Large nuclei (aerosol/CCN) are found to be essential for initiating coalescence in polluted environments. Once coalescence has been initiated, the rate at which cloud liquid water is transferred to precipitation-size drops depends on the amount of drizzle which forms in the cloud. Drizzle production is retarded when accumulation mode aerosol concentration is high, but is promoted when coarse mode aerosol concentration is high. Ultra-giant CCN (diameter >10 micron) accelerate the onset of coalescence but suppress drizzle production, and result in a slower production of raindrops. Larger nuclei slow the rate of drizzle formation but condensate is converted to precipitation-size drops, and may fall out of the cloud before most of the available water has been harvested. In addition, recent dual polarization radar data have shown that the evolution of rain drop spectra could also change as a function of aerosols and microphysical processes which will have impacts on other dynamical processes in the clouds.
These results in turn determine the ice processes in clouds once they grow to temperatures colder than 0oC and affect droplet freezing and secondary ice processes, the overall precipitation formation and possibly efficiency and amount of precipitation that will reach the surface. These results have implications for both advertent and inadvertent weather modification.