The Arrival Rate of Raindrops at the Ground

J. G. Hosking Department of Computer Science, University of Auckland, Auckland, New Zealand

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C. D. Stow Department of Physics, University of Auckland, Auckland, New Zealand

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Abstract

Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.

Abstract

Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.

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