Numerical simulation of canopy flow and CO2 flux at the West Coast Flux station

Net ecosystem-atmosphere exchange (NEE) of carbon dioxide is controlled in part by many factors including atmospheric turbulence and advection. Vertical turbulent fluxes of carbon dioxide can be measured by eddy-correlation (EC) instruments, however, mean horizontal and vertical advection effects on NEE could also be significant in complex (coastal, mountainous) terrain. Katabatic and anabatic winds induced by differential heating and cooling are expected to be strong in complex terrain during fair weather. Furthermore the measurements of eddy flux at the West Coast Flux station on Vancouver Island are anomalous just after sunset; this behavior is as yet not adequately understood.

Understanding wind and turbulent regimes at the site is important for more meaningful interpretations of NEE from the tower data. The broad aim of this research is to investigate the canopy flow and carbon dioxide flux at the West-Coast Flux station, based on a high-resolution mesoscale numerical weather model developed by Terry Clark. Currently, we have included the canopy tree-drag dynamics and modified the surface energy budget to include the effect of canopy elements in the model. Diurnal variation of all variables is computed by taking the canopy radiation balance into consideration. A 2-D simulation was run and appears to be quite useful for understanding the complex effects of tall trees on air circulation in the boundary layer. It captures the first-order effects of diurnal heating/cooling on the sloping terrain. The volume exchange of heat and momentum above and within the canopy located on a hill appears to be strongly affected by the local flows resulting from diurnal thermal forcing which may help explaining the anomalous measurements just after sunset.

The future work includes adding a carbon dioxide budget into the model and simulating the mean vertical advection and horizontal advection effect on the estimates of NEE. If time allows, three-dimensional tests might also be performed to evaluate both along-slope gradients in carbon dioxide concentrations, and the resulting effects of fully 3-D advection on NEE.