The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study
The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study
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DOI：
10.1007/s10546-009-9423-1
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发表时间：
2009-08
影响因子：
4.3
通讯作者：
Jing Huang;M. Cassiani;J. Albertson
中科院分区：
文献类型：
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作者：
Jing Huang;M. Cassiani;J. Albertson
陆地生态系统的特点是树冠植被密度范围很大，众所周知，这会影响树冠-大气界面上的湍流输送过程。在存在致密的水平均匀的冠层时，冠层亚层被描述为类似于平面混合层。在另一个极端，在基本上没有冠层的情况下，冠层亚层通常被假定为类似于粗糙表面上的湍流边界层。然而，随着植被密度的增加，冠层湍流是如何从边界层型转变为混合层型的，目前还不清楚。我们使用大涡模拟来研究五种不同的植被密度，从非常稀疏的树冠到非常密集的树冠。这项研究利用了流动统计和冠层内大尺度相干湍流结构的研究。利用适当的正交分解来识别相干结构。速度分量的偏斜度和特征长度尺度的结果表明，随着植被密度的增加，冠层湍流逐渐经历了类似于粗糙壁边界层到类似于混合层的转变。正如其他人所证明的，我们发现冠层亚层内的相干结构由一个椭圆截面的反向旋转涡对构成的强扫掠/抛射运动构成。随着树冠变得更密，这些重要的结构在这里显示得更高。植被密度似乎对相干结构所代表的总湍流动能的百分比没有显著影响。
Terrestrial ecosystems are characterized by a wide range of canopy vegetation density, which is known to affect turbulent transport processes across the canopy–atmosphere interface. In the presence of a dense and horizontally homogeneous canopy, the canopy sublayer has been described as resembling a plane mixing layer. At the other extreme, where the canopy is essentially absent, the canopy sublayer is typically assumed to be similar to a turbulent boundary layer over a rough surface. However, it remains unclear how the canopy turbulence changes from boundary-layer-like to mixing-layer-like as the vegetation density increases. We use large-eddy simulation to study five different vegetation densities varying from an extremely sparse canopy to an extremely dense canopy. This investigation draws on the study of flow statistics as well as large-scale coherent turbulent structures within the canopy sublayer. The coherent structures are identified through the use of proper orthogonal decomposition. The results of skewness of velocity components and characteristic length scales suggest that, as the vegetation density increases, the canopy turbulence gradually undergoes a transition from resembling a rough-wall boundary layer to being similar to a mixing layer. As demonstrated by others, we found that the coherent structures within the canopy sublayer consist of a strong sweep/ejection motion framed by a counter-rotating vortex pair with elliptical cross-sections. As the canopy becomes denser, these important structures are shown here to be more elevated. Vegetation density does not appear to have a significant effect on the percentage of the total turbulent kinetic energy that is represented by the coherent structures.