Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy
Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy
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夜间低空急流对高大森林冠层涡相关通量的影响
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
E. Mursch
中科院分区：
文献类型：
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作者：
T. Prabha;M. Leclerc;A. Karipot;D. Hollinger;E. Mursch
本文讨论了在美国Howland AmeriFlux观测站观测到的低空急流及其对高大森林冠层上空夜间湍流交换和标量通量的影响。低频运动和湍流爆发的特点是中等强度的LLJ，而低频运动的抑制期间，强LLJ和增强剪切。基于剪切遮挡假设的分析旨在阐明LLJ对通量测量的影响。在没有剪切遮蔽的情况下，大涡流穿透粗糙子层，引起增强的混合，而在剪切遮蔽期间，混合减少。在没有后者的情况下，“倒置”涡是主要负责增强的速度方差，标量和动量通量。林冠上的积分长度尺度大于林冠高度。小波分析的方差谱和协谱表明，大涡（空间尺度大于低空急流高度）与活跃的冠层尺度湍流相互作用，有助于反梯度标量通量。
Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.