An Approximation for the Shapes of Large Raindrops

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  • 1 Department of Atmospheric and Oceanic Science, University of Michigan, Ann Arbor 48104
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Abstract

A simple model is proposed for approximating the shapes of raindrops in terminal fall equilibrium. The equilibrium figures of raindrops are complex, but they can be represented approximately as oblate spheroids, which have axis ratios determined by the balance of surface and gravity forces at the equator. The axis ratios and drop diameters predicted by the model correlate well with experimental and other theoretical results. Using empirical formulas for evaporation from ellipsoids and the results of the model, estimates of ventilation coefficients are made. It is found that drop deformation may cause a small diminution of ventilation coefficient compared with evaporation from an equivalent sphere.

Abstract

A simple model is proposed for approximating the shapes of raindrops in terminal fall equilibrium. The equilibrium figures of raindrops are complex, but they can be represented approximately as oblate spheroids, which have axis ratios determined by the balance of surface and gravity forces at the equator. The axis ratios and drop diameters predicted by the model correlate well with experimental and other theoretical results. Using empirical formulas for evaporation from ellipsoids and the results of the model, estimates of ventilation coefficients are made. It is found that drop deformation may cause a small diminution of ventilation coefficient compared with evaporation from an equivalent sphere.

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