This project will use a new instrument to measure the size distribution and concentration of cloud droplets in trade-wind Cumulus clouds over the ocean. The instrument, based on the technique of phase-Doppler interferometry, will provide better size resolution and more stable calibration than instruments normally used on research aircraft. The project is part of the "Rain in Cumulus over the Ocean" (RICO) field experiment, which will collect measurements during December 2004 and January 2005. One important objective of RICO is to understand the "warm rain" process, in which rain is formed without involvement of the ice phase. Accurate measurement of the droplet size distribution is important if this objective is to be realized, so the measurements from this new instrument promise to provide key data for RICO. Specific topics to be investigated include the importance of "ultra-giant" (>50 micrometer) aerosol particles in the formation of rain, the influence on the cloud droplet size distribution of entrainment of environmental air into the cloud, and the possible enhancement of collision rates among droplets caused by turbulence. The measurements should also be widely used by other RICO investigators in their own studies of related aspects of warm-rain formation. An important potential impact of this research will be the provision of a needed measurement technique for future studies in cloud physics. Other broad impacts include graduate student training in this area of research, planned inclusion of under-represented minorities and undergraduates in the related educational activities, and contributions to a graduate-student seminar series that will be part of RICO.
