Turbulence governs many atmospheric processes including mixing, transport, and energy transfer. Consequently, there is a strong need for the examination and validation of existing turbulence theories. The HOckey-Stick Transition (HOST) hypothesis was proposed to challenge traditional understanding of near-surface turbulence processes derived from Monin-Obukhov Similarity Theory (MOST). Within the MOST framework, the momentum flux entirely depends upon $\partial \overline{U}/\partial z$$\partial \overline{U}/\partial z$ (i.e., the change in mean wind speed ($\overline{U}$$\overline{U}$) with height ($z$$z$)), but this relationship is not as straightforward under HOST. Because HOST was developed using observations over relatively uniform, homogeneous terrain, questions arise regarding HOST's applicability within and above heterogeneous forest canopies where multi-level turbulence measurements are somewhat rare but are essential for developing a unified similarity scaling applicable over complex surfaces. To this end, we used one year (1 January 2016 through 31 December 2016) of turbulence measurements sampled at eight heights along a 60-m tower within and above a mixed deciduous forest at Chestnut Ridge in eastern Tennessee in the southeastern U.S. We examined the diurnal and seasonal variability of selected turbulence parameters (i.e., friction velocity ($u_{*}$$u_{*}$) and turbulence velocity scale ($V_{TKE}$$V_{TKE}$)) to detail the micrometeorological characteristics of the site during the study period. We then used these turbulence measurements to evaluate HOST by determining their relationship with $\overline{U}$$\overline{U}$ and to assess the dependencies of this relationship on time of day, season, wind direction, and atmospheric stability. We found that HOST is most applicable under very stable regimes, whereas the relationships between $u_{*}$$u_{*}$ and $\overline{U}$$\overline{U}$, and between $V_{TKE}$$V_{TKE}$ and $\overline{U}$$\overline{U}$, were more linear above the forest canopy than within the forest canopy and when the canopy was not foliated. Overall, this work builds upon previous studies that have described limitations in MOST and identifies scenarios when the HOST hypothesis may be more applicable than MOST for representing near-surface turbulence processes.

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关于曲棍球棒过渡假说在描述落叶林内部和上方湍流的应用


湍流控制着许多大气过程，包括混合、传输和能量传递。因此，迫切需要对现有的湍流理论进行检验和验证。HOckey-Stick Transition （HOST） 假说的提出是为了挑战源自 Monin-Obukhov 相似性理论 （MOST） 的对近地表湍流过程的传统理解。在 MOST 框架内，动量通量完全取决于 $\partial \overline{U}/\partial z$ （即平均风速 （ $\overline{U}$ ） 随高度 （ $z$ ） 的变化），但在 HOST 下，这种关系并不那么简单。由于 HOST 是使用对相对均匀、均匀地形的观测开发的，因此对 HOST 在异质森林树冠内和之上的适用性提出了问题，其中多级湍流测量有些罕见，但对于开发适用于复杂表面的统一相似性标度至关重要。为此，我们使用了一年（2016 年 1 月 1 日至 2016 年 12 月 31 日）的湍流测量，在美国东南部田纳西州东部栗树岭的落叶混交林内和上方的 60 米塔上沿八个高度采样。我们检查了选定湍流参数（即摩擦速度 （ $u_{*}$ ）和湍流速度尺度 （ $V_{TKE}$ ））的昼夜和季节性变化，以详细说明研究期间现场的微气象特征。然后，我们使用这些湍流测量值通过确定它们与 $\overline{U}$ 一天中的时间、季节、风向和大气稳定性的关系来评估 HOST，并评估这种关系对一天中的时间、季节、风向和大气稳定性的依赖性。 我们发现 HOST 在非常稳定的制度下最适用，而 $u_{*}$ 和 $\overline{U}$ 以及 和 $\overline{U}$ 之间的关系 $V_{TKE}$ 在森林冠层上方比在森林冠层内和冠层没有叶状时更线性。总体而言，这项工作建立在先前的研究基础上，这些研究描述了 MOST 中的局限性，并确定了 HOST 假设可能比 MOST 更适用于表示近地表湍流过程的情景。