Use of the daily-differencing approach to evaluate uncertainties in eddy covariance measurements

Conference (2007, June)

Ten years of CO2 flux measurements at the Vielsalm forest site.

Poster (2007)

Some Methodological Questions Concerning Advection Measurements: A Case Study

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

Mesure et analyse du transport advectif de CO2 dans une forêt sur versant

Doctoral thesis (2007)

The micrometeorological technique of eddy-covariance is used for the estimation of the CO2 exchange between the ecosystems and the atmosphere. Hundreds of instrumented sites, spread all over the world ... [more ▼]

The micrometeorological technique of eddy-covariance is used for the estimation of the CO2 exchange between the ecosystems and the atmosphere. Hundreds of instrumented sites, spread all over the world, use it henceforth to study a great variety of ecosystems. This technique is however affected by a systematic error when applied above tall canopies like forests, in stable atmospheric conditions, i.e. primarily during non windy nights without cloud cover. Indeed, during these periods the turbulent transport would be competed with by another transport mechanism which is called advection. In this work, the presence of advection has been tested on the experimental forested site of Vielsalm (Belgium) and its importance has been evaluated. For this purpose, an experimental set-up allowing the measurements of wind velocity, CO2 concentration and temperature of the air inside the forest has been installed. It has allowed the description, for the stable periods, of gravitational flows being carried out close to the ground due to the cooling of surfaces and the presence of a weak slope. These flows were shown to be responsible for advective CO2 transport. An uncertainty analysis has been carried out using dedicated measurement campaigns. It has conclude with the feasibility of measurements of vertical and especially horizontal CO2 gradients on the site but has shown that the greatest factor of uncertainty related to the estimate of the vertical velocity component above and in the forest. In spite of these uncertainties, a fine analysis of the gravitational episodes has made it possible to highlight a coherent mechanism linking the flow field and the CO2 concentration field and making it possible to better understand how CO2 could be transported laterally and vertically by the gravitational flows. Finally, the feasibility of a correction based on the estimate of the advective terms has been evaluated. It has been shown that uncertainties relating to advection were too important to make it possible to appreciably improve the night assessment of CO2 fluxes by means of this method. This one proves however interesting for better understanding the processes of transport at work in a forest cover. [less ▲]

Coordinate rotations

Conference (2006)

Comparing CO2 storage and advection conditions at night at different carboeuroflux sites

in Boundary-Layer Meteorology (2005), 116(1), 63-94

Anemometer and CO2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While ... [more ▼]

Anemometer and CO2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the 'advection velocity', which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO2 removal from the ecosystem and to CO2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates. [less ▲]

Quality Analysis Applied On Eddy Covariance Measurements At Complex Forest Sites Using Footprint Modelling

in Theoretical and Applied Climatology (2005), 80(2-4),

Europe-Wide Reduction In Primary Productivity Caused By The Heat And Drought In 2003

in Nature (2005), 437(7058),

Horizontal And Vertical Co2 Advection In A Sloping Forest

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

Analyse de la qualité des mesures de flux de CO2 échangés par les écosystèmes forestiers

Master of advanced studies dissertation (2001)