Some methodological questions concerning advection measurements: a case study

Abstract

A dataset from two campaigns conducted at the Vielsalm experimental site in Belgium was used as a basis for discussing some methodological problems and providing intermediate results on estimating CO₂ advection. The analysis focused on the horizontal [CO₂] gradient and on the vertical velocity w, the variables most affected by uncertainty. The sampling error for half-hourly horizontal [CO₂] gradients was estimated to be 1.3 μmol mol⁻¹. Despite this important random error for half-hour estimations of [CO₂], the mean horizontal [CO₂] gradients in advective conditions were shown to be representative at the ecosystem scale and to extend only to the lowest part of a drainage sub-layer, which developed in the trunk space. By contrast, under daytime conditions, this gradient was shown to be more sensitive to local source heterogeneities. The estimation of the short-term averaged vertical velocity ( $\bar{w})$ was the greater source of error when computing advection terms. The traditional correction methods used to obtain $\bar{w}$ are discussed and a (co)sine correction is tested to highlight the instrumental origin of the offset in w. A comparison of measurements by sonic anemometers placed close together above the canopy showed that the uncertainty on $\bar{w}$ was 0.042 m s⁻¹, which is of the same order of magnitude as the velocity itself. In addition, as the drainage sub-layer is limited to the lowest part of the canopy, the representativeness of $\bar{w}$ is questionable. An alternative computation using the divergence of the horizontal wind speed in the trunk space produced a $\bar{w}$ estimation that was four times lower than the single-point measurement. However, this value gives a more realistic estimate of the vertical advection term and improves the CO₂ budget closure at the site.