A Large-Eddy Simulation Study of Water Vapour and Carbon Dioxide Isotopes in the Atmospheric Boundary Layer

A large-eddy simulation model developed at the National Center for Atmospheric Research (NCAR) is extended to simulate the transport and diffusion of (COO)-O-18, H (2) (18) O and (CO2)-C-13 in the atmospheric boundary layer (ABL). The simulation results show that the O-18 compositions of leaf water and the ABL CO2 are moderately sensitive to wind speed. The variations in the O-18 composition of water vapour are an order of magnitude greater than those in the C-13 and O-18 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 O-18 composition of the surface water vapour flux and may be a reasonable approximation to the C-13 and O-18 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.