This project will observe and analyze the stable isotope composition of water vapor and precipitation, primarily at the 300 meter Boulder Atmospheric Observatory tower. The measurements will be made using an optical measurement technology which has only recently become available, and which allows continuous in-situ observations to be made on a practical basis. Further measurements will be taken of the size distribution of precipitation hydrometeors at the top and bottom of the tower, as well as measurements of the surface fluxes of sensible and latent heat (using a sonic anemometer and a high speed hygrometer) and soil heat flux (using a conductive plate). These measurements will complement existing measurements of carbon monoxide (CO), and carbon dioxide (CO2), and meteorology at the tower, and the water vapor measurements would be taken at the same heights as the CO and CO2 measurements, at 22, 100, and 300 meters. In addition, measurements of the isotopic composition of precipitation and soil water would be taken by students at several middle schools in the region and analyzed for isotopic composition. The middle schools will also be supplied with meteorological observing stations, and students will take meteorological measurements and receive instruction on atmospheric processes and the science of isotope hydrology. The research seeks to evaluate 1) the role of rainfall evaporation (virga) in controlling the isotopic composition and humidity of the summertime boundary layer, 2) the partioning of the surface energy and water fluxes, through isotopic constraints on the transpiration fraction of surface latent heating, and 3) the degree to which pairing water isotope measurements with other gas measurements, such as CO2 concentration, yield improved estimates of gas exchange between the surface and the overlying atmosphere. The measurements will be used to constrain and validate simulations from the Community Land and Community Atmosphere Models.
The isotopic composition of precipitation and water vapor contains a wealth of useful information, including the temperature at which evaporation occurs over the ocean, the extent to which evapotranspiration over land serves as a source for subsequent precipitation (i.e. the strength of the land-atmosphere coupling), the pathways of water vapor transport in the atmosphere, and the strength of trace gas exchange (including CO2) between land and atmosphere. But the multitude of factors determining isotopic composition makes it difficult to extract the useful information, particularly given our limited quantitative understanding of some of the physical processes. Research conducted under this project will thus serve the climate science community by helping to deconvolve the factors determining isotopic composition, and unlock the valuable climatological information contained in it. In addition, the participation of middle school students in the project will serve the general public, including underrepresented groups, in surrounding communities by offering an early exposure to scientific research. Educational activities will also take place at the University of Colorado involving graduate and undergraduate students, including a project in which engineering students design and build a mechanical device to collect rain and snow samples.
