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

Abstract

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^-1 =τ_c^-1 + τ_t^-1, and the measurements are in excellent agreement with this finding. This finding underscores the importance of droplet size dispersion for the aerosol indirect effect: increasing aerosol concentration not only suppresses precipitationmore » 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:

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

Publication Date:

Mon Nov 28 00:00:00 EST 2016

Research Org.:

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1333749

Alternate Identifier(s):

OSTI ID: 1340762

Report Number(s):

PNNL-SA-119793
Journal ID: ISSN 0027-8424

Grant/Contract Number:  

AC06-76RL01830; AC05-76RL01830

Resource Type:

Published Article

Journal Name:

Proceedings of the National Academy of Sciences of the United States of America

Additional Journal Information:

Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 113 Journal Issue: 50; Journal ID: ISSN 0027-8424

Publisher:

National Academy of Sciences, Washington, DC (United States)

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; aerosol indirect effect; cloud-droplet size distribution; cloud–turbulence interactions