Attributing differences in the fate of lateral boundary ozone in AQMEII3 models to physical process representations

Liu, Peng; Hogrefe, Christian; Im, Ulas; Christensen, Jesper H.; Bieser, Johannes; Nopmongcol, Uarporn; Yarwood, Greg; Mathur, Rohit; Roselle, Shawn; Spero, Tanya

Increasing emphasis has been placed on characterizing the contributions and the uncertainties of ozone imported from outside the US. In chemical transport models (CTMs), the ozone transported through lateral boundaries (referred to as LB ozone hereafter) undergoes a series of physical and chemical processes in CTMs, which are important sources of the uncertainty in estimating the impact of LB ozone on ozone levels at the surface. By implementing inert tracers for LB ozone, the study seeks to better understand how differing representations of physical processes in regional CTMs may lead to differences in the simulated LB ozone that eventually reaches the surface across the US. For all the simulations in this study (including inline-formulaWRF∕CMAQ, inline-formulaWRF∕CAMx, inline-formulaCOSMO-CLM∕CMAQ, and inline-formulaWRF∕DEHM), three chemically inert tracers that generally represent the altitude ranges of the planetary boundary layer (BC1), free troposphere (BC2), and upper troposphere–lower stratosphere (BC3) are tracked to assess the simulated impact of LB specification.

Comparing inline-formulaWRF∕CAMx with inline-formulaWRF∕CMAQ, their differences in vertical grid structure explain 10 %–60 % of their seasonally averaged differences in inert tracers at the surface. Vertical turbulent mixing is the primary contributor to the remaining differences in inert tracers across the US in all seasons. Stronger vertical mixing in inline-formulaWRF∕CAMx brings more BC2 downward, leading to higher BCT (inline-formula M8inlinescrollmathml BCT = BC1 + BC2 + BC3 123pt10ptsvg-formulamathimg3350802fed1a8326a7c431f656a09431 acp-18-17157-2018-ie00001.svg123pt10ptacp-18-17157-2018-ie00001.png ) and inline-formulaBC2∕BCT at the surface in inline-formulaWRF∕CAMx. Meanwhile, the differences in inert tracers due to vertical mixing are partially counteracted by their difference in sub-grid cloud mixing over the southeastern US and the Gulf Coast region during summer. The process of dry deposition adds extra gradients to the spatial distribution of the differences in DM8A BCT by 5–10 inline-formulappb during winter and summer.

inline-formulaCOSMO-CLM∕CMAQ and inline-formulaWRF∕CMAQ show similar performance in inert tracers both at the surface and aloft through most seasons, which suggests similarity between the two models at process level. The largest difference is found in summer. Sub-grid cloud mixing plays a primary role in their differences in inert tracers over the southeastern US and the oceans in summer. Our analysis of the vertical profiles of inert tracers also suggests that the model differences in dry deposition over certain regions are offset by the model differences in vertical turbulent mixing, leading to small differences in inert tracers at the surface in these regions.

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Liu, Peng / Hogrefe, Christian / Im, Ulas / et al: Attributing differences in the fate of lateral boundary ozone in AQMEII3 models to physical process representations. 2018. Copernicus Publications.

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