Process-Based Intercomparison of Water Isotope-Enabled Models and Reanalysis Nudging Effects

The products from the Stable Water Isotope Intercomparison Group, Phase 2, are currently used for numerous studies, allowing water isotope model-data comparisons with various isotope-enabled atmospheric general circulation model (AGCMs) outputs. However, the simulations under this framework were performed using different parameterizations and forcings. Therefore, a uniform experimental design with state-of-the-art AGCMs is required to interpret isotope observations rigorously. Here, we evaluate the outputs from three isotope-enabled numerical models nudged by three different reanalysis products and investigate the ability of the isotope-enabled AGCMs to reproduce the spatial and temporal patterns of water isotopic composition observed at the surface and in the atmospheric airborne water. Through correlation analyses at various spatial and temporal scales, we found that the model's performance depends on the model or reanalysis we use, the observations we compare, and the vertical levels we select. Moreover, we employed the stable isotope mass balance method to conduct decomposition analyses on the ratio of isotopic changes in the atmosphere. Our goal was to elucidate the spread in simulated atmospheric column delta 18O, which is influenced by factors such as evaporation, precipitation, and horizontal moisture flux. Satisfying the law of conservation of water isotopes, this budget method is expected to explain various fractionation phenomena in atmospheric meteorological and climatic events. It also aims to highlight the spreads in modeled isotope results among different experiments using multiple models and reanalyses, which are primarily dominated by uncertainties in moisture flux and precipitation, respectively. Our study focuses on surface and atmospheric water isotopes, which are crucial for understanding climate and environmental processes. We assessed the performance of different climate models that simulate water isotopes and compared them to real-world observations. To accomplish this, we employed advanced atmospheric models that include isotopes and subjected them to different input data sets. We discovered that the accuracy of the simulations varied depending on the specific model and data used, as well as the vertical levels considered. By performing correlation analyses at different spatiotemporal scales, we obtained insights into how well the models align with the observed isotopic patterns in both surface and airborne water. Additionally, we utilized a stable isotope mass balance method to examine how various factors, such as evaporation, precipitation, and horizontal moisture flux, influence changes in the isotopic composition of the atmosphere. This method enabled us to identify the sources of uncertainty in the model results. Our research emphasizes the need for a standardized experimental design when studying water isotopes with climate models. By identifying the dominant sources of uncertainty, our findings will prove valuable for scientists from various disciplines and enhance the understanding of simulated future climate and water cycle studies. We built a publicly available data set of isotope-enabled nudged simulations from 1979 to 2020, utilizing three models and three reanalysesWe decomposed atmospheric processes impacting water isotopes, spanning evaporation to precipitation, leading to the first global estimationModeled water isotope spreads due to model and reanalysis choices were dominated by moisture flux and precipitation uncertainties