A Study of Shallow Cumulus Cloud Droplet Dispersion by Large Eddy Simulations

Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models (GCMs). This study investigates droplet dispersion in shallow cumulus clouds under different aerosol conditions using three-dimensional large eddy simulations (LES). It is found that cloud droplet mean radius, standard deviation, and relative dispersion generally decrease as aerosol mixing ratio increases from 25 mg-1 (clean case) to 100 mg-1 (moderate case), and to 2000 mg-1 (polluted case). Under all the three simulated aerosol conditions, cloud droplet mean radius and standard deviation increase with height. However, droplet relative dispersion increases with height only in the polluted case, and does not vary with height in the clean and moderate cases. The mechanisms for cloud droplet dispersion are also investigated. An additional simulation without considering droplet collision-coalescence and sedimentation under the aerosol mixing ratio of 25 mg-1 shows smaller values of droplet mean radius, standard deviation, and relative dispersion as compared to the base clean case. This indicates that droplet collision-coalescence plays an important role in broadening droplet spectra. Results also suggest that the impact of homogeneous mixing on cumulus cloud droplet spectra is significant under all the three simulated aerosol conditions. In weak mixing (strong updraft) regions where clouds are closer to be adiabatic, cloud droplets tend to have larger mean radius, smaller standard deviation, and hence smaller relative dispersion than those in stronger mixing (downdraft or weak updraft) regions. The parameterized cloud optical depth in terms of cloud liquid water content, droplet number concentra- tion, and relative dispersion is only slightly smaller than the result calculated from detailed droplet spectra, indicating that current parameterization of cloud optical depth as used in many GCMs is plausible for low clouds.