Flux Footprint Estimation for Vertical Source Distribution

In this study, a Lagrangian stochastic dispersion model is combined with various vertical distributions of canopy source to investigate the flux footprint estimation for tall canopies under different atmospheric stabilities. For understanding the contributions of scalar sources/sinks under the canopy to the scalar flux, the canopy is divided into multi-layers (source from the ground is included in the lowest layer). Three kinds of vertical source profiles, including: uniform distribution, distribution by leaf area density (LAD), and distribution by photosynthesis rate, are considered to test how multi-layers of sources influence the vertical-integrated flux footprint. We found that under all atmospheric stability conditions, it is less necessary to divide the canopy into multi-layers of sources as the measurement height (Zm) is increased to 5 times of the canopy height (h). Also, when the atmosphere is unstable, the importance of dividing the canopy into multi-layers is reduced at all tested measurement heights. When the source/sink distribution is non-uniform, the footprint contribution depends on the source strength of each layer. The source profiles by the LAD and photosynthesis rate we selected concentrate on 0.4h and 0.5h respectively, and consequently the middle source layers of the canopy contribute to the flux mostly. We also compared our results with an analytical model (Lee, 2004) predictions in neutral condition. We found that the locations of the vertical-integrated footprint peaks of these two models get closer when Zm is increased to 5h. Based on the results, we suggest that the vertical source profile of tall canopies should be considered when conducting the flux footprint estimations.