A basic underlying assumption for most of our treatment of the physics of clouds is that drops are distributed in space in a perfectly random manner at sub-grid scales. It has been recognized for some time now that this assumption is clearly violated at cloud edges where entrainment takes place, but the impact of turbulence on drop spatial distributions in cloud interiors, and hence on our understanding of cloud physical processes, has only recently come under intensive scrutiny. The suggestion from theoretical and modeling studies that turbulence should "cluster" drops has proven difficult to confirm from in-cloud observations, mostly because of sampling problems with in situ devices. Wind tunnel experiments performed by the principle investigator seem to provide some support for clustering in that environment. This grant supports continued laboratory studies to investigate the impact of turbulence on drop spatial distributions in well-characterized conditions, with particular emphasis on how this applies to the collision-coalescence process. The principle investigator will use high spatial resolution, high digital resolution, and short exposure Charge Coupled Device (CCD) cameras to make measurements of the three-dimensional spatial distribution of drops using inline holography in a laboratory turbulence chamber. The turbulence chamber will be larger such that the turbulence Reynolds number inside will be closer to and the energy dissipation rates similar to those found in the atmosphere. The flow inside the chamber will be characterized with particle image velocimetry. The spatial distribution of drops, together with the turbulence characterization, allows for investigations of the dependence of clustering on drop size and turbulent intensity. A further addition of a high speed digital camera, loaned from the Max Planck Institute for Dynamics and Self Organization in Germany, will allow measurement of the drop relative velocity distribution, thus closing the measurements needed to evaluate the generalized collection kernel. The laboratory results will be evaluated, and extended to high turbulence Reynolds number regimes, in natural clouds using the same inline holographic system.
Broader Impacts: This work has broad potential impact as it spans the fields of atmospheric sciences and engineering multiphase flows. The principle investigator has strong collaborative ties with people in the engineering multiphase flow community, and thus benefits from and acts as a conduit to the atmospheric sciences community of the greater engineering expertise in multiphase flows. This experimental work is a critical contribution to a field where theoretical and modeling work has outpaced our observational abilities. The principle investigator has a well-established track record working with undergraduate students, which he hopes to continue with this grant, and will also support one graduate and one post-doctoral student.
