Modelling vegetation-atmosphere CO₂ exchange by a coupled Eulerian-Langrangian approach

A Eulerian-Lagrangian canopy microclimate model was developed with the aim of discerning physical from biophysical controls of CO₂ and H₂O fluxes. The model couples radiation attenuation with mass, energy, and momentum exchange at different canopy levels. A unique feature of the model is its ability to combine higher order Eulerian closure approaches that compute velocity statistics with Lagrangian scalar dispersion approaches within the canopy volume. Explicit accounting for within-canopy CO₂, H₂O, and heat storage is resolved by considering non-steadiness in mean scalar concentration and temperature. A seven-day experiment was conducted in August 1998 to investigate whether the proposed model can reproduce temporal evolution of scalar (CO₂, H₂O and heat) fluxes, sources and sinks, and concentration profiles within and above a uniform 15-year old pine forest. The model reproduced well the measured depth-averaged canopy surface temperature, and CO₂ concentration profiles within the canopy volume, CO₂ storage flux, net radiation above the canopy, and heat and mass fluxes above the canopy, as well as the velocity statistics near the canopy-atmosphere interface. Implications for scaling measured leaf-level biophysical functions to ecosystem scale are also discussed.