Evaluating the Model Representation of Asian Summer Monsoon Upper Troposphere and Lower Stratosphere Transport and Composition Using Airborne In Situ Observations

Chemistry Climate Models (CCMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth's climate system. This study demonstrates the use of airborne in situ observations to diagnose the representation of chemical composition and transport by CCMs. Process‐based diagnostics using dynamical and chemical coordinates are presented which minimize the spatial and temporal sampling differences between airborne in situ measurements and CCM grid points. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves as a rapid transport pathway for surface emissions to reach the upper troposphere and lower stratosphere (UTLS). We examine two CCM configurations for their representation of the ASM UTLS using a set of airborne observations from south Asia. The diagnostics reveal good model performance at representing tropospheric tracer distribution throughout the troposphere and lower stratosphere, and excellent representation of chemical aging in the lower stratosphere when chemical loss is dominated by photolysis. Identified model limitations include the use of zonally averaged mole fraction boundary conditions for species with sufficiently short tropospheric lifetimes, which may obscure enhanced regional emissions sources. Overall, the diagnostics underscore the skill of current‐generation models at representing pollution transport from the boundary layer to the stratosphere via the ASM mechanism, and demonstrate the strength of airborne in situ observations toward characterizing this representation. Plain Language Summary The chemical composition of Earth's atmosphere is important to understand for future climate prediction. This study establishes an approach for evaluating the representation of chemical composition in global climate models, and demonstrates the capabilities of the approach using a set of observations collected by research aircraft. We specifically target an evaluation of the Asian summer monsoon, a process with a well‐documented transport pathway for chemical species near the surface to reach the upper atmosphere. In doing so, we identify specific areas where focused model improvement is needed. Key Points Process‐based diagnostics for model evaluation using airborne in situ observations are presented The Asian summer monsoon is explored for its role in impacting global composition and climate The diagnostics use dynamical and chemical coordinates