On the CO2 exchange between the atmosphere and the biosphere: the role of synoptic and mesoscale processes

Estimating global carbon fluxes by inverting atmospheric CO₂ through the use of atmospheric transport models has shown the importance of the covariance between biospheric fluxes and atmospheric transport on the carbon budget. This covariance or coupling occurs on many time scales. This study examines the coupling of the biosphere and the atmosphere on the meso- and synoptic scales using a coupled atmosphere—biosphere regional model covering Canada. The results are compared with surface and light aircraft measurement campaigns at two boreal forest sites in Canada (Fraserdale and BERMS).

Associated with cold and warm frontal features, the model results showed that the biospheric fluxes are strongly coupled to the atmosphere through radiative forcing. The presence of cloud near frontal regions usually results in reduced photosynthetic uptake, producing CO₂ concentration gradients across the frontal regions on the order of 10 parts per million (ppm). Away from the frontal region, the biosphere is coupled to the mesoscale variations in similar ways, resulting in mesoscale variations in CO₂ concentrations of about 5 ppm.

The CO₂ field is also coupled strongly to the atmospheric dynamics. In the presence of frontal circulation, the CO₂ near the surface can be transported to the mid to upper troposphere. Mesoscale circulation also plays a significant part in transporting the CO₂ from the planetary boundary layer (PBL) to the mid-troposphere. In the absence of significant mesoscale or synoptic scale circulation, the CO₂in the PBL has minimal exchange with the free troposphere, leading to strong gradients across the top of the PBL. We speculate that the ubiquity of the common synoptic and mesoscale processes in the atmosphere may contribute significantly to the rectifier effect and hence CO₂ inversion calculations.