Analytic formulation and parametrization of the kinetic potential theory for drizzle formation.

The kinetic potential of nucleation theory is extended to describe cloud droplet growth processes that can lead to drizzle formation. In this model drizzle formation is identified as a statistical barrier crossing phenomenon that transforms cloud droplets to much larger drizzle size with a rate dependent on turbulent diffusion, droplet collection efficiency, and properties of the underlying cloud droplet size distribution. Closed-form expressions for the kinetic potential, critical drop volume, barrier height, and both steady-state and transient barrier crossing drizzle rates are obtained in terms of measurable cloud properties. In an analogy with the theory of phase transformation, clouds are classified into two regimes: an activated metastable regime, in which there is a significant barrier and drizzle initiation resembles nucleation, and an unstable regime where kinetics dominates analogous to the spinodal regime of phase transformation. Observational evidence, including the threshold behavior of drizzle formation and the well-known effect that aerosols have on drizzle suppression, is shown to favor drizzle formation under activated conditions (more similar to nucleation than spinodal decomposition) and under transient conditions rather than steady state. These new applications of the kinetic potential theory should lead to more accurate parametrizations of aerosol-cloud interaction and improved algorithms for weather forecasting and climate prediction.