John C. Lin, Derek V. Mallia, Dien Wu, and Britton B. Stephens
Despite the need for researchers to understand terrestrial biospheric carbon fluxes to account for carbon cycle feedbacks and predict future CO2 concentrations, knowledge of these fluxes at the regional scale remains poor. This is particularly true in mountainous areas, where lack of observations leads to significant uncertainties in carbon fluxes. Yet mountainous regions are also where significant forest cover and biomass are foundareas that have the potential to serve as carbon sinks. As CO2 observations in mountainous areas increase in the future, it is imperative that they are properly interpreted to yield information about carbon fluxes. In this paper, we will present CO2 observations at 3 sites in the mountains of Western U.S., along with atmospheric simulations that attempt to extract information about biospheric carbon fluxes from the CO2 observations, focusing especially on the observed and simulated diurnal cycles of CO2. We show that atmospheric models can systematically simulate the wrong diurnal cycle and significantly misinterpret the CO2 observations, leading to large errors in the retrieved biospheric fluxes, due to erroneous atmospheric flows as a result of terrain that is misrepresented in the model. The problems are exacerbated as the spatial resolution is degraded, and our results indicate that a fine grid spacing of ~4km or less may be needed to simulate a realistic diurnal cycle of CO2 for the sites on top of the steep mountains examined here in the American Rockies. In the absence of higher resolution models, we recommend coarse-scale models to focus on assimilating afternoon CO2 observations on mountaintop sites over the continent.