Collision Rate of Small Graupel and Water Drops

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Abstract
An approach permitting one to calculate the collision efficiency and the collision kernel of spherical particles of different densities for Reynolds numbers up to 100 (300-μm-radius drops, or 700-μm-radius graupel) is presented. It is used for the calculation of graupel–drop collision efficiencies and collision kernels in calm air for low-, medium-, and high-density graupel at 750- and 500-mb pressure levels. Low-density graupel interacts with water droplets in a way similar to ice crystals: there exists a cutoff size, below which graupel cannot collect water droplets. The authors have shown that the cutoff size decreases with the growth of graupel density, so that medium- and high-density graupel is able to collect droplets with the radii exceeding a certain minimum size. The graupel–drop collision efficiency increases with the drop size up to a maximum value and then sharply decreases to zero, when the drops' terminal velocity approaches the terminal velocity of graupel. As soon as the terminal... Abstract An approach permitting one to calculate the collision efficiency and the collision kernel of spherical particles of different densities for Reynolds numbers up to 100 (300-μm-radius drops, or 700-μm-radius graupel) is presented. It is used for the calculation of graupel–drop collision efficiencies and collision kernels in calm air for low-, medium-, and high-density graupel at 750- and 500-mb pressure levels. Low-density graupel interacts with water droplets in a way similar to ice crystals: there exists a cutoff size, below which graupel cannot collect water droplets. The authors have shown that the cutoff size decreases with the growth of graupel density, so that medium- and high-density graupel is able to collect droplets with the radii exceeding a certain minimum size. The graupel–drop collision efficiency increases with the drop size up to a maximum value and then sharply decreases to zero, when the drops' terminal velocity approaches the terminal velocity of graupel. As soon as the terminal...

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