Turbulence-Aerosol-Cloud Interactions: Implications for Drizzle Initiation in the Presence of Ordinary and Giant Cloud Condensation Nuclei

ROBERT MCGRAW, Yangang Liu, Virendra Ghate, Brookhaven National Laboratory

     Abstract Number: 143
     Working Group: Aerosols, Clouds and Climate

Abstract
Nucleation theory forms the foundation for understanding drizzle formation in warm clouds in the kinetic potential (KP) model. The model inputs cloud droplet number concentration, liquid water fraction, and a turbulent diffusion coefficient, from which emerge a critical droplet size, defined by the condition that the rate of droplet growth by condensation and collection is balanced by evaporation, a barrier maximum at the critical size, and a steady-state barrier crossing rate identified with the drizzle formation rate. For the present study, the Köhler activation of water-soluble aerosols is included in the kinetic potential to form an integrated description of the effects of ordinary cloud condensation nuclei (CCN), particles having dry radii < 0.5 micron, and giant CCN (GCCN) on drizzling and non-drizzling clouds. Water-soluble GCCN in the 0.5 to 3 micron radius range are found to significantly reduce the KP drizzle barrier. Near the high end of this range (circa 2 micron for marine clouds) the barrier vanishes and drizzle occurs spontaneously. The combined model yields predictions for drizzle formation and its seasonal dependence consistent with observations. The new, broader framework provides theoretical basis for understanding how turbulence-aerosol-cloud interactions impact drizzle initiation in natural clouds and effectiveness of cloud seeding.