Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
5
3
2008
10.5194/bg-5-657-2008
http://www.biogeosciences.net/5/657/2008/
http://www.biogeosciences.net/5/657/2008/bg-5-657-2008.html
http://www.biogeosciences.net/5/657/2008/bg-5-657-2008.pdf
657
668
2008-05-06
Dependence of CO<sub>2</sub> advection patterns on wind direction on a gentle forested slope
B. Heinesch
heinesch.b@fsagx.ac.be
Y. Yernaux
M. Aubinet
Unité de Physique des Biosystèmes Faculté Universitaire des Sciences Agronomiques de Gembloux 8, avenue de la Faculté, B-5030 Gembloux, Belgium
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 <i>vertical convergence</i>, characterised by a negative
vertical velocity at the canopy top and horizontal velocity divergence below
the canopy, the other corresponding to an <i>equilibrium</i> situation without any vertical
movement. The causes of these two distinct flow patterns were analysed.
These measurements combined with those of the horizontal CO<sub>2</sub>
concentration gradient below the canopy supported the dilution hypothesis
suggested by Aubinet et al. (2003): the horizontal CO<sub>2</sub> 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 CO<sub>2</sub> flux, changes in CO<sub>2</sub> 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.
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