Anthropogenic aerosols are effective cloud condensation nuclei, which determine the initial cloud droplet number concentration, and therefore influence cloud albedo, the so-called first aerosol indirect effect. For a given cloud water content, higher cloud droplet number concentrations imply smaller cloud droplet size. Smaller cloud droplet size also decrease the precipitation efficiency, which may increase cloud liquid water path and cloud fraction, the so-called second aerosol indirect effect. Based on current model estimates, the combined first and second indirect effects range from 0.0 to -4.8 W/{$\rm m^2$}, which can potentially counteract the warming caused by greenhouse gases. However, the magnitude of this effect is highly uncertain. Therefore, it is important to investigate the impact of aerosols on clouds in more detail.

ATHAM (Active Tracer High Resolution Atmospheric Model), a cloud resolving model, along with data collected in the CLOUDY COLUMN (CC) experiment during the second Aerosol Characterization Experiment (ACE-2), is used to investigate the aerosol indirect effect on clouds. Two CC cases, a clean case (pristine marine air) and a polluted case (anthropogenic pollution from Europe), are investigated here. ATHAM can generally reproduce the observed cloud properties although the liquid water path and cloud optical depth are over-estimated and the precipitation rate is under-estimated. The over-estimation of liquid water is sensitive to the use of the cyclic lateral condition, to the grid resolution, and to whether or not the large-scale advection term for liquid water is applied, and the observed drizzle flux may be over-estimated by a factor of 5 to 10. Sensitivity tests have been conducted to examine the impacts of the aerosols on cloud microphysical and optical properties using different precipitation parameterizations, initial conditions and large scale forcing terms. The results show that the mean cloud droplet size and precipitation rate are very sensitive to the aerosol amounts.