title: Aerosol indirect effect from turbulence-induced broadening of cloud-droplet size distributions.
abstract: The influence of aerosol concentration on the cloud-droplet size distribution is investigated in a laboratory chamber that enables turbulent cloud formation through moist convection. The experiments allow steady-state microphysics to be achieved, with aerosol input balanced by cloud-droplet growth and fallout. As aerosol concentration is increased, the cloud-droplet mean diameter decreases, as expected, but the width of the size distribution also decreases sharply. The aerosol input allows for cloud generation in the limiting regimes of fast microphysics ([Formula: see text]) for high aerosol concentration, and slow microphysics ([Formula: see text]) for low aerosol concentration; here, [Formula: see text] is the phase-relaxation time and [Formula: see text] is the turbulence-correlation time. The increase in the width of the droplet size distribution for the low aerosol limit is consistent with larger variability of supersaturation due to the slow microphysical response. A stochastic differential equation for supersaturation predicts that the standard deviation of the squared droplet radius should increase linearly with a system time scale defined as [Formula: see text], and the measurements are in excellent agreement with this finding. The result underscores the importance of droplet size dispersion for aerosol indirect effects: increasing aerosol concentration changes the albedo and suppresses precipitation formation not only through reduction of the mean droplet diameter but also by narrowing of the droplet size distribution due to reduced supersaturation fluctuations. Supersaturation fluctuations in the low aerosol/slow microphysics limit are likely of leading importance for precipitation formation.
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: Kamal Kant
authors.initials: KK
authors.lastname: Chandrakar
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: Will
authors.initials: W
authors.lastname: Cantrell
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: Kelken
authors.initials: K
authors.lastname: Chang
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: David
authors.initials: D
authors.lastname: Ciochetto
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.|Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany.
authors.forename: Dennis
authors.initials: D
authors.lastname: Niedermeier
authors.affiliation: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Ovchinnikov
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931; rashaw@mtu.edu.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: Raymond A
authors.initials: RA
authors.lastname: Shaw
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, MI 49931.|Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931.
authors.forename: Fan
authors.initials: F
authors.lastname: Yang
keywords: aerosol indirect effect; cloud-droplet size distribution; cloud–turbulence interactions