The non-closure of surface energy balance, often encountered in eddy covariance (EC) measurements, raises a critical query: does this non-closure lead to underestimated scalar fluxes, particularly CO2 flux (Fc), when using the same theoretical framework in EC? To address this question, we utilize high-resolution large-eddy simulations (LESs) to explore correlations between energy flux imbalances and Fc imbalances in convective boundary layers, considering both homogeneous and idealized heterogeneous surfaces. Our findings reveal that the unsteady CO2 or storage represents a leading factor influencing Fc imbalance, especially notable when the entrainment ratio for Fc is large. Even in scenarios with uniform surface Fcs, heterogeneous thermally-generated turbulence resulting from variable surface sensible heat flux (H) can induce substantial horizontal flux divergence, magnifying Fc imbalance. While a linear correlation between the energy flux imbalance and Fc imbalance arises under shared causative mechanisms (e.g., storage), complex correlations emerge if their influencing factors differ, contingent upon surface heterogeneity and site location. This complexity underscores the limitations in applying the closing methods for energy flux imbalance to the Fc imbalance.

中文翻译：

---

能量平衡非闭合对二氧化碳通量不确定性的影响——来自对流边界层大涡模拟的见解


在涡度相关差 （EC） 测量中经常遇到的表面能量平衡的非闭合问题提出了一个关键问题：在 EC 中使用相同的理论框架时，这种非闭合是否会导致低估标量通量，尤其是 CO2 通量 （Fc）？为了解决这个问题，我们利用高分辨率大涡模拟 （LES） 来探索对流边界层中能量通量不平衡和 Fc 不平衡之间的相关性，同时考虑均匀和理想化的非均质表面。我们的研究结果表明，不稳定的 CO2 或储存是影响 Fc 不平衡的主要因素，当 Fc 的夹带率较大时尤其明显。即使在表面 Fc 均匀的情况下，由可变表面感热通量 （H） 引起的非均相热产生的湍流也会引起大量的水平通量发散，从而放大 Fc 不平衡。虽然能量通量不平衡和 Fc 不平衡之间的线性相关性是在共同的致病机制（例如存储）下出现的，但如果它们的影响因素不同，则会出现复杂的相关性，具体取决于表面异质性和位置。这种复杂性凸显了将能量通量不平衡的闭合方法应用于 Fc 不平衡的局限性。