Two different physical mechanisms account for the formation of precipitation. In clouds cold enough to contain a mixture of ice crystals and supercooled water droplets, the crystals grow by the diffusion of water vapor to sizes large enough to coagulate with one another to form snowflakes or to accrete cloud droplets and form graupel, snow pellets, or denser forms of ice precipitation. All of these forms of ice-phase precipitation may melt when falling through warmer air and reach the ground as rain. In so-called warm clouds, which are not cold enough for ice to form, precipitation can be created by an all-water process. Cloud droplets grow initially by condensation - though at a rate slower than ice crystals, because the cloud environment is only slightly supersaturated relative to water though it may be strongly supersaturated relative to ice. Once some of the droplets reach a radius of about 20 micrometer, they begin to collide with smaller droplets and grow by coalescence. Either the collision-coalescence process or the ice-crystal process, depending on cloud temperature, can explain the development of precipitation. However, an uncertain link in the collision-coalescence process is the slowness of drop growth by condensation and the consequent long time for drops to reach a size where collisions become important.
The goal of this continuing research project is to develop a physically consistent understanding of the development and evolution of rain in warm clouds. The approach is based on (1) analysis of data from radars, instrumented aircraft, and surface observations, and (2) simulations with a detailed cloud microphysical model to examine physical processes that cannot be inferred from observations alone. Key unresolved questions are: (1) How do giant nuclei (hygroscopic particles larger than 1 micrometer) affect the rate of precipitation onset in different kinds of clouds? (2) What are the microphysical mechanisms that account for the observed radar characteristics of growing cumulus clouds? (3) How do these clouds modify the environment for subsequent cloud development? (4) How do environmental parameters such as wind shear influence the rate of precipitation production? (5) Do giant nuclei affect the total precipitation falling from clouds of different kinds? Answers to these questions are critical for understanding precipitation development and have significance for questions related to artificial cloud modification, global precipitation measurements, and climate studies.
