Collisional Forcing of Raindrop Oscillations

Abstract

An energy model is formulated to compare collisional kinetic energy with viscous dissipation for oscillating raindrops as a function of size, raindrop distribution and rainfall rate. A potential energy model is used to calculate oscillation energies, and is coupled with results from a potential flow model to determine the time-average axis ratios for fundamental mode oscillations. The model findings are found to be consistent with observations of mean and extreme axis ratios. It is concluded that there is sufficient collisional energy to produce large-amplitude oscillations in moderate-to-heavy rainfall, and that time-average axis ratios should depart significantly from equilibrium, thereby altering the radar polarization characteristics of rain.