Aerosol indirect effects are one of the leading contributors to cloud radiative properties relevant to climate. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value, which depends on the particle size and chemical composition. In the traditional formulation of this problem, only average, uniform saturation ratios are considered. Using experiments and theory, we examine the effects of fluctuations, produced by turbulence. Our measurements, from a multiphase, turbulent cloud chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The laboratory measurements demonstrate cloud formation in mean-subsaturated conditions (i.e., relative humidity <100%) in the fluctuation-dominant activation regime. The theoretical framework developed to interpret these measurements predicts a transition from a mean- to a fluctuation-dominated regime, based on the relative values of the mean and standard deviation of the environmental saturation ratio and the critical saturation ratio at which aerosol particles activate or become droplets. The theory is similar to the concept of stochastic condensation and can be used in the context of the atmosphere to explore the conditions under which droplet activation is driven by fluctuations as opposed to mean supersaturation. It provides a basis for future development of cloud droplet activation parameterizations that go beyond the internally homogeneous parcel calculations that have been used in the past.

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Aerosol particles, cloud droplets, and ice crystals, coupled through the supersaturation field, play an important role in the buoyancy and life cycle of convective clouds. This letter reports laboratory observations of copious cloud droplets and ice crystals formed in the wake of a warm, falling water drop, which is a laboratory surrogate for a relatively warm hydrometeor in atmospheric clouds, such as a graupel particle in the wet growth regime. Aerosols were activated in the regions of very high supersaturation due to mixing in the wake. A mechanism is explored for attaining very high supersaturations capable of activating significant fractions of the interstitial aerosols within the lifetime of a convective cloud. The latent heat released from the activation of interstitial aerosols and subsequent growth may provide an additional source of buoyancy for cloud invigoration and may lead to larger concentrations of ice crystals. Plain Language SummaryHail or other large icy hydrometeors like graupel fall through clouds, leaving regions of disturbed turbulent air in their wake. Because graupel particles are very likely to be warm or cold, relative to their surroundings, numerous new cloud droplets and even ice crystals can form in the disturbed air. The heat associated with the condensation of water vapor onto these newly formed droplets or crystals could provide a significant boost to the cloud's buoyancy. Calculations suggest that this mechanism could expose regions of convective clouds to high supersaturations in tens of minutes, providing a source of invigoration and higher concentrations of ice crystals.

The heat transfer performance of R-11 vapor condensing on single horizontal trapezodially shaped integral-fin tubes has been investigated by systematically varying the fin density, the semi-vertex angle, and the fin height. For the nine copper tubes tested, the best performance has been obtained with a tube having a fin density of 1417 fpm, a semi-vertex angle of 10 deg, and a fin height of 1.22 mm. This tube has yielded a maximum value of the heat transfer coefficient of 16,500 W/m2 K at a ΔT of about 3 K, corresponding to an enhancement ratio of 10.3. The performance of the tube has been further improved by fabricating from it “specially enhanced” tubes having axial grooves of varying height. An enhancement ratio of 12.3 has been obtained with this type of tube.