Deuterium Excess of Water Vapor in the Atmospheric Boundary Layer

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This research explores using water vapor isotope observations for attributing the sources of water vapor to remote source regions, local influences, and transport in the atmospheric boundary layer (ABL). The ABL is the lowest ~1 km of the atmosphere and is a large reservoir of atmospheric water. The ABL water pool supports convective cloud formation. Condensation in clouds depletes vapor from the ABL. Entrainment or turbulent diffusion at the top of the ABL, transports vapor to the upper atmosphere. While both these processes deplete moisture from the ABL, new vapor enters this layer from land evapotranspiration and advection of moisture from remote source regions. Understanding how water moves between the ABL, the land, and the upper atmosphere is essential for accurate prediction of precipitation processes and for climate modeling. Deuterium and oxygen-18 are stable isotopes of hydrogen and oxygen. This study will use "deuterium excess" (d-excess) of water vapor, a measure of the relative abundance of the deuterium and oxygen-18 isotopes of water, to track the movement of water among the three pools and to identify contributions of local land and remote oceanic sources to the ABL vapor. A fundamental premise of the project is that both remote water sources and local ABL processes will imprint influences on vapor d-excess, but at very different time scales. The investigators postulate combined use of the analytical tools appropriate for these different time scales will unravel local influences from those originating from water transport from outside of the local domain. The proposal conceptualizes that the local hydrological cycle is composed of three pools of water (terrestrial water, vapor in the ABL, and vapor in the free atmosphere) and that exchanges among these pools will leave distinct signatures on the d-excess temporal variations. The research methodology consists of data analyses and ABL modeling. The experimental data are hourly observations made with laser-based instruments in 11 climate zones in North America and in Asia. An isotopic land surface model will quantify local evapotranspiration effects on temporal variations in vapor d-excess. Equilibrium boundary layer calculations and large-eddy simulations will be used to infer d-excess signal of the vapor in the free atmosphere and the entrainment influence on the ABL d-excess. Trajectory analysis will identify contributions from remote source regions. The project will contribute to capacity-building in isotope hydrology and in boundary-layer meteorology. Despite the recent explosive growth of laser-based vapor isotope measurements, current ability to interpret water transport processes with these measurements remains preliminary. The project could transform the field from descriptive to quantitative. Hands-on modules, designed from large-eddy simulation data and a simplified one-dimensional ABL model, will enhance classroom learning on ABL and isotopic principles. These modules will form the basis of an open online course on ABL techniques. A new data website will promote sharing of high-frequency vapor isotope data. As the volume of laser-based measurement continues to grow, a centralized data depository will facilitate investigations that transcend disciplinary and geographic boundaries.