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See detailNight-time airflow in a forest canopy near a mountain crest
Sedlak, Pavel; Aubinet, Marc ULg; Heinesch, Bernard ULg et al

in Agricultural and Forest Meteorology (2010), 150(5), 736-744

Night-time airflow within a deep and dense canopy near the top of a mountain ridge is investigated based on measurements at Bily Kriz, Czech Republic. The site is characterized by a young Norway spruce ... [more ▼]

Night-time airflow within a deep and dense canopy near the top of a mountain ridge is investigated based on measurements at Bily Kriz, Czech Republic. The site is characterized by a young Norway spruce forest on a 13 degrees slope and the occurrence of almost exclusively upslope or downslope flows. The forest canopy reaches the ground surface. A decoupled two-layer structure of canopy flow typically develops at night. While the above-canopy flow is most frequently an upslope-directed larger-scale flow over the ridge, the lower-canopy flow is downslope (katabatic). However, the lower-canopy flow can be forced upslope when the wind speed above the canopy exceeds a well-defined limit. Less frequently, on the lee slope to the larger-scale flow, both the above-canopy and the lower-canopy flow are usually downslope, although a flow reversal in the lower canopy is also observed, accompanied with a large shear stress (friction velocity) above the canopy. The occurrence of opposing flows is not limited to sunset/sunrise transition periods. In a simplified modelling approach to the dynamics of the nocturnal lower-canopy flow decoupled from above, local equilibrium is assumed of solely two opposing driving forces - one induced by the negative buoyancy (due to radiative cooling of the canopy) and the other by the hydrodynamic pressure gradient (resulting from the larger-scale flow over the ridge) - and the canopy drag as a retarding force. The diagnostic model gives realistic values of the major driving terms for Bily Kriz, and the downslope or upslope direction and speed of the lower-canopy flow that agree well with the measurements. The model contributes to better interpretation of the experimental results, which are in accordance with recent publications on the flow patterns on forested hills. Knowledge of the lower-canopy flow behaviour and of the degree of its decoupling from the flow aloft is necessary for assessing the contribution of advection to the CO2 budget at sloping forest sites, and for analysis of the flux footprint. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

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See detailMicrometeorological survey of air-sea ice CO2 fluxes in arctic coastal waters
Heinesch, Bernard ULg; Tison, Jean-Louis; Carnat, Gauthier et al

in Geophysical Research Abstracts (2010), 12(EGU2010-10570),

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See detailComparison of two drainage flow situations on a gentle forested slope
Heinesch, Bernard ULg; Yernaux, Michel ULg; Aubinet, Marc ULg

Poster (2010)

Drainage flows generated on a gentle slope in stable conditions were analysed at the forested site of Vielsalm (Belgium). Two distinct situations were encountered, one corresponding to vertical ... [more ▼]

Drainage flows generated on a gentle slope in stable conditions were analysed at the forested site of Vielsalm (Belgium). Two distinct situations were encountered, one corresponding to vertical convergence, characterised by a negative vertical velocity at the canopy top, the other corresponding to an equilibrium situation without any vertical movement. The causes of these two distinct flow patterns were analysed. Moreover, combined measurements of vertical velocity above the canopy and horizontal velocity divergence below the canopy were found to comply with the continuity equation: an acceleration of the horizontal flow was indeed observed in the trunk-space in convergence situations while not in equilibrium conditions. These measurements combined with those of the horizontal [CO2] gradient below the canopy were found to met the dilution hypothesis suggested by Aubinet et al.(2003): the horizontal [CO2] gradient was negative in convergence situations while slightly positive in equilibrium conditions. The existence of such patterns allows us to confirm the coherence of advection observations made at the site and help to better understand the mechanisms responsible of night flux exchanges at work in stable conditions in complex terrains. Nevertheless, difficulties were met when trying to obtain reliable estimates of the advection transport terms. The estimation of the vertical velocity in the trunk-space obtained by using the divergence measurements refuted the hypothesis of linearity of the vertical profile of vertical velocity, assumption usually used when computing vertical advection. This problem resulted in a great uncertainty on vertical advection which did not allowed to improve the night-flux estimation by adding advection terms to the turbulent flux and the storage. [less ▲]

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See detailAir-sea ice CO2 fluxes measurement with eddy-covariance micrometeorological technique
Heinesch, Bernard ULg; Aubinet, Marc ULg; Carnat, Gauthier et al

Conference (2010)

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See detailSurvey of air-ice ocean carbon dioxyde exchange over arctic sea-ice
Heinesch, Bernard ULg; Aubinet, Marc ULg; Carnat, Gauthier et al

Conference (2009)

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See detailDependence of CO2 advection patterns on wind direction on a gentle forested slope
Heinesch, Bernard ULg; Yernaux, Michel ULg; Aubinet, Marc ULg

in Biogeosciences (2008), 5(3), 657-668

Gravitational flows generated on a gentle slope in stable conditions were analysed at a forested site at Vielsalm in Belgium. There were two distinct situations at the site, one corresponding to vertical ... [more ▼]

Gravitational flows generated on a gentle slope in stable conditions were analysed at a forested site at Vielsalm in Belgium. There were two distinct situations at the site, one corresponding to vertical convergence, characterised by a negative vertical velocity at the canopy top and horizontal velocity divergence below the canopy, the other corresponding to an equilibrium situation without any vertical movement. The causes of these two distinct flow patterns were analysed. These measurements combined with those of the horizontal CO2 concentration gradient below the canopy supported the dilution hypothesis suggested by Aubinet et al. (2003): the horizontal CO2 concentration gradient is negative in convergence situations but slightly positive in equilibrium conditions. The existence of such patterns allows us to confirm the coherence of advection observations made at the site. However, the sum of turbulent CO2 flux, changes in CO2 storage and advective terms were shown to greatly overestimate the expected net ecosystem exchange in the convergence conditions. The most probable cause was identified as being a poor estimate of the vertical profile of the vertical velocity component. [less ▲]

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See detailSome Methodological Questions Concerning Advection Measurements: A Case Study
Heinesch, Bernard ULg; Aubinet, Marc ULg; Yernaux, Michel ULg

in Boundary-Layer Meteorology (2007), 122(2),

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See detailModelisation de la reponse des flux de respiration d'un sol forestier selon les principales variables climatiques.
Perrin, Dominique ULg; Laitat, E.; Aubinet, Marc ULg et al

in Biotechnologie, Agronomie, Société et Environnement = Biotechnology, Agronomy, Society and Environment [=BASE] (2004), 8(1),

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See detailHorizontal And Vertical Co2 Advection In A Sloping Forest
Aubinet, Marc ULg; Heinesch, Bernard ULg; Yernaux, Michel ULg

in Boundary-Layer Meteorology (2003), 108(3),

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See detailLong Term Carbon Dioxide Exchange Above A Mixed Forest In The Belgian Ardennes
Aubinet, Marc ULg; Chermanne, B.; Vandenhaute, M. et al

in Agricultural and Forest Meteorology (2001), 108(4),

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See detailMesure et calibration d'anémomètres à thermistors pour la mesure de faibles vitesses d'écoulements d'air.
Aubinet, Marc ULg; Yernaux, Michel ULg; Deltour, Jules

in Journal de Physique Iii (1992), 2(12),

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