The Cloud Droplet Relative Dispersion Effect - Does It Exist?

Observations of cloud droplet spectra have suggested that there may be a relationship between the relative dispersion of the droplet size distribution and the cloud droplet number concentration or aerosol concentration. Such a relationship is of interest because a higher relative dispersion of the droplet size distribution causes a decrease in the reflectivity of clouds, which in turn speaks to the role of clouds in the climate system. In recent studies, this impact of the number concentration on the relative dispersion has been termed the “dispersion effect.” These studies usually only consider the impact of a variable relative dispersion on the albedo of the clouds while assuming that all other cloud properties remain unchanged. However, changing the relative dispersion of a cloud droplet distribution will also impact the microphysical process rates and hence the cloud properties themselves.

In this study we use simulations of shallow cumulus clouds to explore the potential magnitude of the dispersion effect. The simulations are run with the RAMS bin-emulating microphysics scheme with different values of aerosol concentration and relative dispersion (a parameter that remains fixed in time and space in the RAMS microphysics scheme). We find that the increase in average cloud albedo with decreasing relative dispersion is partially offset by a decrease in the cloud fraction that is induced primarily by changes in the droplet evaporation rate. The change in cloud fraction is largest for low values of the droplet concentration. These results suggest that should a positive correlation exist between relative dispersion and cloud droplet concentration, the impact of this correlation on cloud albedo would not have as large an impact on the first indirect effect as previously thought for shallow cumulus clouds.