One way to test our understanding of the impact of convective processes on the isotopic composition of water vapor and precipitation is to analyze the isotopic mesoscale variations during organized convective systems such as tropical cyclones or squall lines. The goal of this study is to understand these isotopic mesoscale variations with particular attention to isotopic signals in near-surface vapor and precipitation that may be present in observations and in paleoclimate proxies. With this aim, we run cloud resolving model simulations in radiative-convective equilibrium in which rotation or wind shear is added, allowing us to simulate tropical cyclones or squall lines. The simulations capture the robust aspects of mesoscale isotopic variations in observed cyclones and squall lines. We interpret these variations using a simple water budget model for the sub-cloud layer of different parts of the domain. We find that rain evaporation and rain-vapor diffusive exchanges are the main drivers of isotopic depletion within cyclones and squall lines. Horizontal advection spreads isotopic anomalies, thus reshaping the mesoscale isotopic pattern. Variations in near-surface relative humidity and wind speed have a significant impact on d-excess variations within tropical cyclones, but the evaporation of sea spray is not necessary to explain the observed enrichment in the eye. This study strengthens our understanding of mesoscale isotopic variability and provides physical arguments supporting the interpretation of paleoclimate isotopic archives in tropical regions in terms of past cyclonic activity.

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.

In observations and cloud-resolving model (CRM) simulations, large-scale domains where convection is more aggregated (clustered into a smaller number of clouds) are associated with a drier troposphere. What mechanisms explain this drying? Hypotheses involve dynamical and microphysical processes. The goal of this study is to quantify the relative importance of these processes. We use a series of CRM simulations with different dynamical regimes and different kinds of convective organization forced by external forcings (isolated cumulonimbi, tropical cyclones, squall lines). We interpret the simulation results in the light of a hierarchy of simpler models (last-saturation model, analytical model). In CRM simulations, the troposphere is drier in the environment of more aggregated convection (tropical cyclones and squall lines). A last-saturation model is able to reproduce the drier troposphere even in absence of any microphysical processes or horizontal motions. Cloud intermittence is the key factor explaining this drying: when clouds are more intermittent, subsiding air parcels are more likely to encounter a cloud. An analytical model highlights the key role of the duration of convective systems. Remoistening by microphysical processes contributes to the moister troposphere when convection is less aggregated, though its importance is secondary smaller than that of intermittence. We suggest that the observed anti-correlation between convective aggregation and relative humidity may, at least partially, be mediated by the duration of convective systems.

The isotopic composition of water vapor (e.g. its Deuterium content) evolves along the water cycle as phase changes are associated with isotopic fractionation. In the tropics, it is especially sensitive to convective processes. Consequently the isotopic composition of precipitation recorded in paleoclimate archives has significantly contributed to the reconstruction of past hydrological changes. It has also been suggested that observed isotopic composition of water vapor could help better understand convective processes and evaluate their representation in climate models. Yet, water isotopes remain rarely used beyond the isotopic community to answer today’s pressing climate questions. A prerequisite to better assess the strengths and weaknesses of the isotopic tool is to better understand what controls spatio-temporal variations in water vapor isotopic composition through the tropical atmosphere. A first step towards this better understanding is to understand what controls the isotopic composition of the sub-cloud layer water vapor over the ocean. Isotopic measurements show that the water vapor is the most enriched in trade-wind regions, and becomes more depleted as precipitation increases. To understand this pattern, we use global simulations with the isotope-enabled general circulation model LMDZ, large-eddy simulation in radiative-convective equilibrium and with large-scale ascent or descent, with the isotope-enabled model SAM and simple analytical models. We show that increased precipitation rate is associated with increased isotopic depletion if it is associated with stronger large-scale ascent, but with decreased isotopic depletion if it is associated with warmer surface temperature. As large-scale ascent increases, the isotopic vertical gradient in the lower troposphere is steeper, which makes downdrafts and updrafts more efficient in depleting the sub-cloud layer water vapor. The steeper gradient is caused mainly by the larger quantity of snow falling down to the melting level, forming rain whose evaporation depletes the water vapor.