New understanding and quantification of the regime dependence of aerosol-cloud interaction for studying aerosol indirect effects

Aerosol indirect effects suffer from large uncertainty in climate models and among observations. This study focuses on two plausible factors: regime dependence of aerosol-cloud interactions and the effect of cloud droplet spectral shape. We show, using a new parcel model, that combined consideration of droplet number concentration (N_c) and relative dispersion (ε, ratio of standard deviation to mean radius of the cloud droplet size distribution) better characterizes the regime dependence of aerosol-cloud interactions than considering N_c alone. Given updraft velocity (w), ε increases with increasing aerosol number concentration (N_a) in the aerosol-limited regime, peaks in the transitional regime, and decreases with further increasing N_a in the updraft-limited regime. This new finding further reconciles contrasting observations in literature and reinforces the compensating role of dispersion effect. The nonmonotonic behavior of ε further quantifies the relationship between the transitional N_a and w that separates the aerosol- and updraft-limited regimes.