Oct 25, 2019
Whitaker Hall, room 100
Dr. Raymond Shaw, Professor
Department of Physics
Michigan Technical University
Whitaker Hall, room 100
How turbulence influences cloud optical properties and precipitation: Laboratory studies of turbulent moist Rayleigh-Benard convection
Aerosol particles, such as sea salt, dust and anthropogenic pollution, influence the optical properties of clouds and the tendency of a cloud to form precipitation through droplet collisions. We have investigated cloud droplet growth in a turbulent environment under varying levels of aerosol concentration. The results reveal a surprising role of turbulence in cloud droplet growth that leads to two regimes: a polluted cloud regime in which thermodynamic conditions are rather uniform and cloud droplet sizes are similar, and a clean cloud regime in which thermodynamic conditions are highly variable and cloud droplet sizes are very diverse. The narrowing of droplet size range under polluted conditions introduces a new stabilizing factor by which increased aerosol concentration can suppress precipitation and enhance cloud brightness.
Cloud droplet growth in a turbulent environment is studied by creating turbulent moist Rayleigh-Benard convection in a laboratory chamber (the Pi Chamber). Cloud formation is achieved by injecting aerosols into the water-supersaturated environment created by the isobaric mixing of saturated air at different temperatures. In steady state, the injection and activation of aerosol particles to form cloud droplets is balanced by cloud droplet growth through vapor condensation and loss by gravitational settling. A range of steady-state cloud droplet number concentrations is achieved by supplying aerosols at different rates. As steady-state droplet number concentration is decreased the mean droplet size increases as expected, but also the width of the size distribution increases. This increase in the width is associated with larger supersaturation fluctuations due to the slow droplet microphysical response (sink of the water vapor) compared to the fast turbulent mixing (source of the water vapor). The boundary between the two regimes can be identified with a cloud Damkoehler number of order unity.
Raymond Shaw is Distinguished Professor of Physics and co-founder and director of the interdepartmental doctoral program in Atmospheric Sciences at Michigan Technological University. He is an atmospheric physicist with interests in clouds, nucleation, and turbulence. Current research includes laboratory studies of ice nucleation, and field studies of clouds with an airborne digital holographic cloud imager (HOLODEC). He led an NSF MRI project to develop the "Pi Cloud Chamber" facility at Michigan Tech, and has been actively involved with Pi Chamber science activities (see http://phy.sites.mtu.edu/cloudchamber/). He has published approximately 100 papers in peer reviewed journals in the fields of atmospheric sciences, fluid dynamics, optics, and mainline physics. He received his BS from Brigham Young University in 1993 and PhD from Penn State University in 1998, and was a postdoctoral fellow in the Advanced Study Program at the National Center for Atmospheric Research. Since then he has been on the faculty at Michigan Tech. His honors include: NASA New Investigator Program Award, NSF CAREER Award, Henry G. Houghton Award of the American Meteorological Society, and a Humboldt Research Fellowship at the Institute for Tropospheric Research (TROPOS) in Leipzig, Germany. He has been a visiting professor at Cornell University and Peking University, and has been a visiting scientist at TROPOS and the Max Planck Institute for Dynamics and Self-Organization. He has been a member of the editorial board of New Journal of Physics (2008-2016); an elected member of the International Commission on Clouds and Precipitation (2008-2016); a member of the AMS Cloud Physics Committee (2000-2006, 2015-2019); a member of the Scientific Advisory Board of TROPOS (2013-2021); and currently serves as an elected member of the Executive Committee of the APS Topical Group on Physics of Climate (2015-2017, 2019-2021).
Organizer / Host: Dr. Wang