Natural oscillations of small raindrops

Abstract

REMOTE sensing of the rate of rainfall and the characteristics of hydrometeors with polarimetric radars is based on the assumption that the eccentricity of raindrops increases predictably with drop size^1–3. Field observations, however, have shown that a particular raindrop size can have a wide range of shapes^4,5 because of oscillations thought to be caused by drop collisions. Here we report observations of the shapes of small raindrops in circumstances where oscillations might occur sympathetically with eddy shedding in the drop's wake. The analysis of oscillations was aided by measurements of oscillation frequency, based on variations in rainbow angle and comparisons of mode energies. Our observations agree with quiescent shapes only for drops <1 mm in diameter. For 1–1.5 mm we discovered significant shape variability of distinct types corresponding to drop oscillations in axisymmetric and transverse modes. Our finding of reduced average distortion explains the mysterious shift away from quiescent distortion for small raindrops, postulated from radar observations⁶, and indicates the existence of an important class of raindrop oscillations—promoted not by collisions but by intrinsic aerodynamic forces.