Turbulence--Cloud Interactions: Laboratory and Field Studies

Emphasis area: Atmospheric Physics

PI: Raymond Shaw

Sponsor/Agency: National Science Foundation

Total Funding: $435,111

Period funded: 2005-2007

Details:

Atmospheric clouds are a crucial part of the Earth System, interacting with electromagnetic radiation emitted by the sun and the Earth, catalyzing chemical reactions in the atmosphere, and redistributing water and energy through the hydrologic cycle. The physical characteristics of clouds are determined in part by the rate at which the constituent cloud droplets collide and coalesce and it is thought that this process depends strongly on the spatial distribution of cloud particles and the interaction of cloud particles with their local turbulent environment. Unfortunately, there is a dearth of reliable data on the spatial distribution of particles in turbulence. This is troublesome because measurements are not available to compare with theoretical and computational work. We propose to address one aspect of the role of turbulence in cloud microphysical processes by quantifying droplet spatial correlations in turbulent flows using digital holography. Additionally, we will use a highframe-rate camera to record three-dimensional droplet tracks, thereby obtaining relative droplet velocities, and droplet accelerations. The work is an extension of our recent development of an airborne digital inline holographic system (Holodec), which was tested during the IDEAS-3 field project, and our previous laboratory and computational studies of particle-turbulence interactions. The second-generation holographic instrument will allow for investigations of scale-dependent droplet spacing in all three spatial dimensions, thereby providing a statistically robust clustering signature with modest numbers of droplets. The primary study will be carried out in a laboratory turbulence chamber seeded with cloud droplets, which allows us to study spatial correlations and droplet dynamics in a statistically stationary system with well-characterized turbulence and droplet properties. In the latter stages of the work we will deploy the system at a ground-based observatory where droplet clustering will be observed in natural clouds — by comparing these data to the laboratory data we can evaluate the possible influence of intermittency due to the difference in Turbulence Reynolds numbers. We expect that the measurements will provide a unique view of the interactions of droplets with turbulence due to droplet inertia, and that they will complement data sets from both the field and laboratory taken with other instruments (i.e., instruments that measure one dimensional particle spacing such as the Fast FSSP or phase Doppler interferometer). The main advantage of the laboratory setting is that it enables us to investigate specific mechanisms in an environment where physical parameters can be externally controlled.