Turbulence statistics above and within two Amazon rain forest canopies
Turbulence statistics above and within two Amazon rain forest canopies
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DOI：
10.1023/a:1002401829007
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发表时间：
2000-02-01
影响因子：
4.3
通讯作者：
Grace, J
中科院分区：
文献类型：
--
作者：
Kruijt, B;Malhi, Y;Grace, J
湍流结构在两个亚马逊雨林的特点是一系列的冠层以上的稳定性条件，并与以前的研究结果相比，在其他森林冠层和最近的理论产生的湍流涡旋以上的森林冠层。三维风速和温度波动的数据同时收集在五个层次的内部和以上的两个冠层的30-40米高的森林，在三个独立的时期。我们分析了每小时的统计，联合概率分布，长度尺度，空间相关性和连贯性，以及功率谱的垂直和水平的风速。白天的结果表明，急剧衰减的湍流在顶部三分之一的树冠，导致非常小的运动，几乎高斯概率分布的风速，在较低的树冠。这与强烈的倾斜和kurtotic分布在上层冠层。到了晚上，衰减甚至更强，即使在上层树冠中，偏斜也消失了。在较低的冠层的功率谱峰移动到较低的频率相对于上冠层，空间相关性和相干性低，整个冠层。在树冠顶部的垂直风速的积分长度尺度是小的，约0.15小时相比，0.28小时的值，预计从剪切长度尺度在树冠顶部，基于假设，树冠上部的空气表现为一个平面混合层。所有这些都表明，虽然交换没有被完全抑制，但热带雨林的冠层与其他森林的不同之处在于，快速、连贯的向下扫掠不会穿透到较低的冠层，并且长度尺度受到抑制。这与该区域的稳定性与冠层以上条件相比持续反转有关。逆温很可能是由集中在冠层顶部附近的叶片中的强热吸收维持的，通常较弱的湍流不能破坏大冠层深度上的温度梯度。
The turbulence structure in two Amazon rain forests was characterised for a range of above-canopy stability conditions, and the results compared with previous studies in other forest canopies and recent theory for the generation of turbulent eddies just above forest canopies. Three-dimensional wind speed and temperature fluctuation data were collected simultaneously at up to five levels inside and above two canopies of 30-40 m tall forests, during three separate periods. We analysed hourly statistics, joint probability distributions, length scales, spatial correlations and coherence, as well as power spectra of vertical and horizontal wind speed.The daytime results show a sharp attenuation of turbulence in the top third of the canopies, resulting in very little movement, and almost Gaussian probability distributions of wind speeds, in the lower canopy. This contrasts with strongly skewed and kurtotic distributions in the upper canopy. At night, attenuation was even stronger and skewness vanished even in the upper canopy. Power spectral peaks in the lower canopy are shifted to lower frequencies relative to the upper canopy, and spatial correlations and coherences were low throughout the canopy. Integral length scales of vertical wind speed at the top of the canopy were small, about 0.15 h compared to a value of 0.28 h expected from the shear length scale at the canopy top, based on the hypothesis that the upper canopy air behaves as a plane mixing layer. All this suggests that, although exchange is not totally inhibited, tropical rain forest canopies differ from other forests in that rapid, coherent downward sweeps do not penetrate into the lower canopy, and that length scales are suppressed. This is associated with a persistent inversion of stability in that region compared to above-canopy conditions. The inversion is likely to be maintained by strong heat absorption in the leaves concentrated near the canopy top, with the generally weak turbulence being unable to destroy the temperature gradients over the large canopy depth.