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  • Author or Editor: H. G. Leighton x
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H. G. Leighton

Abstract

A comparison is made between the evolution of initially exponential snowflake size distributions growing by deposition and aggregation as calculated with a numerical model and with the approximate analytical model of Passarelli. The agreement between the two models implies that to a good approximation the assumptions of the analytical model are consistent with the additional assumption that the distribution maintains an exponential shape.

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H. G. Leighton

Abstract

The variation in the evolution of a cloud droplet spectrum with collision efficiency is examined for sets of efficiencies based on different fluid mechanical models. The results indicate that the variation of collision efficiencies for droplet radii ≲30 µm are large enough to give appreciable variation in the droplet spectrum development. This is in contrast to the relative insensitivity to variations in the collision efficiencies of larger drops.

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H. G. Leighton
and
R. R. Rogers

Abstract

The growth of cloud droplets by condensation and coalescence in a strong updraft is investigated for different cloud conditions and different initial droplet distributions. Growth by coalescence is determined by solving the stochastic collection equation, and growth by condensation is calculated from the diffusion equation.

The results indicate that under suitable conditions, starting from an initial droplet distribution centered at about 8 µm radius with a dispersion of 0.2, an appreciable number of cloud droplets will grow to precipitation sizes in times of the order of 10–15 min. The results are sensitive to the initial droplet concentration and moisture content of the cloud. The importance of combining coalescence and condensation raises the question of the validity of calculations of rain formation by coalescence only.

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A. S. Koziol
and
H. G. Leighton

Abstract

The significance of the influence of turbulence on collisions and coalescence of small cloud droplets is still an outstanding problem. In particular, the growth of droplets in the radius range 10 to 15 µm is not well understood. The present research examines whether or not turbulence affects the growth rate of such small drops by simulating trajectories of two hydrodynamically interacting droplets in a turbulent field. The trajectories were calculated with a model based on linear Stokes hydrodynamics. Turbulence was modeled in the form of random Fourier modes with both the space and time spectra prescribed. Both spectra were characterized by Kolmogorov scaling. The space spectrum was modeled in the inertial and dissipation subranges. On the basis of scale analysis, only small-scale time variations were allowed, and the so-called Eulerian–Lagrangian time spectrum was applied. The results show that most collision rates increase moderately in a turbulent flow characterized by rates of energy dissipation of the order of 1, 10, and 100 cm2 s−3.

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