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

Distinctively different Z-R relations for initial convective and transition rain at the surface were found during the Tropical Ocean and Global Atmosp here Coupled Ocean-Atmosphere Response Experiment. Initial convective rain (of 20-30-min duration) is marked by nearly constant median volume diameter of D-o approximate to 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 rat e indicates an equilibrium distribution that accounts for the near linearit y between Z and R. The form of the distribution differs from those previous ly reported, however. In contrast, the airborne raindrop measurements at 3 km in climatologically similar conditions show size spectra closely resembl ing the equilibrium collision-coalescence-breakup spectra of Hu and Srivast ava and others at R > 20 mm h(-1). The center of the plateau (of near-const ant 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 wh ich the updraft decreases the rate of fall of the drops relative to the sur face, thus extending the residence time for collisions and reducing the dep th of fall required for equilibrium to be reached. At the same time the upd raft separates the large fast-falling drops from the smaller ones. The larg e ones fall within the convergent core of the convective cell to form the n arrow equilibrium drop spectra observed at the surface, while the small one s rise into the divergent air above the updraft maximum and fall out elsewh ere 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 collis ion rate is maximized. The warm convective clouds of the western tropical P acific Ocean appear to be well tuned to this process. The implications for radar measurements of rainfall are also treated.