Dispersion of Cloud Droplet Size Distributions, Cloud Parameterizations and Indirect Aerosol Effects

Most studies of the effect of aerosols on cloud radiative properties have considered only changes in the cloud droplet concentration, neglecting changes in the spectral shape of the cloud droplet size distribution. However, it has been shown that that the spectral dispersion of the cloud droplet size distribution (defined as the ratio of the standard deviation to the mean radius of the droplet size distribution) has a significant role in determining cloud radiative properties (e.g., Liu and Daum 2000). It has also been shown that the addition of anthropogenic aerosols to a marine air mass enhances not only the droplet concentration, but also the spectral dispersion, and that the increased spectral dispersion acts to offset the cooling of the first indirect aerosol effect by as much as 10% to 80% (Liu and Daum 2002). As for the droplet concentration, the spectral dispersion is a function not only of pre-cloud aerosols, but is also affected by the dynamical properties of clouds such as updraft velocity and turbulence. These dynamical effects cause uncertainties in the relationship between the spectral dispersion and droplet concentration. Here we seek to differentiate between the effects of cloud dynamics and pre-cloud aerosols on the spectral dispersion. We also address the effect of the spectral dispersion on the parameterization of warm rain microphysics, and on the second indirect aerosol effect.