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Abstract The role played by fluctuations of supersaturation in the growth of cloud droplets is examined in this study. The stochastic condensation framework and the three regimes of activation of cloud droplets— namely, mean dominant, fluctuation influenced, and fluctuation dominant—are used for analyzing the data from high-resolution large-eddy simulations of the Pi convection-cloud chamber. Based on a detailed budget analysis the significance of all the terms in the evolution of the droplet size distribution equation is evaluated in all three regimes. The analysis indicates that the mean-growth rate is a dominant process in shaping the droplet size distribution in all three regimes. Turbulence introduces two sources of stochasticity, turbulent transport and particle lifetime, and supersaturation fluctuations. The transport of cloud droplets plays an important role in all three regimes, whereas the direct effect of supersaturation fluctuations is primarily related to the activation and growth of the small droplets in the fluctuation-influenced and fluctuation-dominant regimes. We compare our results against the previous studies (experimental and theory) of the Pi chamber, and discuss the limitations of the existing models based on the stochastic condensation framework. Furthermore, we extend the discussion of our results to atmospheric clouds, and in particular focus on recent adiabatic turbulent cloud parcel simulations based on the stochastic condensation framework, and emphasize the importance of entrainment/mixing and turbulent transport in shaping the droplet size distribution.
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The Effect of Turbulence, Gravity, and Noncontinuum Hydrodynamic Interactions on the Drop Size Distribution in Clouds
Abstract The evolution of micrometer-sized droplets in clouds is studied with focus on the “size-gap” regime of 15–40-μm radii, where condensation and differential sedimentation are least effective in promoting growth. This bottleneck leads to inaccurate growth models, and turbulence can potentially rectify disagreement with in situ cloud measurements. The role of turbulent collisions, mixing of droplets, and water vapor fluctuations in crossing the size gap has been analyzed in detail. Collisions driven by the coupled effects of turbulent shear and differential sedimentation are shown to grow drizzle sized droplets. Growth is also promoted by turbulence-induced water vapor fluctuations, which maintain polydispersity during the initial-condensation-driven growth and facilitate subsequent growth by differential-sedimentation-driven coalescence. The collision rate of droplets is strongly influenced by noncontinuum hydrodynamics, and so the size evolution beyond the condensation regime is found to be very sensitive to the mean-free path of air. Turbulence-induced inertial clustering leads to a moderate enhancement in the growth rate, but the intermittency of the turbulent shear rate does not change the coalescence rate significantly. The coupled influence of all these phenomena is evaluated by evolving a large number of droplets within an adiabatically rising parcel of air using a Monte Carlo scheme that captures turbulent intermittency and mixing. Significance StatementThis study is directed toward improving descriptions of the microphysical determinants of the time for rain formation in clouds. Existing models predict significantly longer times than the tens of minutes observed in warm clouds. There is a growing body of evidence that turbulence plays a key role in resolving this discrepancy. We incorporate accurate turbulent collision dynamics and assess the interplay of the various underlying physical factors facilitating growth to rain-sized droplets. Our study, in addition to providing important insight into cloud microphysics, will pave the path to the next generation of large-scale rain cloud evolution studies.
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- Award ID(s):
- 2206851
- PAR ID:
- 10621754
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 82
- Issue:
- 8
- ISSN:
- 0022-4928
- Format(s):
- Medium: X Size: p. 1647-1659
- Size(s):
- p. 1647-1659
- Sponsoring Org:
- National Science Foundation
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