Dependence of CO₂ advection patterns on wind direction on a gentle forested slope

B. Heinesch (1) , M. Yernaux (1) , M. Aubinet (1)

1 Unité de Physique des Biosystèmes Faculté Universitaire des Sciences Agronomiques de Gembloux 8

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.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

B. Heinesch, M. Yernaux, M. Aubinet. Dependence of CO₂ advection patterns on wind direction on a gentle forested slope. Biogeosciences Discussions, 2007, 4 (6), pp.4229-4260. ⟨hal-00297941⟩