Comparison of Regional Simulation of Biospheric CO2 Flux from the Updated Version of CarbonTracker Asia with FLUXCOM and Other Inversions over Asia
Authors: Takele Kenea Samuel , Labzovskii Lev , Goo Tae‐Young , Li Shanlan , Oh Young‐Suk , Byun Young‐Hwa 
Published: March 2020

&lt;p&gt;There are still large uncertainties in the estimates of net ecosystem exchange of CO&lt;sub&gt;2&lt;/sub&gt;&lt;br&gt;(NEE) with atmosphere in Asia, particularly in the boreal and eastern part of temperate Asia. To&lt;br&gt;understand these uncertainties, we assessed the CarbonTracker Asia (CTA2017) estimates of the&lt;br&gt;spatial and temporal distributions of NEE through a comparison with FLUXCOM and the global&lt;br&gt;inversion models from the Copernicus Atmospheric Monitoring Service (CAMS), Monitoring&lt;br&gt;Atmospheric Composition and Climate (MACC), and Jena CarboScope in Asia, as well as&lt;br&gt;examining the impact of the nesting approach on the optimized NEE flux during the 2001&amp;#8211;2013&lt;br&gt;period. The long&amp;#8208;term mean carbon uptake is reduced in Asia, which is &amp;#8722;0.32 &amp;#177; 0.22 PgC yr&lt;sup&gt;&amp;#8208;1&lt;/sup&gt;,&lt;br&gt;whereas &amp;#8211;0.58 &amp;#177; 0.26 PgC yr&lt;sup&gt;&amp;#8208;1&lt;/sup&gt; is shown from CT2017 (CarbonTracker global). The domain&lt;br&gt;aggregated mean carbon uptake from CTA2017 is found to be lower by 23.8%, 44.8%, and 60.5%&lt;br&gt;than CAMS, MACC, and Jena CarboScope, respectively. For example, both CTA2017 and CT2017&lt;br&gt;models captured the interannual variability (IAV) of the NEE flux with a different magnitude and&lt;br&gt;this leads to divergent annual aggregated results. Differences in the estimated interannual&lt;br&gt;variability of NEE in response to El Ni&amp;#241;o&amp;#8211;Southern Oscillation (ENSO) may result from&lt;br&gt;differences in the transport model resolutions. These inverse models&amp;#8217; results have a substantial&lt;br&gt;difference compared to FLUXCOM, which was found to be &amp;#8211;5.54 PgC yr&lt;sup&gt;&amp;#8208;1&lt;/sup&gt;. On the one hand, we&lt;br&gt;showed that the large NEE discrepancies between both inversion models and FLUXCOM stem&lt;br&gt;mostly from the tropical forests. On the other hand, CTA2017 exhibits a slightly better correlation&lt;br&gt;with FLUXCOM over grass/shrub, fields/woods/savanna, and mixed forest than CT2017. The land&lt;br&gt;cover inconsistency between CTA2017 and FLUXCOM is therefore one driver of the discrepancy in&lt;br&gt;the NEE estimates. The diurnal averaged NEE flux between CTA2017 and FLUXCOM exhibits&lt;br&gt;better agreement during the carbon uptake period than the carbon release period. Both CTA2017&lt;br&gt;and CT2017 revealed that the overall spatial patterns of the carbon sink and source are similar, but&lt;br&gt;the magnitude varied with seasons and ecosystem types, which is mainly attributed to differences&lt;br&gt;in the transport model resolutions. Our findings indicate that substantial inconsistencies in the&lt;br&gt;inversions and FLUXCOM mainly emerge during the carbon uptake period and over tropical&lt;br&gt;forests. The main problems are underrepresentation of FLUXCOM NEE estimates by limited eddy&lt;br&gt;covariance flux measurements, the role of CO&lt;sub&gt;2&lt;/sub&gt; emissions from land use change not accounted for&lt;br&gt;by FLUXCOM, sparseness of surface observations of CO&lt;sub&gt;2&lt;/sub&gt; concentrations used by the assimilation&lt;br&gt;systems, and land cover inconsistency. This suggested that further scrutiny on the FLUXCOM and&lt;br&gt;inverse estimates is most likely required. Such efforts will reduce inconsistencies across various&lt;br&gt;NEE estimates over Asia, thus mitigating ecosystem&amp;#8208;driven errors that propagate the global&lt;br&gt;carbon budget. Moreover, this work also recommends further investigation on how the&lt;br&gt;changes/updates made in CarbonTracker affect the interannual variability of the aggregate and&lt;br&gt;spatial pattern of NEE flux in response to the ENSO effect over the region of interest.&lt;/p&gt;