Cloud droplet spectral dispersion and the indirect aerosol effect: Comparison of two treatments in a GCM

Two parameterizations of cloud droplet spectral dispersion and their impact on the indirect aerosol effect are compared in a global climate model. The earlier scheme specifies β , the ratio of droplet effective radius to volume‐mean radius, in terms of N , the cloud droplet number concentration. The new scheme specifies β in terms of mean droplet mass ( L / N ), where L is liquid water content, to account for the effect of variations in L . For low to moderate N , the new scheme gives a stronger increase of β with increasing N than the old scheme. In a present‐climate simulation, the new scheme shows a stronger gradient between remote regions (small β ) and polluted/continental regions (large β ). The new scheme also offsets the first indirect aerosol forcing (Δ F ) more strongly: Δ F = −0.65 W m −2 with constant β , −0.43 W m −2 with the old β , and −0.38 W m −2 with the new β .