Abstract

Giant raindrops (4–8 mm diameter) have been observed recently within warm convective rainbands with depths ≤2 km off the coast of the big island of Hawaii. The presence of giant raindrops in these clouds was initially surprising since model and laboratory results support the view that collisional breakup should rapidly destroy such drops in natural clouds. A mechanism proposed by Beard et al. and others to explain the giant raindrops is that exceptionally large aerosol particles act as nuclei for very large cloud droplets near cloud base. Such rare large cloud drops then grow rapidly into giant raindrops by accretion of smaller cloud droplets in the updraft. We show here that an additional set of circumstances is required to explain the existence of giant raindrops in Hawaiian rainband clouds, one which prolongs their growth and separates them from smaller raindrops. We present data from the 1985 Joint Hawaiian Warm Rain Project that suggest that giant raindrops result when selected small raindrops recirculate from downdrafts into updrafts within eddies generated along updraft/downdraft shear zones near cloud top. We show that this recirculation mechanism is consistent with observed cloud structure both at early and late stages of cloud lifetime in Hawaiian rainbands.

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