The Remarkable Condensational Growth of Cloud Droplets Grown on Giant Sea-salt Aerosol Particles

Condensational growth of cloud drops is often assumed to be a well-understood process, using the drop growth equation: After activation of cloud condensation nuclei into cloud drops at cloud base, model studies often assume that drops quickly become dilute and consist of pure water. As a consequence, adiabatic ascent predicts that the growth rate of these drops is inversely proportional to their radius, and consequently that the drop spectrum will narrow with altitude. Here we demonstrate that drop growth on giant sea-salt aerosol particles (GCCN, dry radius > 0.5 μm) behaves very differently. For typical marine stratocumulus updrafts (0.4 m s-1), and for dry radius > 2 μm, these drops typically remain concentrated solution drops, and their condensational growth is so fast that they accelerate away from the main drop peak, and thus very rapidly attain precipitation drop sizes through condensation only. We show how many studies, that have attempted to include the effects of GCCN by initiating large pure water drops at cloud base, may have seriously underestimated the effects of GCCN on warm rain formation. We also speculate on possible solutions to this issue.