The role of canopy-boundary layer interactions in scalar transport over forests

Transport of scalars from forests is of significant interest for a variety of reasons. Forests represent an important sink of CO2 from the atmosphere. Measuring and quantifying these CO2 fluxes is an important part of improving our estimation of future trends in CO2 concentrations. Similarly, forests may also alter surface moisture fluxes and be an important source or sink of other trace gases.

Many forested regions are also hilly or mountainous. Recent theoretical and modelling results have shown a significant impact of the dynamical interactions between forests and the boundary layer over complex terrain. This talk investigates the effect of these flow interactions on scalar transport through a series of idealised numerical simulations of flow over a forested hill. Experiments include tracers with a fixed source release rate and also with a simple CO2 model where the CO2 uptake depends on the local CO2 concentration. Sensitivity to the detailed turbulence characteristics will also be investigated by comparing results from different mixing length turbulence schemes and from large-eddy simulations. Enhanced flow into and out of the forest canopy and increased flow separation lead to significant variations in the concentration of scalars across the hill. With the CO2 model, where the sink term depends on the local concentration, this can lead to differences is the total uptake by the forest, although this tends to be smaller. The importance of including these dynamical effects in flux measurements is demonstrated using results from the model.