Laboratory Measurements of Collection Efficiencies for Accretion

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Abstract
Collection efficiencies for accretion were measured for six pairs of nearly unchanged drops. Cloud droplets of 11 and 17 μm and collector drops between 100 and 400 μm radius were used. The resulting efficiencies were in the 51–70% range and all values were significantly below computed collision efficiencies for rigid spheres. Inferred coalescence efficiencies between 54 and 82% were found to decrease with increasing collector drop and cloud droplet sizes. Drop separation was attributed to the grazing bounce mechanism whereby an air film nullifies the relative closure velocity allowing the tangential velocity of the cloud droplet to carry it past the collector drop. Abstract Collection efficiencies for accretion were measured for six pairs of nearly unchanged drops. Cloud droplets of 11 and 17 μm and collector drops between 100 and 400 μm radius were used. The resulting efficiencies were in the 51–70% range and all values were significantly below computed collision efficiencies for rigid spheres. Inferred coalescence efficiencies between 54 and 82% were found to decrease with increasing collector drop and cloud droplet sizes. Drop separation was attributed to the grazing bounce mechanism whereby an air film nullifies the relative closure velocity allowing the tangential velocity of the cloud droplet to carry it past the collector drop.

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