LAUSR

Abstract We investigated the sub-diurnal variability of the carbon dioxide and water vapour isotopologues by modelling a representative case measured above the Harvard Forest. To this end, we developed a model that couples the local processes governed by soil and vegetation conditions to non-local atmospheric processes such as entrainment and long-range advection. The model formulation is based on solving the stable isotopologues 12CO2, 13CO2, C18OO, H216O and H 2 18 O as conserved variables. It also includes simultaneously solving the meteorological state variables coupled with their respective surface fluxes. Our model results indicate the need for a comprehensive observational data-set to ensure that the essential processes and interactions between the boundary-layer dynamics of a forest and the atmospheric boundary layer are satisfactorily reproduced. We present and discuss the temporal evolution of the budgets of 13CO2 and C18OO, in order to quantify the individual contributions made by soil, plant and entrainment dynamics. All these contributions turn out to be relevant, as they enable us to quantify how the energy, water and carbon fluxes on sub-daily scales are partitioned. Regarding the role played by entrainment, we carried out a set of three systematic experiments in which air, with different CO2 and H2O isotopic compositions originating in the residual layer, mix with the boundary-layer air. Our findings show that both the C18OO and H 2 18 O isotopic ratios and their respective isofluxes are influenced by the entrainment event. This result indicates that high frequency and accurate isotopologues surface measurements (seconds or minutes) can be used to quantify how non-local atmospheric processes modify isotopic composition at sub-daily scales.