This project results from the principal investigator's participation in the Rain in Cumulus over the Ocean (RICO) cloud field study in the Caribbean region in 2005. It is a continuation of analysis of RICO field data. The coalescence and collection of drops leading to precipitation in these clouds depends on several key physical processes including characteristics of cloud condensation nuclei (CCN) in the air; surface fluxes of heat, water vapor, particles and gases; vertical profiles of temperature, humidity, and winds; cloud geometry; and entrainment and mixing. The analysis to date has focused on the latter and its effect on the cloud droplet spectrum. Here observations from the particle volume monitor (PVM) cloud probe on the National Center for Atmospheric Research C-130 research aircraft were used to provide evidence for how this mixing occurs. It is the only microphysics probe used during RICO that has a sufficiently large sensitive volume to give useable sampling statistics for cloud scales as small as 10 cm. Key findings based on analysis of several dozen clouds sampled on RICO Research Flight 12 include probability density functions of sizes of droplet-free environmental air parcels entrained into the sampled clouds. These parcels were much smaller than hitherto suggested, and the observations indicate that inhomogeneous-type mixing process dominate following entrainment of outside air into these clouds.
The effort of this research will be to extend high resolution data analysis using the PVM and other C-130 data to other RICO flights which have less vertical wind shear and greater amounts of precipitation drops than found for Flight 12. This will help answer the question: are our results to date peculiar for conditions producing the clouds sampled on Flight 12, or are they also applicable to cumuli formed under different conditions? This same type of analysis also will be extended to observations with the same PVM instrument obtained in a 1995 study of Florida cumulus clouds. These Florida clouds were much more robust than the typical RICO trade-wind cumuli and allow another test of the degree to which RICO results can be generalized. In addition the principal investigator will continue collaboration with researchers at the University of Utah and their efforts with a detailed numerical model of entrainment and mixing that extends the interaction of explicit microphysics and turbulent dissipation to the Kolmogorov scale. Here the PVM high speed data can serve as a constraint and also as an input. Further collaboration is also planned with researchers at the Desert Research Institute and their RICO measurements of CCN for the purpose of estimating supersaturations in the Flight 12 cumuli. And finally, the principal investigator collaborate with several RICO modelers performing a numerical large-eddy simulation of clouds developing in conditions encountered on Flight 12, based on his detailed analysis of clouds observed on that flight.
The intellectual merit of the work is its contribution towards resolving the critical issue of how mixing occurs following entrainment of outside air into clouds. This has been debated for many years without reaching a resolution. The most recent literature dealing with this issue emphasizes the importance of understanding this process given its strong effect on the droplet spectra, coalescence, and cloud optics. The work takes advantage of an excellent and unique data set from the RICO field study to clarify this issue. The broader impact of the work is its contribution to more quantitative understanding of droplet spectral evolution and the formation of precipitation in cumulus clouds. This understanding needs to be improved and used to improve cloud models so that they can predict the role of clouds in our climate system more accurately. Graduate student research at the University of Utah and Nevada's Desert Research Institute will also be strengthened through collaboration with the principal investigator.
