Short-lived Climate Forcers and Age of Air: Diagnostics for the Performance of Atmospheric Models

This thesis evaluates the ability of atmospheric models to simulate the chemistry and dynamics of short-lived climate forcers and other trace gases. Modelling is a critical tool for studying the impacts of these trace gases on the climate and making predictions that can inform policy decisions. It is also challenging; the atmosphere involves a large number of physical and chemical processes that must be approximated to varying degrees depending on the specific goals of the model in question. A critical component of climate modelling is therefore determining how well models match measurements of the atmosphere in order to understand their limitations. The work in this thesis contributes to this effort through investigations in three key areas: ozone-related chemistry, the implications of methane distributions, and changes in atmospheric transport. This was mainly done through comparisons of the specified dynamics run of the Canadian Middle Atmosphere Model (CMAM39) with satellite measurements from the Atmospheric Chemistry Exper- iment Fourier Transform Spectrometer (ACE-FTS). In addition, these comparisons were extended to other atmospheric models to gain perspective on the current state of atmospheric modelling and how CMAM39 performs relative to similar models. Measurements from additional satellite instruments were also incorporated to complement the conclusions drawn from the comparisons with ACE-FTS. In the first two studies, ozone-related chemistry in CMAM39 was explored through the parti- tioning of total inorganic chlorine gases, which are primarily responsible for ozone depletion. For this purpose, a suite of climatologies was developed using measurements of stratospheric chlorine gases from ACE-FTS and three other satellite instruments. It was found that CMAM39 generally underestimates the activation of chlorine gases that lead to ozone depletion. The following two studies involved a comprehensive examination of trace gas concentrations in CMAM39 and multiple other models. This began with a larger multi-model assessment as part of an Arctic Monitoring and Assessment Programme project, which focused on comparing ten models with each other. The second study was a more specific investigation of CMAM39 and three other models, focusing on diagnosing issues in atmospheric transport using the distributions of methane and other trace gases such as OH, N2O, and CO. As a global model with detailed stratospheric chemistry, CMAM39 was found to be one of the best-performing models. In the final study, an “age of air” product was developed using ACE-FTS measurements of SF6in order to test model predictions that the general circulation of the atmosphere is accelerating. This was a significant contribution to the collection of observation-based estimates of age of air due to its global coverage, vertical resolution, and relatively long 17-year time series. The dataset was used to detect a significant decrease in age of air in the lower stratosphere between 2004 and 2020, which indicates that part of the Brewer-Dobson Circulation is accelerating as predicted by models.

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