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CAM-chem is based off the MOZART Chemical Transport model, which used meteorological reanalysis 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 CESM 1 used NCEP reanalysis. At that time, interpolations of the reanalysis fields to CAM-chem vertical levels produced discontinuities in the XXX. 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 reanalysis on 32 layers could produce better results because the internal model physics, which is performed on 32 layers, is aligned better with the nudging. 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
56L
32L
 56L-32L
CH4_LIFETIME (yr)
7.478
7.631 
-0.153
CH3CCL3_LIFETIME (yr) 
4.478
4.582
-0.103
CO_BURDEN (Tg)
287.389 
293.319
-5.930
CO_EMIS (Tg/yr)
1063.802
1063.172 
0.630
CO_TDEP (Tg/yr)
138.416
138.496
-0.080
CO_CHEM_LOSS (Tg/yr)
2348.777
2315.937
32.840
CO_LIFETIME (yr) 
0.116 
0.120
-0.004
O3_BURDEN (Tg)
338.290
324.627
13.662
O3 Strat BURDEN (Tg/yr)
2847.368
2817.248
30.120
O3_STE (Tg/yr)
476.645
490.927
-14.282
ISOP_EMIS (Tg/yr) 
464.215 
460.233 
3.981
LNO_PROD (TgN/yr)
3.943 
3.063
-0.881
Total optical depth
0.161
0.154
-0.007

See all comparisons here: 2003-2012 comparisons

Figure 1: Cloud and ozone differences between 32 and 56 layer simulations for June 2013.


Figure 2: Add in CloudSat comparisons here.



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