Advanced modeling and observational techniques are used to observe the evolution of cloud droplet sizes in cumulus clouds. Observations are collected by flying a research aircraft into cumulus clouds, with instrumentation attached to the wings to measure cloud droplet sizes. A holographic instrument (HOLODEC) is used, which samples thousands of droplets in one instant to provide fine-scale spatial resolution. High resolution Large Eddy Simulation (LES) with advanced representation of cloud droplets is used to simulate a cumulus congestus cloud in great detail.

We find that although there are fewer droplets and smaller liquid water content in regions where dry air has been mixed into the cloud, the droplets that are present trend towards larger diameters than those in the undiluted part of the cloud. We use theoretical equations for cloud droplet growth rates to explain this observation. These findings contribute to a more complete understanding of how clouds develop, and may be relevant to the development of drizzle and precipitation.