Download Assessment of Uncertainties in Atmospheric Transport and Surface Flux of Carbon from the North American Terrestrial Biosphere Book in PDF, ePub and Kindle
The North American terrestrial biosphere acts as a strong sink of atmospheric CO2 and therefore plays a key role in the global carbon cycle. The atmospheric inversion approach is used to quantify the magnitude and distribution land-atmosphere carbon exchange, and requires accurate atmospheric transport and surface flux prior. We evaluate the relative sensitivity of simulated atmospheric total, biological, and fossil fuel CO2 mole fractions in the atmospheric boundary layer and integrated column over North America to changes in transport model and surface fluxes. We run three versions of a mesoscale model that incorporate different physics parameterization schemes and identical surface fluxes; we run the same mesoscale transport model with two different surface fluxes. All simulations are conducted for North America during 2008. Observed CO2 mole fractions reveal that seasonal amplitude ranges from 13 ppm in the West to over 34 ppm in the Midcontinent, and the models tested match these amplitudes to within a few ppm. Biology drives both the magnitude of the seasonal amplitude and regional differences in amplitude. Fossil fuels exhibit a seasonal cycle that is smaller than biological CO2, but not trivial. During the growing season, variations in surface fluxes yield mean differences in regionally, seasonally averaged atmospheric boundary layer total CO2 mole fractions that are larger for all regions than those resulting from varied transport model. The relative contributions of biological and fossil fuel to total mean difference CO2 show distinct quantitative patterns for varied flux and transport, and can provide information for attributing model-model differences in total CO2. Seasonal amplitude is much greater in the ABL than in the integrated column. Simulated total biological, and fossil fuel integrated column XCO2 are about 1/10th the magnitude of their signal in the atmospheric boundary layer. Flux and transport differences are also found in the integrated column at approximately 1/10th their atmospheric boundary layer values. While transport error is a significant problem for identifying terrestrial carbon fluxes, it is not an overwhelming one. Our work indicates that there is potential for remotely sensed integrated column XCO2 to distinguish between the flux signal and transport errors. Understanding transport error deserves more study, motivating current and future observational campaigns and modeling.While reducing transport uncertainty in atmospheric inversions has received considerable attention in recent years, quantification of carbon surface flux uncertainty remains a challenge. Model-observation studies can help identify model temporal and spatial limitations. To this end, we organize 166 CO2 flux tower measurement sites across North America by region, climate, and vegetation type into 23 groupings. The data span from 2000 through 2014 and are compared to output from eight atmospheric inverse estimates and 17 terrestrial biosphere models. We generate a mean year of observed and simulated net ecosystem exchange for each regional vegetation group and for each model. The NOAA CarbonTracker inverse estimates, major carbon flux inverse products, almost always underestimate amplitude of the seasonal cycle (biased positive relative to observations) and have a small spread. Furthermore, the inversions dont typically improve upon the prior with respect to the observations. Groups characterized by large seasonal amplitudes are not well represented by the models. For these groups, drawdown is underestimated. The terrestrial biosphere models often encompass the observations, but may have too much model-model variability. No one model is best everywhere. Model performance varies by vegetation and location. Certain biomes are well represented, certain biomes are not, and some models are reliably better than others. In general, evergreen forests in the north and east are better represented by the models than grasslands or crops in the midcontinent and southwest. Our large-scale, regional approach to model-observation analyses provides insight into the vegetation- and location-dependent performance of many inverse and terrestrial biosphere model estimates of land-atmosphere carbon exchange. This can help inform selection and application of surface flux priors in future inversions.
