Laboratory measurements of raindrop oscillations

Oscillations are measured from modulations in recorded streaks for drops falling through the primary and secondary rainbows in artificial light. Measurements have been made using fall columns from 3 to 28 meters.

Raindrop fall streaks modulated by oscillations.

Numerical Raindrop Shapes

• Raindrop shapes are calculated from a surface force balance for gravity, aerodynamic pressure, and surface tension.
• Oscillation shapes are calculated by mapping spherical harmonic perturbations onto the raindrop shapes.
• Drop images are calculated for a cosine shading with the brightness mapped onto an image plane.

These particular modes are excited by vortex shedding as a drop falls through the air.

Electrostatic shape of falling drops.

Field measurements of raindrop oscillations

Fall speed and sizes are obtained from the strobe dots, oscillation frequencies, and amplitudes from the streak gaps and lengths. Measurements have been made in Hawaiian and Midwestern showers.

Aircraft measurements of raindrop size and shape using a laser 2D probe

• 1985 Joint Hawaiian Warm Rain Project
• 1990 Hawaiian Rainband Project (HaRP)
• 1991 CaPE Project near Cape Kennedy, Florida

Raindrop images from a shower containing the largest drop ever measured (8.2 mm diameter). The magnified record is a time sequence with each vertical line denoting the passage of 0.1 seconds. The scale at bottom shows relative size.