Relations between the Hydrologic Cycle and Removal of Stable Isotopes of Water Vapor by Tropical Precipitation Systems

The mechanisms by which precipitation scavenges the heavy isotopes of water (HDO and H₂¹⁸O) from the ambient vapor are elucidated. It is shown that extensive, long-lived precipitation systems with organized convergent flow in the atmospheric boundary layer reduce stable isotope concentrations markedly without any concomitant reduction in specific humidity. The stable isotope concentrations of water vapor therefore serve as markers of the vigor of the hydrologic cycle in the tropics that must be taken into account in any interpretation of past temperature changes. Trajectory analysis is applied to data collected from field experiments in order to provide a measure that relates activity of tropical rain systems to the reduction of stable isotope concentrations in water vapor.