Mechanisms of aerosol indirect effects on glaciated clouds simulated numerically

Various improvements were made to a state-of-the-art aerosol-cloud model and it was validated against observations from field campaigns. The robustness of this
aerosol-cloud model is in its ability to explicitly resolve all the known modes of heterogeneous cloud droplet activation and ice crystal nucleation. The model links
cloud particle activation with the aerosol loading and chemistry of seven different aerosol species. Continental and maritime cases were simulated for the purposes of
validating the aerosol-cloud model, and investigating the salient microphysical and dynamical mechanisms of aerosol indirect effects (AIE) from anthropogenic solute
and solid aerosols, focusing mainly on glaciated clouds.

The results showed that increased solute aerosols reduced cloud particle sizes and inhibited warm rain processes, thus, enhancing chances of homogeneous cloud droplet and aerosol freezing. Cloud fractions and their optical thicknesses increased quite substantially in both cases. Although liquid mixing ratios were boosted, there
was however a substantial reduction of ice mixing ratios in the upper troposphere owing to the increase in snow production aloft. Convective updrafts became weaker
mainly in the continental case, while weak vertical velocities strengthened slightly in the upper troposphere. With an increase in solid aerosols, clouds became slightly
more extensive over the continents, while the cloudiness diminished over the oceans.

The total AIE of clouds from solute aerosols was two times higher in the oceanic than in the continental case, because the sensitivity of the cloud properties to perturbation
in aerosol concentrations diminishes with increasing background aerosol concentrations. Also, the AIEs of glaciated clouds were greater than those of water-only
clouds by a factor of two in the continental case while both cloud types were equally important in the maritime case. The radiative importance of glaciated clouds lied
in their large collective spatial extent and existence above water-only clouds. The glaciation AIE from solid aerosols had a cooling effect in continental clouds because
of an increase in cloud fraction and a warming effect in maritime clouds because of a decrease in cloud fraction. In addition to the traditional AIEs (glaciation, riming
and thermodynamic), sedimentation, aggregation and coalescence were new AIEs identified. Importantly, it was discovered that these individual AIEs interact, compensate
and buffer each other, hence, the relative importance of contributions from responses of various processes vary during the climate change. Finally, meteorology
was identified to have little effect on the mechanisms of aerosol-cloud interaction.