Sensitivity of the First Indirect Aerosol Effect to an Increase of Cloud Droplet Spectral Dispersion with Droplet Number Concentration

Observations show that an increase in anthropogenic aerosols leads to concurrent increases in the cloud droplet concentration and the relative dispersion of the cloud droplet spectrum, other factors being equal. It has been suggested that the increase in effective radius resulting from increased relative dispersion may substantially negate the indirect aerosol effect, but this is usually not parameterized in global climate models (GCMs). Empirical parameterizations, designed to represent the average of this effect, as well as its lower and upper bounds, are tested in the CSIRO GCM. Compared to a control simulation, in which the relative dispersion of the cloud droplet spectrum is prescribed separately over land and ocean, inclusion of this effect reduces the magnitude of the first indirect aerosol effect by between 12% and 35%.