Coherent Eddy Structures Over Plant Canopies
Coherent Eddy Structures Over Plant Canopies
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植物冠层上的相干涡流结构
DOI：
10.1002/9781118527221.ch10
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
J. Finnigan
中科院分区：
文献类型：
--
作者：
R. Shaw;E. Patton;J. Finnigan
二十多年来，在植被冠层和大气表层最低层之间的界面上一直有相关的流动模式的报道，这一区域被称为粗糙度亚层。统计分析提供了这种组织的第一批迹象。后来，林地上的仪表塔在标量的时间痕迹上显示出了类似斜坡的模式。当与快速响应风速仪测量的速度场耦合时，形成了一幅重复的下游喷流和上游扫掠组合的图像，形成了流向收敛和标量微锋的形成。大涡模拟(LES)既能再现冠层和粗糙度次层流的统计特性，又能再现场中观测到的湍流结构。此外，LES还阐明了冠层流动结构的涡旋性质，揭示了迎头涡和头低涡的组合，前者与抛射有关，后者与扫掠有关。最近的模拟表明，在热不稳定条件下，在整个边界层底部的森林树冠内的流动显示了地表加热对粗糙层结构和地表热量、质量和动量交换机制的影响。随着浮力不稳定性的增加，动量通量和标量通量在空间中变得越来越分离。对流单体模式产生了增强的冠顶切变和大尺度下沉气流下的冠层阻力增加的区域。在上升气流区域，冠顶羽状物聚集形成开放单体中尺度结构的壁面，并减小冠层内阻力。
For more than two decades, coherent flow patterns have been reported at the interface between vegetation canopies and the lowest levels of the atmospheric surface layer, a region called the roughness sublayer. Statistical analyses provided the first indications of such organization. Later, instrumented towers in forested lands revealed ramplike patterns in time traces of scalar quantities. When coupled with the velocity field measured by fast‐response anemometry, a picture was formed of a repeated downstream ejection and upstream sweep combination creating streamwise convergence and the formation of scalar microfronts. Large‐eddy simulation (LES) has been shown to reproduce both the statistics of canopy and roughness sublayer flow, and the turbulence structures detected in the field. Further, LES has allowed elucidation of the vortical nature of canopy flow structures and revealed a combination of head‐up and head‐down vortices, the former associated with the ejection and the latter with the sweep. Recent simulations that resolve flow within a forest canopy at the base of a full boundary layer under thermally unstable conditions illustrates the impact of surface heating on roughness sublayer structure and on mechanisms for heat, mass and momentum exchange at the surface. With increasing buoyant instability, momentum and scalar fluxes become increasingly disassociated in space. Convective cellular patterns produce regions of enhanced canopy‐top shear and increased canopy drag beneath large‐scale downdrafts. In updraft areas, canopy‐top plumes coalesce to form the walls of the open‐cell mesoscale structure, and diminished within‐canopy drag.