A Large-Eddy Simulation Study of Water Vapour and Carbon Dioxide Isotopes in the Atmospheric Boundary Layer
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A large-eddy simulation model developed at the National Center for Atmospheric Research (NCAR) is extended to simulate the transport and diffusion of C18OO, H218O and 13CO2 in the atmospheric boundary layer (ABL). The simulation results show that the 18O compositions of leaf water and the ABL CO2 are moderately sensitive to wind speed. The variations in the 18O composition of water vapour are an order of magnitude greater than those in the 13C and 18O compositions of CO2 both at turbulent eddy scales and across the capping inversion. In a fully-developed convective ABL, these isotopic compositions are well mixed as with other conserved atmospheric quantities. The Keeling intercepts determined with the simulated high-frequency turbulence time series do not give a reliable estimate of the 18O composition of the surface water vapour flux and may be a reasonable approximation to the 13C and 18O compositions of the surface CO2 flux in the late afternoon only after a deep convective ABL has developed. We suggest that our isotopic large-eddy simulation (ISOLES) model should be a useful tool for testing and formulating research hypotheses on land–air isotopic exchanges.
KeywordsIsotopes Keeling plot Kinetic fractionation Land-surface model Large-eddy simulation
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