Influence of convective processes on the isotopic composition (<i>δ</i>¹⁸O and <i>δ</i>D) of precipitation and water vapor in the tropics: 2. Physical interpretation of the amount effect

TLDR

In tropical precipitation, heavier water isotopes are anticorrelated with precipitation amount, yet the physical mechanisms driving this amount effect remain poorly understood. The study aims to investigate the physical processes underlying the amount effect and to develop a method for quantifying their relative contributions. Using a single‑column model that incorporates the Emanuel convection parameterization, the authors simulate stable isotope dynamics, focus on convective processes, and analyze variability across timescales with a TOGA‑COARE unidimensional experiment. They find that reevaporation of falling rain and diffusive exchanges, together with recycling of sub‑cloud vapor by convective fluxes, dominate the amount effect, highlighting the need for detailed rain‑evaporation representation and showing that the effect is most evident at intraseasonal or longer timescales, linked to water residence time.

Abstract

In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so‐called amount effect are still poorly understood and quantified. In the present study, stable water isotopes (H 2 18 O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large‐scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean–Global Atmosphere–Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.