Integrating Diel Land-Atmosphere Interactions to Model Carbon Dioxide and Water Vapor Isotopologues

Plant transpiration and soil evaporation, which taken together determine net evapotranspiration, partly regulate the moisture budget, along to the air mass entrained from the residual and free tropospheric layers. In turn, this budget determines the diurnal variability of key processes associated with the development of the boundary layer and clouds. Under land and atmospheric conditions, and constrained by the net available radiative energy, the partitioning of this energy between the sensible heat flux and evapotranspiration is crucial in determining the formation, spatial characteristics and vertical development of boundary-layer clouds. Combining the net ecosystem exchange of carbon and net evapotranspiration fluxes with fluxes of the carbon dioxide and water vapor isotopologues, in other words the isofluxes, enables us to quantify the relative contributions of soil evaporation and plant transpiration to the net evapotranspiration, as well as the strength of the coupling between the carbon and water cycles. With the development of faster and more accurate instrumental techniques to measure these stable isotopes, we design a modelling tool to support the interpretation of these observations. This modelling framework includes representations of the essential local (surface) and non-local (entrainment and advection) processes that govern the diel behavior of these isotopologues.

To integrate the diurnal interactions above the forest and the dynamics of the atmospheric boundary layer, the model is developed to reproduce a complete observational data set gathered at the Harvard forest (USA). Under cloudless and convective conditions, the mean profiles in the atmospheric boundary layer of the state variables and non-reactant stable isotopes are constant on with height and characterized by linear turbulent fluxes. We extend our land-atmosphere mixed-layer model to include the evolution of the isotopologues CO₂,¹³CO₂, C¹⁸OO, H₂O and H₂¹⁸O. By aggregating four successive observed days, we compare the data-set of observations with the mixed-layer results. Our findings confirm that the model reproduces the main characteristics of the observations above the canopy top, demonstrating it to be a useful tool for the interpretation of measurement data, and for testing new formulations of the isofluxes. With the aim of improving our understanding of these processes, we present and discuss the individual contributions (local and non-local) to the isotopic and total mass budgets of CO₂ and H₂O.