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- μ m 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 Statement This 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.