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Atmosphere 2015, 6(1), 60-87; doi:10.3390/atmos6010060

Wind Regimes above and below a Temperate Deciduous Forest Canopy in Complex Terrain: Interactions between Slope and Valley Winds

1
Center for Ecological Research, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China
2
School of Forestry and Bio-technology, Zhejiang A & F University, 88 North Road of Huancheng, Lin’an 311300, China
3
Forest Analysis and Inventory Branch, Ministry of Forests, Lands, and Natural Resource Operations, 727 Fisgard Street, Victoria, BC V8W-9C2, Canada
*
Author to whom correspondence should be addressed.
Academic Editor: Albert A. M. Holtslag
Received: 15 August 2014 / Accepted: 15 December 2014 / Published: 25 December 2014
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Abstract

The thermally driven wind over mountainous terrains challenges the estimation of CO2 exchange between forests and the atmosphere when using the eddy covariance technique. In this study, the wind regimes were investigated in a temperate deciduous forested valley at the Maoershan site, Northeast China. The wind direction above the canopy was preferentially up-valley in the daytime and down-valley in the nighttime, corresponding to the diurnal patterns of above-canopy temperature gradient and stability parameter. In both leaf-on and -off nighttime, a down-valley flow with a maximum velocity of 1~3 m∙s−1 was often developed at 42 m above the ground (2.3-fold of the canopy height). However, the below-canopy prevailing wind was down-slope in the night, contrast to the below-canopy temperature lapse and unstable conditions. This substantial directional shear illustrated shallow slope winds were superimposed on larger-scale valley winds. As a consequence, the valley-wind component becomes stronger with increasing height, indicating a clear confluence of drainage flow to the valley center. In the daytime, the below-canopy wind was predominated down-slope due to the temperature inversion and stable conditions in the leaf-on season, and was mainly up-valley or down-slope in the leaf-off season. The isolation of momentum flux and radiation by the dense canopy played a key role in the formation of the below-canopy unaligned wind and inverse stability. Significant lateral kinematic momentum fluxes were detected due to the directional shear. These findings suggested a significant interaction between slope and valley winds at this site. The frequent vertical convergence / divergence above the canopy and horizontal divergence/convergence below the canopy in the nighttime / daytime is likely to induce significant advections of trace gases and energy flux. View Full-Text
Keywords: mountain wind system; valley terrain; directional shear; forest canopy; temperature gradient; eddy covariance mountain wind system; valley terrain; directional shear; forest canopy; temperature gradient; eddy covariance
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Wang, X.; Wang, C.; Li, Q. Wind Regimes above and below a Temperate Deciduous Forest Canopy in Complex Terrain: Interactions between Slope and Valley Winds. Atmosphere 2015, 6, 60-87.

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