The Influence of Surface Flux Distribution and Magnitude on the Atmospheric Concentration Signals at Towers within North America

This presentation examines the ability of the atmospheric CO2 monitoring network to detect differences in the spatial distribution and magnitude between plausible scenarios of land-atmosphere carbon flux. The net ecosystem exchange (NEE) estimates of four terrestrial biospheric models (TBMs) are used to represent different “realizations” of surface flux distributions, which are compared in terms of their resulting atmospheric signals. The analysis focuses on North America, and uses the nine towers of the 2004 network. The study includes four test cases designed to isolate the influence of the following on the atmospheric measurements: (1) overall flux differences; (2) magnitude differences in flux across large regions; (3) differences in the flux patterns within given regions; and (4) the relative influence of flux variability in the near- and far-field of measurement locations. Using a Chi-squared test of variance, the CO2 signals generated from the different representations of surface flux distribution are compared, and the concentration measurements are found to be able to distinguish between overall differences in flux distributions. The differences observed are driven primarily by differences in flux magnitude over large-scales, and the fine-scale (mostly temporal) variability of fluxes within the near-field of observation locations. However, differences in the spatial distribution of fluxes within individual ecoregions do not appear to translate into significant differences in the observed signals at the towers. Thus, given the types of variation in flux represented by the four TBMs, the atmospheric data may be most informative in the evaluation of aggregated fluxes over large spatial scales (e.g., ecoregions), as well as in the improvement of how the diurnal cycle of fluxes is represented in TBMs, particularly in areas close to tower locations.