Influence of Turbulent Fluctuations on Cloud Droplet Size Dispersion and Aerosol Indirect Effects

Aerosols influence macroscopic cloud properties by altering microphysical processes. For example, the number concentration and size distribution of aerosol particles affect the corresponding cloud droplet properties, which in turn affect properties like cloud reflectivity and rate of precipitation formation. Taken together, these factors are known as the first and second indirect effects [Twomey, 1977, Albrecht, 1989].

Previous interpretations of precipitation suppression by aerosols have been based primarily on a reduction in the mean droplet size. However, recent studies show clear evidence of a change in relative dispersion with an increase in aerosol loading [Martin et al., 1994, Liu and Daum, 2002, Feingold and Siebert, 2009, Lu et al., 2012]. Since the droplet collision rate is also a function of relative dispersion, the aerosol effect on relative dispersion will contribute the cloud lifetime, or second indirect, effect.

Beyond the influence of size dispersion on the precipitation rate, it also directly influences cloud optical properties [Pontikis and Hicks, 1992, Liu and Daum, 2002, Feingold and Siebert, 2009]. It is expected that an increase in the relative dispersion has a warming effect, opposite to the cooling effect induced by an increase in the cloud droplet number [Liu and Daum, 2002, Feingold and Siebert, 2009]. The actual effect of aerosol loading on cloud droplet relative dispersion are contradictory, however; some studies suggest an increase in the relative dispersion with a decrease in the aerosol loading (e.g. [Lu et al., 2007, 2012]) while other observations show the opposite trend (e.g. [Martin et al., 1994, Liu and Daum, 2002, Pawlowska et al., 2006]).

In recent work it was found that cloud droplet size distributions become broader as a result of supersaturation variability, and that the sensitivity of this effect is inversely related to cloud droplet number density [Chandrakar et al., 2016]. The subject is investigated in further detail using an extensive dataset from a laboratory cloud chamber capable of producing steady-state turbulence. Stochastic theory is found to successfully describe properties of the droplet size distribution, including an analytical expression for the relative dispersion. The latter 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. Experiments spanning fast to slow microphysics regimes are reported. The observed dispersion is used to estimate time scales for autoconversion, demonstrating the important role of the turbulence-induced broadening effect on precipitation development. Moreover, the effects that changes in dispersion with aerosol loading have on effective radius (and its parameterization) are also explored.

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