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

• Main Authors: F. Winterstein, P. Jöckel
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-02-01
• Series: Geoscientific Model Development
• Online Access: https://gmd.copernicus.org/articles/14/661/2021/gmd-14-661-2021.pdf

<p>Climate projections including chemical feedbacks rely on state-of-the-art chemistry–climate models (CCMs). Of particular importance is the role of methane (<span class="inline-formula">CH₄</span>) 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.</p> <p>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 <span class="inline-formula">CH₄</span> with predefined fields of the hydroxyl radical (OH), excited oxygen (O(<span class="inline-formula">¹</span>D)), and chlorine (Cl), as well as photolysis and the reaction products limited to water vapour (<span class="inline-formula">H₂O</span>). This chemical production of <span class="inline-formula">H₂O</span> is optionally fed back onto the specific humidity (<span class="inline-formula"><i>q</i></span>) of the connected general circulation model (GCM), to account for the impact onto SWV and its effect on radiation and stratospheric dynamics.</p> <p>The submodel CH4 is further capable of simulating the four most prevalent <span class="inline-formula">CH₄</span> isotopologues for carbon and hydrogen (<span class="inline-formula">CH₄</span> and <span class="inline-formula">CH₃D</span>, as well as <span class="inline-formula">¹²</span><span class="inline-formula">CH₄</span> and <span class="inline-formula">¹³</span><span class="inline-formula">CH₄</span>). Furthermore, the production of deuterated water vapour (HDO) is, similar to the production of <span class="inline-formula">H₂O</span> in the <span class="inline-formula">CH₄</span> 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 <span class="inline-formula">CH₄</span> age and emission classes is possible, the output of which can be used for offline inverse optimization techniques.</p> <p>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.</p>