Terrestrial biosphere models (TBMs) play a key role in the detection and attribution of carbon cycle processes at local to global scales and in projections of the coupled carbon-climate system. TBM evaluation commonly involves direct comparison to eddy-covariance flux measurements. We use atmospheric CO₂ mole fraction ([CO₂]) measured in situ from aircraft and tower, in addition to flux-measurements from summer 2016 to evaluate the Carnegie-Ames-Stanford-Approach (CASA) TBM. WRF-Chem is used to simulate [CO₂] using biogenic CO₂ fluxes from a CASA parameter-based ensemble and CarbonTracker version 2017 (CT2017) in addition to transport and CO₂ boundary condition ensembles. The resulting “super ensemble” of modeled [CO₂] demonstrates that the biosphere introduces the majority of uncertainty to the simulations. Both aircraft and tower [CO₂] data show that the CASA ensemble net ecosystem exchange (NEE) of CO₂ is biased high (NEE too positive) and identify the maximum light use efficiency E_max a key parameter that drives the spread of the CASA ensemble in summer 2016. These findings are verified with flux-measurements. The direct comparison of the CASA flux ensemble with flux-measurements confirms missing sink processes in CASA. Separating the daytime and nighttime flux, we discover that the underestimated net uptake results from missing sink processes that result in overestimation of respiration. NEE biases are smaller in the CT2017 posterior biogenic fluxes, which assimilate observed [CO₂]. Flux tower analyses reveal an unrealistic overestimation of nighttime respiration in CT2017 which we attribute to limited flexibility in the inversion strategy.

中文翻译：

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联合 CO2 摩尔分数和通量分析证实了北美 CASA 陆地碳吸收过程的缺失

陆地生物圈模型 (TBM) 在局部到全球范围内碳循环过程的检测和归因以及耦合碳气候系统的预测中发挥着关键作用。TBM 评估通常涉及与涡流协方差通量测量的直接比较。除了 2016 年夏季的通量测量值外，我们还使用从飞机和塔架原位测量的大气 CO ₂摩尔分数 ([CO ₂ ]) 来评估卡内基-艾姆斯-斯坦福方法 (CASA) TBM。除了运输和 CO ₂之外，WRF-Chem 还使用来自 CASA 参数的集合和 CarbonTracker 2017 版 (CT2017) 的生物 CO ₂通量来模拟 [CO ₂ ]边界条件集合。由此产生的建模 [CO ₂ ] 的“超级集合”表明，生物圈为模拟引入了大部分不确定性。飞机和塔[CO ₂ ]的数据显示，CO的CASA合奏净生态系统交换（NEE）₂被偏置为高（NEE太正），并确定最大光利用效率ë_最大2016 年夏季推动 CASA 集合传播的关键参数。这些发现已通过通量测量得到验证。CASA 通量集合与通量测量的直接比较证实了 CASA 中缺少汇过程。分离白天和夜间通量，我们发现低估的净吸收是由于缺少汇过程导致高估呼吸。在 CT2017 后生物源通量中，NEE 偏差较小，可同化观察到的 [CO ₂ ]。通量塔分析揭示了 CT2017 中对夜间呼吸的不切实际的高估，我们将其归因于反演策略的灵活性有限。