Role of Aerosols in Governing Optical Depth of Marine Low-level Clouds

Marine low-level clouds have a significant cooling effect on the earth radiation budget. Their optical properties are influenced by aerosols through complex interactions involving aerosol properties as initial condensation nuclei (CN), increases in CN concentration due to anthropogenic activities, aerosol activation to cloud droplets, and cloud dynamics. The aerosol indirect effect on global climate change by altering cloud microphysics is far from being precisely evaluated. The purpose of this study is to examine the role of aerosols in governing marine cloud optical thickness in a global model, combined with in situ and satellite measurements. The global distribution of aerosols including sulfate, sea salt, mineral dust, black and organic carbons is simulated with a three-dimensional global aerosol transport model (Umich/IMPACT), which is driven by the ERA-40 re-analysis meteorological fields for the year 2001. The model predicted seasonal and geographical variations of aerosol number concentrations agree reasonably well with surface observations. Cloud droplet number is parameterized depending on aerosol properties, temperature, and an observed updraft velocity distribution at cloud base. Increase in cloud drops as aerosol number concentration increases is generally captured by the model, compared to in situ aircraft measurements for low-level marine clouds at different locations. The ERA-40 cloud liquid water content constrained by the satellite microwave measurements (SSM/I) of cloud liquid water path is used to compute cloud effective radius and optical depth. The combination of the observed cloud liquid water path includes the aerosol effect on precipitation and cloud feedback implicitly. Finally, the modeled cloud effective radius and optical depth are compared with airborne measurements and satellite (MODIS) retrieved optical properties for low-level marine clouds during the model simulated time periods. Sensitivity study on sea salt aerosol emissions show a large uncertainty (>50%) in the predicted cloud droplet number over the remote oceans. By combining and comparing with the available observations for aerosols and clouds to reduce uncertainties in the model, we present a better way to quantify the marine cloud optical thickness based on modeling of the aerosol-cloud interaction.