Several field projects over the past two decades have measured the microphysical character of warm Cumulus clouds. This talk seeks to address two puzzling aspects of those observations (and many more extending over 50 y): (1) Why are droplet size distributions so broad; and (2) How can rain form via warm processes in less than 20 minutes, as indicated by many of the observations? The central premise developed in this talk is that entrainment and associated mixing is part of the answer to both questions. Turbulent diffusion of cloud droplets causes droplets having different growth histories to contribute to measured size distributions, leading to realistic broadening. Such mixing can also lead to the formation of larger cloud droplets than would be present without entrainment, and the subsequent growth of those largest droplets contributes to the early formation of rain via coalescence. A model framework that combines cloud dynamics and detailed cloud microphysics is used to investigate this premise quantitatively. Other effects are also important, including the presence of giant nuclei and turbulent enhancement of the collection process. The calculations with all these effect included readily produce realistic rainfall and provide a framework for evaluating many other influences on the warm-rain process, including the importance of cloud lifetime, sensitivity to collection efficiencies, variations with aerosol populations, the importance of entrained CCN, and the small-scale nature of the mixing processes.
Thursday, April 21, 2011, 3:30PM
Refreshments 3:15 PM
3450 Mitchell Lane
Bldg 2 Auditorium (Rm1022)