CAM-chem is based off the MOZART Chemical Transport Model, which used meteorological reanalyses to explicitly describe the model dynamics and consequently used the reanalysis vertical layers. CAM-chem is a component of an Earth system model, and consequently is coupled to an atmosphere component that drives model dynamics. Reanalysis is used to nudge the model dynamics towards reality for model experiments that require analysis of real-world events. 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. Recently, we hypothesized that interpolating the reanalysis to 32 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.
Table 1: Selected results from CAM-chem simulations with different vertical levels.
|Global Average values 2003-2012|
O3 Strat BURDEN (Tg/yr)
Total optical depth
See all comparisons here: 2003-2012 comparisons
Figure 1: Add in 56L and 32L vertical levels shown in pressure coordinates.
Figure 2: Cloud and ozone differences between 32 and 56 layer simulations for August, 2013 showing that regionally, large difference may occur.
Figure 3: Add in CloudSat comparisons to show the difference of 32 and 56 layers against the observational cloud field.