This project will use models of clouds to study how mixing affects the size distribution of cloud droplets in cumulus clouds. It is part of the broader "Rain in Cumulus over the Ocean" (RICO) experiment, a collaborative experiment involving more than 15 institutions that plans to observe properties of trade-wind cumulus clouds in December 2004 and January 2005 and then analyze those measurements for several years. A central goal of RICO is to understand the processes leading to precipitation in tropical cumulus clouds, and an important question to be addressed is how cloud droplets collide and grow to raindrop size. One candidate explanation is that mixing increases the range of droplet sizes present in the cloud and so increases the rate of collisions among droplets of different size. This project will investigate that possibility and others through use of a modeling technique that embeds a one-dimensional representation of the fine-scale structure within a three-dimensional model of larger-scale cloud structure. In this way, the investigators obtain realistic trajectories and cloud fields while also simulating the fine-scale structure produced by turbulence. With this model, the investigators will evaluate the relative importance of several potential contributors to the broadening of droplet size distributions. As part of this work, they will also evaluate and improve the large-eddy simulation (LES) model that produces the three-dimensional cloud fields.
The expected long-range impact of RICO and of this project is that the effects of trade-wind cumulus clouds and of rain from warm clouds will be represented more accurately in weather and climate models, leading to improved predictions of rainfall. The research will also result in an assessment of the relative importance of various processes that cause cloud droplet size distributions to broaden, and it will lead to new techniques for representing fine-scale turbulent fields in models of cumulus clouds. The project will also contribute to graduate education through participation by the investigators in the RICO Graduate Student Seminar Series and through training of graduate students as they participate in the research.
