Linking CO2 Surface Fluxes to Concentrations in the Atmospheric Boundary Layer Over Mountainous Terrain

Mountain forests represent a large portion of gross primary productivity within the United States and a significant potential net CO2 sink. Therefore, there is a need to develop methods to estimate regional fluxes of CO2 in mountainous terrain. We present results from a combined modeling and observational study of regional CO2 fluxes in mountainous terrain and discuss the major challenges. We use data from the Airborne Carbon in the Mountains Experiment (ACME), conducted in May and July of 2004. The NCAR C-130 aircraft few over a large region (~350x350 km) of the Colorado Rocky Mountains on ten days, making continuous measurements of CO2, CO, O3, and water vapor concentrations among other measurements. The flights were conducted according to a combination of experimental designs, including morning to afternoon Lagrangian measurements, and morning sampling of nocturnally respired CO2. Applying a simple budget method to the aircraft data, we estimated CO2 drawdowns of several ppm in the mountain boundary layer, representing significant CO2 uptake by the forests. These results agree surprisingly well with local flux measurements at a sub-alpine location. To interpret and understand the observations, we use a modeling framework consisting of the Regional Atmospheric Modeling System (RAMS) and its adjoint. The mesoscale model RAMS is run at very high resolution and comparison with available observations shows that the model is able to capture meteorology well under the strongly forced conditions in complex terrain. We prescribe various scenarios of a CO2 flux at the surface and atmospheric conditions resulting in a variety of spatial and temporal behaviors of CO2 concentration in and above the mountain boundary layer. This enables the calculation of surface CO2 fluxes using the same approach as in the observations, while comparison with the prescribed fluxes allows a detailed investigation of the reliability and applicability of the budget approach in mountainous terrain.