Evaluating the Modeled Representation of the Asian Summer Monsoon UTLS using Airborne In Situ Observations

The Asian summer monsoon (ASM) rapidly lofts anthropogenic species from the Asian boundary layer into the upper troposphere and lower stratosphere (UTLS), where they may have implications for changes in composition including (but not limited to) the depletion of stratospheric ozone. Accurate simulation in this region is essential for characterizing the past, present, and future implications of such composition changes. We evaluate the representation of ASM composition using airborne in situ observations taken during the 2017 StratoClim campaign based in Kathmandu, Nepal. Along the way, diagnostics are established which can evaluate the ability for models to represent the chemical signatures found within the ASM UTLS layer. This evaluation utilizes two configurations of the NCAR Community Earth System Model (CESM): the Whole Atmosphere Community Climate Model (WACCM) with a horizontal grid spacing of 1° and the recently developed Multi-scale Infrastructure for Chemistry and Aerosols (MUSICA) with a refined grid spacing of 0.25° over South and East Asia. Our results find that the two model configurations generally represent the spatial and chemical signatures observed within the ASM UTLS during StratoClim. There is particular skill in representing chemical relationships between longer-lived species (lifetime > 10 years), while relationships with shorter-lived species are not as well represented. Furthermore, there are enhanced concentrations of tropospheric tracers in the MUSICA simulation with horizontal grid refinement, which improves the modeled representation compared to observations. These results provide a benchmark for the evaluation of other model configurations at representing the composition of the ASM UTLS layer, and set the stage for model evaluation using observations from the 2022 Asian summer monsoon Chemical and Climate Impact Project (ACCLIP).