Investigation of Aerosol-Cloud Interactions Using a Chemical Transport Model Constrained by Satellite Observations

This study investigates aerosol-cloud interactions in marine clouds, using a combined approach of global chemical transport modeling for cloud droplet concentrations and satellite remote sensing for cloud liquid water path. The simulated aerosol and cloud droplet concentrations are validated with long-term surface and in situ aircraft measurements for a variety of polluted and relatively clean conditions. The column-integrated cloud liquid water path is constrained by the SSM/I satellite microwave measurements. As a result, the calculated cloud optical thickness (COT) for marine warm clouds displays similar geographical patterns and seasonal variations to two independent retrievals by MODIS and CERES satellite experiments. The fundamental difficulty in validating the simulated COT values is that the two satellite data differ significantly (by 42% to 110%) from each other even for zonal means, and the satellite derived cloud effective radius (Re) is systematically larger (by a factor of 1.5 to 2) than the in-situ aircraft data. The simulated Re is in closer agreement with aircraft data, thus the calculated zonal mean COTs are larger than CERES and MODIS values by factors of 1.5 to 3. The modeled hemispherical mean cloud properties show large systematic differences, i.e., the cloud drop number density is a factor of 2 larger; the liquid water path (LWP) is larger by 12%; and COT is about 30% larger in the northern hemisphere (NH). Most of these asymmetries are driven by anthropogenic aerosols; however, they are not indicated by the satellite observations. Assuming that uncertainties in satellite retrievals are not masking the inter-hemispheric asymmetries, we speculate that this is due to the biogenic source of marine organic aerosols over the southern hemisphere (SH). At constant LWP, a same perturbation in aerosol concentration leads to the largest enhancement in COT over the remote SH oceans. Therefore, the neglected marine biogenic organic aerosols may play an important role in the estimated SH cloud radiative forcing. We also showed that the aerosol dispersion effect on cloud droplet size spectrum reduces the Twomey effect substantially; and the smaller cloud susceptibility obtained at larger cloud LWP implies a possible negative correlation between cloud geometric thickness and droplet concentration.