Aerosol indirect effect from turbulence-induced broadening of cloud-droplet size distributions

Kamal Kant Chandrakar; Will Cantrell; Kelken Chang; David Ciochetto; Dennis Niedermeier; Mikhail Ovchinnikov; Raymond A. Shaw; Fan Yang

doi:10.1073/pnas.1612686113

Edited by John H. Seinfeld, California Institute of Technology, Pasadena, CA, and approved October 14, 2016 (received for review August 1, 2016)

Significance

Atmospheric aerosol concentration is linked to cloud brightness and lifetime through the modulation of precipitation. Generally speaking, larger cloud droplets and wider-droplet size distributions form precipitation more efficiently. We create steady-state clouds in the laboratory through a balance of constant aerosol injection and cloud-droplet removal due to settling. As aerosol concentration is decreased, the cloud-droplet mean diameter increases, as expected, but also the width of the size distribution increases sharply. Theory, simulations, and measurements point to greater supersaturation variability as the cause of this broadening in what can be considered a low aerosol/slow microphysics limit.

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 (τc<τt) for high aerosol concentration, and slow microphysics (τc>τt) for low aerosol concentration; here, τc is the phase-relaxation time and τt 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 τs−1=τc−1+τt−1, 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.

Footnotes

↵¹To whom correspondence should be addressed. Email: rashaw{at}mtu.edu.

Author contributions: K.K.C., W.C., K.C., D.C., D.N., and R.A.S. designed research; K.K.C., K.C., D.C., D.N., M.O., and F.Y. performed research; M.O. and F.Y. performed simulations; K.K.C., W.C., R.A.S., and F.Y. analyzed data; and K.K.C. and R.A.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1612686113/-/DCSupplemental.

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