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In cold clouds, small crystals of size ~ 100 μm settling in a turbulent fluid need to aggregate through collisions to form large hail or graupel particles. Using fully resolved direct numerical simulations, the role of the two main physical processes responsible for collisions is clarified, namely, turbulence and the difference in settling velocity due to different crystals orientations.
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Jucha et al. (2018) studied this question.
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