Two-dimensional simulation of airflow and carbon dioxide transport over a forested mountain: Part II. Carbon dioxide budget analysis and advection effects

We apply a high-resolution atmospheric model to assess the influence of mesoscale advection of CO2 on the estimation of net ecosystem exchange (NEE) using eddy-covariance CO2 flux measurements at a Fluxnet-Canada forest site located on sloping terrain on Vancouver Island, Canada. The numerical simulation is performed for fair-weather conditions over an idealized two-dimensional mountain bounded by water. The model is enhanced to include a CO2 budget with a treatment of canopy photosynthesis and soil respiration. The simulation captures the transport of CO2 by nocturnal drainage flows and weak land breezes. The resulting vertical profiles and time evolution of CO2 concentration show a significant variation near the ground, associated with stability changes in the atmospheric boundary layer. The simulated vertical CO2 gradients are found to be large around sunset and sunrise. The decrease of CO2 concentration over land after midnight and the CO2 accumulation over the neighboring water surface indicate CO2 advection. A CO2 budget analysis of the numerical-model output shows that the mean horizontal and vertical advection have significant fluctuations and opposite signs during daytime, with the net result that they largely counteract each other. At night, mean advection results in the underestimation by 20% of the nocturnal respiration. The estimated NEE at night is dominated by sub-grid-scale vertical flux in this simulation. Further evaluation using 3D simulations with higher resolution is needed to see if our results hold where vertical fluxes are much better resolved.