computational simplicity of a GCM, in comparison to a full-featured CCM.