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The initial implementation in CAM-chem into CCSM used NCEP reanalysis (e.g. Hess and Zbinden, ACP, 2013). At that time, interpolation of the reanalysis fields to CAM-chem vertical levels resulted in excessive stratosphere-troposphere exchange, and better performance was found using the vertical levels of the reanalysis fields. However, updated components in CESM2, including CLUBB, improve the representation of clouds and are potentially more sensitive to differences between physics and dynamics model levels. By default, the Free Running CAM-Chem compset (FCHIST) has 32 layers and the Specified Dynamics compset (FCSD) has 56 layers, because it is using the GEOS grid. Recently, we hypothesized that interpolating the reanalysis to 32 running a FCSD case with 32 instead of 56 layers could produce better results because the internal model physics is tuned to that layer structure. We performed an experiment to compare meteorological vertical levels of 32 and 56 layers, and found that there was little difference between resulting chemistry averaged over 2003-2012 (Table 1). However, we find differences in the modeled clouds, which are exacerbated regionally (Figure 1 and 2). We particularly see large differences over the Tibetan Plateau. This has the potential for a large effect on ozone, with larger regional differences.

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