From Droplet Nucleation to Precipitation: Following in Sean Twomey's Giant Footsteps (Invited Presentation)

Sean Twomey's contributions to the field of atmospheric physics in general and to cloud physics in particular cannot be overstated. His name is synonymous with one of the most important climate effects of the atmospheric aerosol - namely the influence of aerosol particles on cloud albedo - but his work has had a profound influence on our understanding of precipitation formation, radiative transfer, and mathematical inversion methods, to name a few.

In this talk I will summarize research that builds on the foundation stones of Twomey's work. The talk will briefly cover Twomey's 1959 work on droplet nucleation - an elegant analytical solution that manifests simplicity while at the same time revealing the most important droplet nucleation controlling parameters. I will discuss the bright, linear cloud features (shiptracks) that are perhaps the most distinct manifestation of aerosol-generated brightening of clouds. I will introduce aspects of shiptracks that have received less attention - such as the dynamical feedbacks that follow on from the microphysical perturbation, and which might influence their detectability. With recent talk of intervening to modify planetary albedo through seeding of shallow maritime clouds I will present results that show that such exercises would likely be undetectable because of the high degree of variability in local planetary albedo. Finally, I will show how we have applied Twomey's albedo susceptibility concepts to precipitation as a framework for identifying the conditions under which the aerosol may control the amount and probability of rain.