Laboratory Cloud-Convection Chambers: What Have We Learned and What Are the Opportunities for the Future?
Michigan Technological University
Inspired by early convection-tank experiments (e.g., Deardorff and Willis) and diffusion-chamber experiments, Michigan Tech has developed a cloud chamber that operates on the principle of isobaric mixing within turbulent Rayleigh-Bénard convection. The “Pi cloud chamber” at Michigan Tech has been online for five years and has been used for studies of atmospheric aerosol and cloud processes. Highlights of what we have learned are: cloud microphysical and optical properties are representative of those observed in stratocumulus; aerosol number concentration plays a critical role in cloud droplet size dispersion, i.e., dispersion indirect effect; aerosol-cloud interactions can lead to a condition conducive to accelerated cloud collapse; realistic and persistent mixed-phase cloud conditions can be sustained; large-eddy simulations based on the pi chamber are able to capture the essential features of the turbulent convection and warm-phase cloud microphysical conditions.
It is worth considering what more could be learned with a larger-scale cloudy-convection chamber. Turbulence Reynolds numbers and Lagrangian-correlation times would be scaled up, therefore allowing more enhanced role of fluctuations in the condensation-growth process. Larger vertical extent (of order 10 m) would approach typical collision mean free paths, thereby allowing for direct observation of the transition from condensation-growth to coalescence-growth. This would be an opportunity for microphysical model validation, and for synergistic learning from model-measurement comparison under controlled experimental conditions.