Effects of topography on in‐canopy transport of gases emitted within dense forests
Effects of topography on in‐canopy transport of gases emitted within dense forests
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DOI：
10.1002/qj.3546
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发表时间：
2019-05
影响因子：
8.9
通讯作者：
Bicheng Chen;M. Chamecki;G. Katul
中科院分区：
文献类型：
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作者：
Bicheng Chen;M. Chamecki;G. Katul
考虑到在气象学、风能、空气污染、大气化学和生态学中大量应用的相关性，位于丘陵地形的密集树冠内的空气流动的重要性是没有争议的。虽然这种流动的数学描述是复杂的，但通过实验、数学建模和最近的大涡模拟（LESs）之间的相互作用，已经取得了进展。在这项贡献中，LES被用于研究地形引起的流场变化以及这些变化如何传播到冠层内的标量传输。LES运行是在一个位于二维正弦山丘上的高大茂密的森林冠层上方的中性大气边界层中进行的。树叶分布是使用在亚马逊雨林中收集的叶面积密度测量来指定的。通过一系列增加坡幅的LES运动来干扰平坦地形状态的水流。LES运行成功地再现了树冠区山背风侧的再循环区域和流动分离，与先前的实验室和LES研究一致。模拟结果表明，在冠层内部释放的空气包裹有两条优先逃离冠层区域的路径：与平坦地形相似的“局部”路径和靠近流动分离区域的“平流”路径。进一步的分析表明，在流动分离区上方的优先逃逸位置导致了“烟囱”效应，这种效应在靠近森林地面的空气团释放时被放大。本文的研究表明，剪切层湍流是两种途径中向外输出空气包裹的主要机制。然而，与平坦地形相比，地形诱导的流动分离时的平均上升气流显著缩短了低层冠层释放的空气包裹在冠层内的停留时间，从而提高了反应气体的出口分数。
The significance of air flow within dense canopies situated on hilly terrain is not in dispute given its relevance to a plethora of applications in meteorology, wind energy, air pollution, atmospheric chemistry and ecology. While the mathematical description of such flows is complex, progress has proceeded through an interplay between experiments, mathematical modelling, and more recently large‐eddy simulations (LESs). In this contribution, LES is used to investigate the topography‐induced changes in the flow field and how these changes propagate to scalar transport within the canopy. The LES runs are conducted for a neutral atmospheric boundary layer above a tall dense forested canopy situated on a train of two‐dimensional sinusoidal hills. The foliage distribution is specified using leaf area density measurements collected in an Amazon rain forest. A series of LES runs with increasing hill amplitude are conducted to disturb the flow from its flat‐terrain state. The LES runs successfully reproduce the recirculation region and the flow separation on the lee‐side of the hill within the canopy region in agreement with prior laboratory and LES studies. Simulation results show that air parcels released inside the canopy have two preferential pathways to escape the canopy region: a “local” pathway similar to that encountered in flat terrain and an “advective” pathway near the flow‐separation region. Further analysis shows that the preferential escape location over the flow‐separation region leads to a “chimney”‐like effect that becomes amplified for air parcel releases near the forest floor. The work here demonstrates that shear‐layer turbulence is the main mechanism exporting air parcels out the canopy for both pathways. However, compared to flat terrain, the mean updraught at the flow separation induced by topography significantly shortens the in‐canopy residence time for air parcels released in the lower canopy, thus enhancing the export fraction of reactive gases.