What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large‐Eddy Simulations

Abstract

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.

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

“…The amount effect can be seen only if the precipitation increase is associated with a change in the largescale circulation (Bony et al, 2008;Dee et al, 2018;Risi et al, 2020). To compare ctrl to simulations with larger and smaller precipitation rate, we thus run simulations with a prescribed large-scale vertical velocity profile, ω LS .…”

“…Frameworks do exist to interpret the δD v in the subcloud layer (SCL), such as the (Merlivat & Jouzel, 1979) closure assumption, later extended to account for mixing with free tropospheric air (Benetti et al, 2015) and for updrafts and downdrafts (Risi et al, 2020). This latter framework highlighted the need to know the steepness of the relationship between δD v and specific humidity q as they evolve with altitude.…”

Amazonian terrestrial water balance inferred from satellite-derived water vapor isotopes

“…To quantify the bias towards low cloud conditions, we use large-eddy simulations (LES) enabled with isotopic physics 59,65 . We consider two simulations of radiative convective equilibrium: one without any large-scale vertical velocity and one with large-scale ascent with the large-scale vertical velocity profile peaking at 500hPa with a value of -60hPa d -1 .…”

“…It is noted that evaporation has a depleting effect on the vapor only if the rain evaporated fraction is very small (first order approximation in Worden et al 2007). If, in contrast, the rain evaporated fraction is not small enough, rain evaporation has an enriching effect on the water vapor [59][60][61] (not shown). Microphysical processes within different regions of the atmospheric profile may thus have somewhat cancelling effects on the δD_004 sensitivity to ET-P. On the other hand, the reliance of convection on the convergence of remote moisture unambiguously extends the δD_004 range to lower values (orange arrow) 21 .…”

“…We consider two simulations of radiative convective equilibrium: one without any large-scale vertical velocity and one with large-scale ascent with the large-scale vertical velocity profile peaking at 500hPa with a value of -60hPa d -1 . The simulations are run with a horizontal resolution of 750 m that allows us to explicitly simulate convective clouds and their associated updrafts and downdrafts 59 . Ten snapshots of the simulations, corresponding to the last 10 days of the simulation, are analyzed here.…”