Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy |
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Authors: | Thara V Prabha Monique Y Leclerc Anandakumar Karipot David Y Hollinger Erich Mursch-Radlgruber |
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Institution: | (1) Laboratory for Environmental Physics, The University of Georgia, 1109 Experiment Street, Griffin, GA 30223, USA;(2) Department of Biological and Agricultural Engineering, The University of Georgia, Griffin, GA, USA;(3) USDA Forest Service, Durham, NH, USA;(4) Institute fuer Meteorologie und Physik (IMP-BOKU), Vienna, Austria |
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Abstract: | 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. |
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Keywords: | Carbon-dioxide flux Countergradient fluxes Eddy covariance Low-level jet Shear sheltering Wavelet analysis |
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