Evaluate the Representation of Methane in the GFDL Atmospheric Chemistry Model

Global chemistry-climate models (CCMs) that explicitly resolve the different sources and sinks of methane provide a means of quantifying uncertainties in the global methane budget and predicting future methane levels. In this work, we evaluate the representation of methane in the NOAA GFDL Chemistry-Climate Model (AM4 prototype). We perform two sets of simulations for the year 2000 - one with prescribed surface global mean methane concentration and the other driven by methane emissions. We evaluate the simulated methane distributions against a suite of observations including, ground-based, aircraft and satellite retrievals. Initial evaluation indicates that the methane concentration driven simulation shows lower spatial and temporal variability, while the methane emission driven demonstrates better performance in capturing spatial and temporal (seasonal) variation and vertical distribution. We will present results from additional simulations that test the sensitivity of methane distribution and lifetime to different model processes including, emissions, chemistry, and convection, that will help better quantify uncertainties in the simulated methane distributions and lifetime.