Date of Award

2019

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Atmospheric Sciences (PhD)

Administrative Home Department

Department of Physics

Advisor 1

Raymond A. Shaw

Committee Member 1

Will H. Cantrell

Committee Member 2

Alex B. Kostinski

Committee Member 3

Scott Wunsch

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. In chapter 2, moist Rayleigh-Bénard convection with water saturated boundaries is explored using a one-dimensional-turbulence model. This study provides some background about supersaturation statistics in moist convection. Chapters 3 - 7 discuss the experimental and theoretical investigation of aerosol-cloud interactions and cloud droplet size-distributions in turbulent conditions. The experiments are performed in a way so that steady-state microphysics are 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 is also observed to decrease 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 the larger variability of supersaturation due to the slow microphysical response. A stochastic theory developed based on the Langevin 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. These experiments are discussed in chapters 3 and 4.

This effect of varying cloud dropletsize-distributionwidth underscores the importance of droplet size dispersion for aerosol indirect effects. An application of this coupling of aerosol and supersaturation fluctuations during the 'cloud-cleansing' process is discussed in chapter 5. Cloud droplet relative dispersion, defined as the standard deviation over the mean cloud droplet size (d =σr / < r >), is of central importance in determining and understanding aerosol indirect effects. The analytical expression of d obtained from the stochastic theory is found to depend on the cloud droplet removal time, which in turn increases with the cloud droplet number density. The results show that relative dispersion decreases monotonically with increasing droplet number density, consistent with some recent atmospheric observations. The albedo susceptibility due to turbulence broadening has the same sign as the Twomey effect and augments it by order 10%. These results, along with the test of a commonly-used effective radius parameterization, are presented in chapter 6.

In chapter 7, theoretical expressions for cloud droplet size-distribution shape are evaluated using measurements from controlled experiments in the Pi-Chamber. Three theoretical distributions obtained from a Langevin drift-diffusion approach to stochastic condensation are tested. Statistical techniques of χ2 test, sum of squared errors of prediction, and residual analysis are employed to judge relative success or failure of the theoretical distributions to describe the experimental data. In relative comparison, the most favorable comparison to the measurements is the expression for stochastic condensation with size-dependent droplet removal rate.

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