Abstract
Abstract. 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.
Highlights
IntroductionEstimating the non-turbulent components of the CO2 mass balance (horizontal and vertical advection, FH A and FV A respectively) has become a major challenge for micrometeorologists
Estimating the non-turbulent components of the CO2 mass balance has become a major challenge for micrometeorologists
Concurrent with the multiplication of sites and the desire to measure above all kinds of ecosystems, a considerable number of sites are situated in complex terrain. In these cases and especially under nocturnal conditions associated with weak winds, non-turbulent components of the mass balance can represent a non-negligible part of the net ecosystem exchange (NEE)
Summary
Estimating the non-turbulent components of the CO2 mass balance (horizontal and vertical advection, FH A and FV A respectively) has become a major challenge for micrometeorologists. These terms are assumed to be negligible over ideal flat and homogeneous terrain and under good turbulent mixing. Concurrent with the multiplication of sites and the desire to measure above all kinds of ecosystems, a considerable number of sites are situated in complex terrain In these cases and especially under nocturnal conditions associated with weak winds, non-turbulent components of the mass balance can represent a non-negligible part of the NEE. It is necessary to understand and quantify advection processes at a variety of sites
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