The eddy covariance (EC) technique has emerged as the method of choice for observing ecosystem–atmosphere interactions across biomes and climate zones. However, EC measurements are biased when the turbulent flow is decoupled from the underlying surface, severely limiting the applicability of the technique in observing surface–atmosphere fluxes. Friction velocity (u∗) is typically used to detect and filter these periods from EC flux time series. The processes that control decoupling are understood qualitatively, including the strength of vertical turbulent mixing, stable stratification and canopy drag. However, the standard practice utilising u∗ misses most of these processes, resulting in a significant uncertainty in detecting decoupling. Consequently, a quantitative metric, Ω, which encapsulates all these processes in a unified framework, was recently proposed. However, it has not yet been systematically tested over a range of ecosystems and site characteristics. The objectives of this study were therefore to test the efficacy of Ω at a diverse range of EC sites, to quantify the processes controlling decoupling across sites, and to compare Ω against other decoupling metrics, such as u∗. A similar Ω threshold value for coupling was observed at all the 45 tested EC sites, with a value of 0.66 ± 0.06 (mean ± standard deviation). This indicates that the Ω metric captured the essential features of decoupling across sites, thereby enabling deeper analyzes of the causes of decoupling. For example, Ω indicates that (1) flows above dense forest canopies can be decoupled from the forest floor also during the daytime due to canopy drag and that (2) during stable stratification decoupling is more likely with tall towers. These findings significantly enhance our scientific understanding of the underlying causes of decoupling, will inform improved analyzes of EC data and support near-surface turbulence transport analyzes in open and forested landscapes.

中文翻译：

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迈向在涡流协方差通量观测中检测垂直流解耦的增强度量


涡度相关 （EC） 技术已成为观察跨生物群落和气候区的生态系统-大气相互作用的首选方法。然而，当湍流与下垫表面解耦时，EC 测量会有偏差，严重限制了该技术在观察表面-大气通量方面的适用性。摩擦速度 （u∗） 通常用于从 EC 磁通量时间序列中检测和过滤这些周期。控制脱耦的过程是定性的，包括垂直湍流混合的强度、稳定的分层和冠层阻力。然而，使用 u∗ 的标准做法错过了大部分这些过程，导致检测脱钩存在很大的不确定性。因此，最近提出了一种定量指标 Ω，它将所有这些过程封装在一个统一的框架中。然而，它还没有在一系列生态系统和场地特征上进行系统测试。因此，本研究的目的是测试Ω在不同 EC 站点的有效性，量化控制跨站点脱钩的过程，并将Ω与其他脱钩指标（如 u∗）进行比较。在所有 45 个测试的 EC 位点中观察到相似的偶联 Ω 阈值，值为 0.66 ± 0.06 （平均值±标准差）。这表明 Ω 指标捕获了跨站点解耦的基本特征，从而能够更深入地分析解耦的原因。例如，Ω 表示 （1） 由于树冠阻力，茂密森林树冠上方的水流也可以在白天与森林地面分离，并且 （2） 在稳定的分层期间，高塔更有可能脱钩。 这些发现显着增强了我们对脱钩根本原因的科学理解，将为改进 EC 数据的分析提供信息，并支持开阔和森林景观中的近地表湍流输运分析。