Collaborative Research: Testing Controls on Source, Sink, and Lifetime of Atmospheric Water with Numerical Tags and Stable Isotope Ratios

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H2O is instantly recognizable as the chemical formula for water, but this simple formula masks the fact that not all hydrogen atoms are alike. A small fraction of the water on earth could be more accurately called HDO, where D stands for deuterium, the heavier hydrogen isotope in which the nucleus contains a neutron as well as a proton. Likewise, some water molecules contain 18O, an isotope of oxygen which is heavier than ordinary oxygen by the addition of two neutrons. These heavier forms of water share a subtle but important difference from ordinary H2O: they evaporate more sluggishly and condense more readily, preferring the solid and liquid phases to the vapor phase. When water evaporates from the land or ocean surface the vapor is slightly depleted of the heavier isotopes, and each time rain falls from a cloud the water vapor remaining in the atmosphere is slightly lighter than the water that rained out. The removal of heavier isotopes by precipitation suggests that the abundances of heavier isotopes in water vapor and precipitated water contain important clues for understanding the hydrological cycle and its connections to environmental conditions. Results of research in this area are promising but they are complicated by the numerous factors that can influence isotopic composition. Among these are the temperature at the location where the precipitation occurred, the height of the cloud that produced the precipitation, the relative humidity over the ocean where the water vapor evaporated, and the evaporation of lighter isotopes from raindrops as they fall through dry air. Moreover, much of the research on water isotopes is based on correlations between isotopic composition and variables relevant to physical mechanisms (temperature in the region of rainout, for instance). These correlations can be quite informative but correlations by themselves cannot definitively establish causality.Work under this award seeks to establish direct, causal relationships between the physical mechanisms relevant to isotopic composition and the actual composition of water vapor and precipitated water. The research is conducted using a model, the Community Earth System Model (CESM), in which the mechanisms and their influence on isotopic composition are known exactly. CESM is already equipped to simulate the physical processes that determine the ratio of heavier to lighter isotopes, and it also tracks the isotopic composition of water vapor as it circulates in the simulated atmosphere. This project develops a new "water tagging" capability which allows simulated water vapor to record information about conditions under which it evaporated, for instance where and when it evaporated and whether it was processed through clouds. This information is also transferred to the precipitation produced when water vapor condenses. The tag information can be directly compared to isotopic composition, allowing the influence of the physical conditions and processes represented by the tags to be directly assessed. Some simulations use a data assimilation system (known as DART) to ensure that the simulated climate and circulation accurately represent their real-world counterparts. Additional work uses a one-dimensional energy balance model, which provides a reduced-complexity setting to develop new ideas and test hypotheses.The work has scientific broader impacts due to the widespread use of water isotopes as a window on the functioning of climate and the hydrological cycle. For instance isotope ratios in glacial ice cores have been used to estimate temperature differences between ice ages and interglacial warm periods, thus providing a valuable point of reference for understanding current climate change. The water tagging algorithms developed here will be made available to the worldwide user community of CESM, providing users with a powerful tool for the interpretation of isotope data and its application to a variety of research questions.In addition, researchers supported under this award conduct educational outreach through a program focused on the Decker Lake Youth Center, a facility housing post-conviction youth in the Salt Lake Valley. The outreach involves ozone bioindicator gardens which demonstrate the hazards of ozone pollution, a common problem in the Salt Lake Valley. Educational opportunities in correctional facilities have been shown to reduce recidivism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.