Atmospheric Processes Controlling Photosynthesis: Large-Eddy Simulation Experiments

Canopy functionality is critically modulated by interactions to atmospheric conditions occurring at very short spatiotemporal scales. Due to photosynthesis the CO2 concentration is rapidly depleted in the roughness layer of the canopy, and this effect can become larger in the vicinity of single leaves. By resolving processes at small and short spatiotemporal scales, the LES (large-eddy simulation) technique is now enabling to characterize and better understand the interactions between canopies and the local atmosphere, including the adaption time of the plant to rapid changes of the atmospheric conditions driven by turbulence or the presence of shallow cumulus clouds. Our LES experiments are based on coupling explicitly the diurnal atmospheric dynamics to a plant physiology model. We show LES applications to predict the magnitude and the direction of carbon exchange, highlighting the importance of scale effects in resolving interactions between vegetation and the atmosphere.

First, we present the ability of LES in reproducing the surface energy including photosynthesis and CO2 soil respiration coupled to the dynamics of a convective boundary layer. LES results are compared with a complete set of surface and upper-air meteorological and carbon dioxide observations gathered during a representative day observed at the 213-meter meteorological tower at Cabauw. Second, we perform a sensitivity analysis study on plant physiological processes influenced by rapid atmospheric conditions. More specifically, sensitivity on light use efficiency, time response of stomatal aperture, time response of assimilation internal CO2 concentration, temperature and water vapour pressure dependence. We discuss how these mechanisms influenced by atmospheric conditions can lead to limitations on the CO2 assimilation.