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

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 suggests a mechanism in which the updraft decreases the rate of fall of the drops relative to the surface, thus extending the residence time for collisions and reducing the depth of fall required for equilibrium to be reached. At the same time the updraft separates the large fast-falling drops from the smaller ones. The large ones fall within the convergent core of the convective cell to form the narrow equilibrium drop spectra observed at the surface, while the small ones rise into the divergent air above the updraft maximum and fall out elsewhere if they survive. Also, the updrafts in warm tropical convective clouds that produce R > 20 mm h?1 are commonly 4?5 m s?1, the speeds necessary to support the 0.5?1.5-mm-diameter range of drop sizes at which the collision rate is maximized. The warm convective clouds of the western tropical Pacific Ocean appear to be well tuned to this process. The implications for radar measurements of rainfall are also treated.