Warm-Rain Initiation: An Overview of Microphysical Mechanisms

Abstract

Rain triggering mechanisms are evaluated in three microphysical steps: droplet activation on cloud condensation nuclei, droplet growth by condensation, and droplet growth by coalescence. Although considerable progress has been made since the pioneering work of Squires, crucial questions in each of the above steps remain unresolved: Under what conditions do giant particles trigger rain by acting as coalescence nuclei? What is the contribution of stochastic condensation to the growth of large droplets in regions of entrainment? What are the collection efficiencies for droplet sizes critical to the onset of colaescence growth? Such questions cannot be answered without better observations. Aircraft instruments are becoming available with the potential to measure the very largest particles and cloud droplets at the concentration of raindrops. Recent advances in sampling and analysis techniques have extended observations of cloud microstructure to smaller scales, providing new insight on the growth of droplets by mixing. Continued progress in laboratory research should furnish collection efficiencies for the droplets sizes critical to warm-rain initiation. With such improved observations, careful evaluations using available microphysical and dynamical models should provide answers to key questions about warm-rain initiation.