Abstract Concurrent below and above canopy sonic anemometer vertical velocity (w) measurements reveal frequent decoupling events between the air masses below and above the canopy at a dense spruce forest stand in mountainous terrain. Decoupling events occurred predominantly during nighttime but not exclusively. Several single-level approaches based on steady state and integral turbulence characteristic tests as well as friction velocity (u*) filtering and two-level CO2 flux filtering methods are tested. These tests aimed at evaluating the filtering schemes to address decoupling and its effect on above canopy derived eddy covariance net ecosystem CO2 exchange (NEE). In addition to the already existing two-level filtering approach based on the correlation of σw above and below canopy, two new filtering methods are introduced based on w raw data below and above the canopy. One is a telegraphic approximation agreement, which assumes coupling when w both above and below canopy are pointing in the same direction. Another one evaluates the cross-correlation maximum between below and above canopy w data. This study suggests that none of the single-level approaches can detect decoupling when compared to two-level filtering approaches. It further suggests that the newly introduced two-level approaches based on w raw data may have advantages in comparison to the conventional σw approach regarding their flexibility on shorter time scales than one year. We tested the correlation of the newly introduced filtering approaches with the parameters u*, global radiation, buoyancy forcing across the canopy and wind shear across the canopy. In any case, this correlation was not existing or weakly positive, suggesting that concurrent below and above canopy measurements are necessary for addressing decoupling. Sonic anemometer measurements near the forest floor and above the canopy are adequate to apply the new procedures and can be implemented in a routine manner at any forest site globally.

中文翻译：

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评估复杂地形中挪威云杉林冠层上方和下方气团的解耦

摘要 冠层下方和上方同时进行的声波风速计垂直速度 (w) 测量显示，在山区茂密的云杉林林分中，冠层下方和上方的气团之间频繁发生解耦事件。脱钩事件主要发生在夜间，但并非完全如此。测试了几种基于稳态和积分湍流特性测试以及摩擦速度 (u*) 过滤和两级 CO2 通量过滤方法的单级方法。这些测试旨在评估过滤方案以解决解耦问题及其对以上冠层衍生的涡流协方差净生态系统 CO2 交换 (NEE) 的影响。除了已经存在的基于冠层上方和下方 σw 相关性的两级过滤方法，基于冠层下方和上方的 w 原始数据，引入了两种新的过滤方法。一种是电报近似协议，当冠层上方和下方的 w 都指向同一方向时，它假设耦合。另一个评估低于和高于冠层 w 数据之间的互相关最大值。这项研究表明，与两级过滤方法相比，没有一种单级方法可以检测到解耦。这进一步表明，与传统的 σw 方法相比，新引入的基于 w 原始数据的两级方法可能具有优势，因为它们在比一年更短的时间尺度上具有灵活性。我们测试了新引入的过滤方法与参数 u*、全局辐射、穿过冠层的浮力和穿过冠层的风切变的相关性。在任何情况下，这种相关性都不存在或呈弱正相关，这表明同时进行冠层下方和上方的测量对于解决脱钩问题是必要的。靠近森林地面和冠层上方的声波风速计测量足以应用新程序，并且可以在全球任何森林地点以常规方式实施。