Uncertainty of aerosol direct radiative forcing associated with aerosol vertical distribution

Aerosols remain the largest source of uncertainty in anthropogenic forcing of climate change. To constrain aerosol forcing within a column, one needs to know aerosol loading, their optical properties and vertical profiles. In this study, we explore the impact of aerosol vertical distribution on their radiative forcing using radiative transfer models and CMIP6 simulation results. Sensitivity studies show that the magnitude of radiative forcing from both scattering and absorbing aerosols increases with altitude, with higher sensitivity for absorbing aerosols. We also noticed a non-linear effect when absorbing and scattering aerosols coexist in the same column, i.e., the absorption by absorbing aerosols may be enhanced (reduced) when scattering aerosols are located below (above) absorbing aerosols. Using simulated aerosol mass density fields from 7 CMIP6 models, we find that the difference in the aerosol vertical distribution cause uncertainties in aerosol direct forcing as large as 3 W m^-2 (Figure 1) and accounts for ~30% of the total uncertainty, and the non-linear interaction between absorbing aerosols and scattering aerosols can be significant for some regions, such East Asia, Southeast Asia and South America. We also evaluated the model aerosol vertical distribution with CALIPSO and found an overall low bias of ~1 km in aerosol layer height over land. This bias will cause ~0.5 W m-2 aerosol forcing uncertainty globally.