Impact of an imposed wall-jet on canopy turbulence and the net ecosystem-atmosphere exchange of carbon dioxide

The net ecosystem-atmosphere exchange (NEE) of carbon dioxide (CO2) as influenced by an imposed wall-jet velocity profile is investigated using three-dimensional and time-dependent large-eddy simulation (LES). This wall-jet velocity profile mimics that of a nocturnal drainage flow. A distributed, plane source is located within the canopy in order to simulate the transport of CO2. The NEE may be obtained by inte-gration of the rate of change of the concentration of CO2 over the computational domain. Numerical re-sults of the instantaneous and mean velocity fields are presented and their impact on the evolution and propagation of CO2 concentration within and above the canopy. We examine the influence of different canopy types, and source strengths on the accumulation and transport of CO2 in the atmospheric boundary layer. We compare horizontal and vertical CO2 advection terms and analyze the budget of NEE of CO2 in detail. The results show that 10-20% of NEE is drained out by advection when strong turbulence and low heat flux is present. The results also show that the turbulence component in the transverse direction is sig-nificant.