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.

中文翻译：

---

根据大涡模拟和双柱模型，热带对流层水蒸气同位素变化中心的降雨蒸发、融雪和夹带


我们的目标是开发一个简单的模型作为海洋热带对流层水蒸气同位素变化的解释框架。我们使用辐射对流平衡中无组织对流的大涡模拟来证明这个简单模型的基本假设，约束其输入参数并评估其结果。我们还旨在解释对流层中低层和中层水蒸气同位素组成随着降水增加而减少，这是热带海洋观测的一个显着特征。这一特征对我们的解释框架的相关性构成了严格的考验。先前的研究基于观测或参数化对流模型，强调了深对流和中尺度下沉气流、降雨蒸发、雨汽扩散交换和混合过程的作用。我们开发的解释框架在无组织对流的情况下有效，是一个两列模型，代表云中的净上升和环境中的净下降。我们表明，随着降水率的增加，对流层消耗的机制都源于较高的对流层相对湿度。首先，当相对湿度较大时，融化前升华的雪较少，蒸发的雨水也较少。这两种效应都会导致更多的雨水蒸发，并最终导致更多的水蒸气耗尽。这种机制在大规模上升的情况下占主导地位。其次，干燥空气夹带进入云层降低了垂直同位素梯度并限制了对流层水蒸气的消耗。这种机制在大规模下降的制度中占主导地位。