Wake-excited raindrop oscillations

Abstract

Interpretation of microwave scattering in rain depends on the shape of raindrops, normally considered to have an ‘‘equilibrium’’ axis ratio from a force balance at each point on the surface between surface tension, hydrostatic and aerodynamic pressure. Data from the Illinois State Water Survey raindrop cameras and also observations using a high resolution, dual‐polarization radar suggest a pronounced shift from equilibrium axis ratios for small raindrops. New laboratory measurements have been made for small water drops falling at terminal velocity. The experimental findings agree well with equilibrium axis ratios from perturbation theory for d≤1 mm, but scatter significantly above the theoretical curve for d≳1 mm. The scatter is in the same direction as the shifted axis ratios in the field observations. The one‐sided scatter is consistent with wake‐excited oscillations, but only for modes having a single nodal meridian through the poles (corresponding to spherical harmonics of degeneracy, m=1). These iden...