A Field Study of Raindrop oscillations. Part I: Observation of Size Spectra and Evaluation of Oscillation Causes

Abstract

The size spectra of oscillating raindrops were determined from photographic measurements in Illinois showers at night. The oscillations were detected from modulations in the fall streaks produced by backscattered light near the primary rainbow. Drop sizes were determined from the fall speed using strobe lights. A continuous record of raindrop size distributions was obtained from a disdrometer located beneath the camera sample volume. Results show that oscillations begin near 1-mm diameter, at the onset size for vortex shedding. This finding is consistent with the authors' recent laboratory and field studies for small raindrops. The size spectra show that all raindrops above 1 mm are oscillating, out to the largest size measured of 4.2-mm diameter. Extrinsic sources of drop oscillations were evaluated using a collision model with viscous decay and using the pressure forcing from turbulence and wind shear in the surface layer. Based on the disdrometer size distributions, the number of oscillating drops produced by collisions was negligible at all rainfall rates compared to the observed number of oscillating drops. Forcing by turbulence and wind shear at the observed wind speeds was found to be well below the threshold of detection. The most plausible explanation for the observed oscillations must involve intrinsic mechanisms capable of maintaining oscillations against viscous decay. Likely sources are the positive feedback to particular oscillation modes caused by vortex shedding and caused by the aerodynamic pressure and drag fluctuations associated with the oscillations themselves.