Cloud-borne aerosols (aerosols that are attached to clouds) are formed by aerosol activation, aqueous-phase chemistry in clouds, or coagulation of aerosols with cloud droplets. The representation of cloud-borne aerosols is important for simulating aerosol-cloud interactions. However, most of the current GCMs including E3SM neglect the large-scale transport of cloud-borne aerosols in the model for the sake of saving computational time, which could result in biases in aerosol and cloud simulations. As the grid spacing of GCMs is reduced, the importance of cloud-borne-aerosol transport is expected to become more significant. Therefore, the objective of this study is to investigate the impact of neglected large-scale transport of cloud-borne aerosols in the GCM and to explore how the impact change with the resolution of GCMs.

First, we perform a pair of control simulations with/without considering cloud-borne aerosol advection at ne30 (approximately 1 degree) resolution, and we find that on a global basis, including the cloud-borne aerosol advection increases the cloud-borne aerosol burden approximately by 20%, but decreases the accumulation-mode aerosol number and cloud-droplet number by about 3% and 1.8%, respectively. On the other hand, the regional differences in aerosol and CDNC numbers can be over 30% (e.g., over East Asia, Europe, and Southern Ocean). For the aerosol-radiative forcing, however, the impact of cloud-borne aerosol advection is limited. Furthermore, we run additional set of sensitivity simulations with/without considering cloud-borne aerosol advection at ne120 (approximately 0.25 degree) resolution and compared them to the control simulations. When the resolution is changed to ne120, the impact of cloud-borne-aerosol advection on the aerosol burden is changed little on a global basis, while the regional differences, particularly over the outflow regions are noticeable.