New Transfer Functions for Correcting Turbulent Water Vapour Fluxes

in Boundary-Layer Meteorology (2010), 137(2), 205-221

We address the problem of the high-frequency correction of water vapour fluxes measured by eddy covariance with a closed-path infrared gas analyser (IRGA). Different transfer functions are compared and ... [more ▼]

We address the problem of the high-frequency correction of water vapour fluxes measured by eddy covariance with a closed-path infrared gas analyser (IRGA). Different transfer functions are compared and evaluated at a forested (Vielsalm, Belgium) and an agricultural (Lonz,e, Belgium) site. Classical functions, usually applied to correct CO2 fluxes (Gaussian, Lorentzian), are found to be unsuited to water vapour cospectral corrections, being characterised by too sharp a decrease at high frequency. Two other functions characterised by a lower decreasing slope are found to better fit experimental transfer functions. They were calibrated and validated on experimental transfer functions and their dependency on air humidity is parameterised. On this basis, new correction coefficients are estimated. The coefficients are found to be larger than those based on the classical functions, even when the dependency of the latter on air humidity is taken into account. The difference amounts to 10 at the forested site and to 5 larger at the crop site. The study highlights the necessity of characterising the water transfer function shape and taking it into account in the correction factor at each site equipped with a closed path IRGA. [less ▲]

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 ▲]

Footprint Analysis For Measurements Over A Heterogeneous Forest

in Boundary-Layer Meteorology (2000), 97(1),