The distribution of raindrops speeds

Abstract

A general discussion on the drops speeds and their distribution in rainfall accounts quantitatively for the anomalous features discovered by Montero-Martinez et al. (2009), who demonstrate that a non negligible fraction of drops have an anomalously large fall velocity owing to their diameter, the ratio between the observed, and expected speeds being all the more large that the drop diameter is small. While these authors attribute this anomaly to breakup between colliding drops, we show on the basis of detailed laboratory experiments that it can be fully accounted for by a scenario which has successfully represented the distribution of drops sizes in rainfall, namely the spontaneous breakup of large drops. In particular, this scenario predicts that a fraction of drops in rainfall should have a velocity approximately 2.5 times larger than their equilibrium fall velocity given their size, consistent with measurements. The trajectories of the bursted drop fragments, and the distribution of the norm of their velocity at bursting are determined. The average fragment velocity induced by bursting is found to be much smaller that the velocity of the initial mother drop. Moreover, the fraction of drops affected by the velocity anomaly is estimated. Copyright 2011 by the American Geophysical Union.