The interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes.

中文翻译：

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温和地形对森林-大气气体交换的影响及其对涡度协方差测量的影响

森林上基于塔的涡流-协方差（EC）湍流通量测量的解释取决于三个关键假设：（1）流量统计的稳定性，（2）标量源或汇的平面同质性，以及（3）流量统计。大型涡流模拟（LESs）用于控制前两个，以便探索第三个对理想化和真实平缓地形的分解，例如在亚马逊地区遇到的地形。LES运行是在覆盖复杂地形的同质森林内使用均匀分布的源进行的，以将标量湍流的空间模式与地形特征联系起来。结果表明，地形引起的流动特征强烈地调节了通量，包括具有负通量的大面积区域，可补偿通量比景观通量大近一个数量级的“烟囱”区域。显着的空间异质性一直持续到至少两个冠层高度，其中大多数涡度-协方差测量是在高大的森林上方进行的。引入了异质性指数来表征和对比不同的情况，并且地形分类显示在识别负通量和增强通量区域方面具有预测能力。