An observationally based conceptual model of warm oceanic convective rain in the tropics

During TOGA COARE we found distinctively different Z-R relations for initial convective and transition rain at the surface. Initial convective rain (of 20-30 min duration) is marked by nearly constant median volume diameter of Doª2 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. But the form of the distribution differs from those previously reported. In contrast, the airborne raindrop measurements at 3 km 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. This model is supported by the finding that the maximum rate of collisions of raindrops occurs at a diameter of 1 mm, corresponding identically to the center of the plateau of the equilibrium drop spectra aloft. Also, the updrafts in warm tropical convective clouds which produce rain rates greater than 20 mm h-1 are commonly 4 – 5 m s-1, the speeds necessary to support the 0.5 to 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.