An Observationally Based Conceptual Model of Warm Oceanic Convective Rain in the Tropics

Abstract

Abstract Distinctively different Z–R relations for initial convective and transition rain at the surface were found during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. Initial convective rain (of 20–30-min duration) is marked by nearly constant median volume diameter of Do ≈ 2 mm and narrow drop size spectra, while R rises to >50 mm h−1. The constant form of the drop spectra independent of rain rate indicates an equilibrium distribution that accounts for the near linearity between Z and R. The form of the distribution differs from those previously reported, however. In contrast, the airborne raindrop measurements at 3 km in climatologically similar conditions show size spectra closely resembling the equilibrium collision–coalescence–breakup spectra of Hu and Srivastava and others at R > 20 mm h−1. The center of the plateau (of near-constant size) of these spectra repeatedly occurs at a drop size of 1 ± 0.1 mm whose fall speed equals the updraft speed. This suggest...