The Impact of Source Distribution on Scalar Transport over Forested Hills

A significant motivation for studying canopy flows is to understand the impact they have on scalar transport. Previous studies have highlighted the impact of advection due to terrain-induced canopy flows on the above canopy scalar flux even under conditions of neutral flow. This is important in terms of interpreting eddy covariance measurements at sites even with moderate terrain. Here idealised numerical simulations are used to investigate the role of the source distribution in determining the scalar transport within and above a forest canopy. Results show that both the scalar concentration and the scalar fluxes depend on how the scalar source is distributed within the forest canopy. These results can be understand by looking at the canopy-induced flow. The results suggest that even if the canopy flow is well characterised, it is not possible to scale up above-canopy flux measurements to estimate the mean source term in the canopy without an a priori knowledge about the source distribution.