The goal of this study is twofold. First, we aim at developing a simple
model as an interpretative framework for the water vapor isotopic
variations in the tropical troposphere over the ocean. We use large-eddy
simulations to justify the underlying assumptions of this simple model,
to constrain its input parameters and to evaluate its results. Second,
we aim at interpreting the depletion of the water vapor isotopic
composition in the lower and mid-troposphere as precipitation increases,
which is a salient feature in tropical oceanic observations. This
feature constitutes a stringent test on the relevance of our
interpretative framework. Previous studies, based on observations or on
models with parameterized convection, have highlighted the roles of deep
convective and meso-scale downdrafts, rain evaporation, rain-vapor
diffusive exchanges and mixing processes. The interpretative framework
that we develop is a two-column model representing the net ascent in
clouds and the net descent in the environment. We show that the
mechanisms for depleting the troposphere when precipitation rate
increases all stem from the higher tropospheric relative humidity.
First, when the relative humidity is larger, less snow sublimates before
melting and a smaller fraction of rain evaporates. Both effects lead to
more depleted rain evaporation and eventually more depleted water vapor.
This mechanism dominates in regimes of large-scale ascent. Second, the
entrainment of dry air into clouds reduces the vertical isotopic
gradient and limits the depletion of tropospheric water vapor. This
mechanism dominates in regimes of large-scale descent.