Collaborative Research: Investigating the role of non-continuum hydrodynamic interactions in the growth of droplets in warm cumulus clouds

Warm, rain-bearing clouds play a vital role in the global water and energy balances that help regulate Earth's weather system. Predicting the effects of cumulus clouds (low level clouds with a puffy cotton-like appearance) on weather requires precise quantification of water droplet sizes and their growth rates. Current cloud models are limited by an inability to predict droplet growth rates and measured droplet size distributions (DSDs). This award will combine an accurate numerical model of cloud dynamics with theoretical models for predicting the growth of water droplets into rain drops in tropical maritime clouds. The project will train undergraduate and graduate students in interdisciplinary research. Outreach activities to high school and undergraduate students will be conducted. This award is uniquely positioned to advance cloud physics research on Earth and other planets. Elucidating the physics of droplet growth in the 15–40 micrometer “size-gap” range is essential for accurately predicting the DSDs of droplets and the formation of rain-size drops in warm clouds. Drops grow by processes spanning a wide range of scales from the turbulence-induced fluctuations in cloud thermodynamic properties to the continuum and non-continuum hydrodynamic interactions between droplets. Capturing this physics requires the combination of state-of-the-art large-eddy simulations (LESs) and precise theoretical models for the droplet-scale hydrodynamics including continuum breakdown on close approach. An integrated LES-theory cloud model will be developed to achieve transformative insights into the interactions between large-scale processes such as turbulent fluctuations in supersaturation and temperature and small-scale processes including non-continuum hydrodynamic, van der Waals and electrostatic forces. Varying macroscopic conditions such as the cloud height, temperature and pressure at the cloud base, cloud-boundary fluxes of heat and water vapor, and nuclei concentration while monitoring the resulting changes in turbulent dissipation rate, supersaturation fluctuations, and drop collision rates will reveal the origins of the DSD evolution. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2535828 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Donald Koch | Institution: Cornell University, ITHACA, NY | Award Amount: $340,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2535828 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2535828.html