Impact of Aerosol Vertical Distribution on the Clear Sky Direct Aerosol Radiative Forcing Uncertainty in CMIP6 Models

In this study, we investigate the uncertainty of direct aerosol radiative forcing (DARF) associated with aerosol vertical distribution using simulation results from 14 global models within the Coupled Model Intercomparison Project Phase 6 (CMIP6). The column mass loading for each aerosol species is first normalized to the multi-model average for each model, and a unified radiative transfer model and set of aerosol optical parameter are used, so that the differences in the calculated DARF is solely attributed to the difference in aerosol vertical profiles. The global mean DARF in 2014 with respect to 1850 before and after mass-normalization are -0.77 0.52 and -0.81 0.12 respectively, assuming external mixing, which indicates that the inter-model difference in aerosol vertical distribution accounts for ~20% of the total DARF uncertainty. We further conduct two separate experiments by normalizing aerosol optical depth (AOD) and aerosol single scattering albedo (SSA) profiles respectively, and find that the vertical distribution of SSA results in larger DARF uncertainty than that of AOD. Finally, compared with CALIPSO observation, CMIP6 models tend to produce higher aerosol layers. The bias in modeled aerosol profile with respect to CALIPSO leads to stronger DARF, especially for land regions.