A field study of small raindrop oscillations

Abstract

Measurements were made at the ground in Hawaiian showers to detect oscillations of small raindrops as predicted by recent laboratory experiments. Oscillation frequencies and amplitudes were determined from photographs using backscattered light of the primary rainbow. A strobe provided highlights so that the fall speeds and corresponding sizes of each drop could be calculated. The drop‐size distributions were measured independently with a collocated disdrometer. The observations of 0.3 mm to 2.0 mm diameter raindrops clearly showed the onset of oscillations near 1 mm, at the same size where eddy shedding begins. Alternative causes of drop oscillations from drop collisions and wind shear were eliminated using estimates based on drop‐size distributions and winds. Our inferred oscillation sizes, modes and amplitudes were consistent with several previous measurements where small raindrops were found to be generally more spherical than the non‐oscillating shapes predicted by theory. This field study firmly establishes eddy shedding as the major cause of natural shape variations for small raindrops in precipitation—and provides further evidence that significant corrections are needed in estimating the scattering of microwaves in light to heavy rain showers (e.g., for interpreting polarization radar data and evaluating rain outages in communication links).