The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the “amount effect.” Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large‐eddy simulations. Results from large‐eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large‐scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large‐scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently.

中文翻译：

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是什么控制着热带海洋表面附近的水蒸气同位素组成？受大涡模拟约束的分析模型的结果


这项研究的目的是了解控制热带海洋表面附近水蒸气同位素组成的机制，规模约为一百公里一个月。在热带地区，人们长期以来观察到，当降水率较高时，地表附近的雨水和水汽的同位素成分会更加贫化。这就是所谓的“数量效应”。先前的研究基于参数化对流的观测或模型，强调了深对流和中尺度下沉气流以及降雨蒸发的作用。但这些过程的相对重要性从未被量化。我们假设它可以使用受大涡模拟约束的分析模型来量化。大涡模拟的结果证实，只有当降水率变化是由大尺度环流的变化引起时，才能模拟经典的量效应。我们发现，与海洋平衡相比，消耗水蒸气的主要过程是上升气流源自水蒸气更丰富的区域。造成数量效应的主要过程是当大规模上升增加时，同位素垂直梯度更陡，因此上升气流和下降气流更有效地消耗亚云层。