Rain Evaporation, Snow Melt, and Entrainment at the Heart of Water Vapor Isotopic Variations in the Tropical Troposphere, According to Large‐Eddy Simulations and a Two‐Column Model

Abstract

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 of disorganized convection in radiative‐convective equilibrium to justify the underlying assumptions of this simple model, to constrain its input parameters and to evaluate its results. We also aim at interpreting the depletion of the water vapor isotopic composition in the lower and midtroposphere 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 mesoscale downdrafts, rain evaporation, rain‐vapor diffusive exchanges, and mixing processes. The interpretative framework that we develop, valid in case of disorganized convection, 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 as the 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.

Shear‐Convection Interactions and Orientation of Tropical Squall Lines

“…At this stage, we can conclude that the theory of the conservation of the shear projection on the direction perpendicular to the squall line allows to predict the orientation angle of the convective bands. We note in passing that the results are robust to reasonable changes in the relaxation timescale used to impose wind shear and to a prescribed large-scale upward vertical velocity, following (Risi et al, 2021).…”

What Controls the Skill of General Circulation Models to Simulate the Seasonal Cycle in Water Isotopic Composition in the Tibetan Plateau Region?

“…In addition, observations and model simulations suggest that isotopic variations at different altitudes in the troposphere vary in concert (e.g., Bony et al, 2008;He et al, 2015;Shi et al, 2020). This is physically justified by the fact that deep convection efficiently mixes the vapor in the vertical as demonstrated by cloud-resolving model simulations (Risi et al, 2021). The water vapor δD from TES significantly correlates with the observed δ 18 O in precipitation from GNIP (r = 0.6, p < 0.05).…”

A Lagrangian Perspective on Stable Water Isotopes During the West African Monsoon

“…If a liquid hydrometeor falls into unsaturated air, evaporation takes place and acts as a reversed distillation process (Bony et al, 2008). If the evaporated fraction of the rain drop is large, then this process has an enriching effect on the surrounding water vapor (e.g., Risi et al, 2021;Tremoy et al, 2014). In contrast, if the evaporated fraction is small (i.e., partial evaporation), this leads to a depletion in  D v E , as lighter isotopes evaporate preferentially Lee & Fung, 2008;Noone, 2012).…”