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

Here, the influence of aerosol concentration on 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 (τ c < τ t) for high aerosol concentration, and slow microphysics (τ c > τ t) for low aerosol concentration; here, τ c is the phase relaxation time and τ t 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 τ s -1c -1 + τ t -1, and the measurements are in excellent agreement with this finding. This finding underscores the importance of droplet size dispersionmore » for the aerosol indirect effect: increasing aerosol concentration not only suppresses precipitation formation through reduction of the mean droplet diameter, but perhaps more importantly, through 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.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [1] ;  [1]
  1. Michigan Technological Univ., Houghton, MI (United States)
  2. Michigan Technological Univ., Houghton, MI (United States); Leibniz Institute for Tropospheric Research, Leipzig (Germany)
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Publication Date:
Report Number(s):
PNNL-SA-119793
Journal ID: ISSN 0027-8424; KP1701000
Grant/Contract Number:
AC05-76RL01830; AC06-76RL01830
Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 113; Journal Issue: 50; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Research Org:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; aerosol indirect effect; cloud-droplet size distribution; cloud–turbulence interactions
OSTI Identifier:
1333749
Alternate Identifier(s):
OSTI ID: 1340762