Sub-diurnal variability of the carbon dioxide and water vapor isotopologues at the field observational scale

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, H₂ ¹⁶O and H₂ ¹⁸O 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 H₂ ¹⁸O 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.