Abstract
Abstract. We present a performance assessment of the European Integrated Carbon Observing System (ICOS) atmospheric network for constraining European biogenic CO2 fluxes (hereafter net ecosystem exchange, NEE). The performance of the network is assessed in terms of uncertainty in the fluxes, using a state-of-the-art mesoscale variational atmospheric inversion system assimilating hourly averages of atmospheric data to solve for NEE at 6 h and 0.5° resolution. The performance of the ICOS atmospheric network is also assessed in terms of uncertainty reduction compared to typical uncertainties in the flux estimates from ecosystem models, which are used as prior information by the inversion. The uncertainty in inverted fluxes is computed for two typical periods representative of northern summer and winter conditions in July and in December 2007, respectively. These computations are based on a observing system simulation experiment (OSSE) framework. We analyzed the uncertainty in a 2-week-mean NEE as a function of the spatial scale with a focus on the model native grid scale (0.5°), the country scale and the European scale (including western Russia and Turkey). Several network configurations, going from 23 to 66 sites, and different configurations of the prior uncertainties and atmospheric model transport errors are tested in order to assess and compare the improvements that can be expected in the future from the extension of the network, from improved prior information or transport models. Assimilating data from 23 sites (a network comparable to present-day capability) with errors estimated from the present prior information and transport models, the uncertainty reduction on a 2-week-mean NEE should range between 20 and 50 % for 0.5° resolution grid cells in the best sampled area encompassing eastern France and western Germany. At the European scale, the prior uncertainty in a 2-week-mean NEE is reduced by 50 % (66 %), down to ~ 43 Tg C month−1 (26 Tg C month−1) in July (December). Using a larger network of 66 stations, the prior uncertainty of NEE is reduced by the inversion by 64 % (down to ~ 33 Tg C month−1) in July and by 79 % (down to ~ 15 Tg C month−1) in December. When the results are integrated over the well-observed western European domain, the uncertainty reduction shows no seasonal variability. The effect of decreasing the correlation length of the prior uncertainty, or of reducing the transport model errors compared to their present configuration (when conducting real-data inversion cases) can be larger than that of the extension of the measurement network in areas where the 23 station observation network is the densest. We show that with a configuration of the ICOS atmospheric network containing 66 sites that can be expected on the long-term, the uncertainties in a 2-week-mean NEE will be reduced by up to 50–80 % for countries like Finland, Germany, France and Spain, which could significantly improvement (and at least a high complementarity to) our knowledge of NEE derived from biomass and soil carbon inventories at multi-annual scales.
Highlights
Accurate information about the terrestrial biogenic CO2 fluxes is needed at the regional scale to understand the drivers of the carbon cycle (Ciais et al, 2014)
Due to varying transport conditions, standard deviation of the prior uncertainty at the grid scale, and observation error, the spatial distribution of uncertainty reduction is found to vary from summer to winter
The results obtained with the various experiments from this study indicate an uncertainty reduction, which ranges between ∼ 50 and 80 % for the full European domain, between ∼ 70 and 90 % for large countries in western Europe, where the Integrated Carbon Observing System (ICOS) network is denser, but below 50 % in many cases for eastern countries where there are few ICOS sites even with the ICOS66 configuration
Summary
Accurate information about the terrestrial biogenic CO2 fluxes (hereafter net ecosystem exchange – NEE) is needed at the regional scale to understand the drivers of the carbon cycle (Ciais et al, 2014). The spread of the results from the different global inversion studies and the diagnostics by some of these studies demonstrates that the uncertainties remain large at the 1 month and continental scale (Peylin et al, 2013). Such large uncertainties are mainly due to the lack of observations over the continents or to the limited ability of global systems to account for dense observation networks in addition to errors in large-scale atmospheric transport models. With an increasing number of continuous atmospheric CO2 observations, primarily in North America and Europe, and with the development of regional inversion systems using high-resolution mesoscale atmospheric transport models and solving for NEE at typical resolutions of 10 to 50 km (Lauvaux et al, 2008, 2012; Schuh et al, 2010; Broquet et al, 2011; Meesters et al, 2012), there is an increasing ability to constrain NEE at continental to regional scales
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