Three-dimensional aspects of droplet nucleation

Correctly representing cloud droplet concentration in numerical models is critical in evaluating aerosol indirect effects and requires a realistic treatment of droplet nucleation. Results from large eddy simulation (LES) employing size-resolved microphysics suggest aspects of aerosol activation in marine stratocumulus not captured by nucleation schemes based on simple empirical relations or parcel theory. Classical theory predicts that CCN activation occurs at or near cloud base, where supersaturation is maximum. In contrast, we argue that all regions of positive supersaturation, including those inside the cloud, more completely represent the droplet nucleation zone. Furthermore, since nucleation does not occur uniformly but rather only in supersaturated updraft regions, the number of CCN activated should be considered an upper bound on grid-mean droplet concentration.