Abstract
Abstract. An analysis of flow tilt angles from a fetch-limited beech forest site with clearings is presented in the context of vertical advection of carbon dioxide. Flow angles and vertical velocities from two sonic anemometers by different manufacturers were analyzed. Instead of using rotations, where zero-flow angles were assumed for neutral flow, the data was interpreted in relation to upstream and downstream forest edges. Uncertainties caused by flow distortion, vertical misalignment and limited sampling time (statistical uncertainty) were evaluated and found to be highly significant. Since the attack angle distribution of the wind on the sonic anemometer is a function of atmospheric stratification, an instrumental error caused by imperfect flow distortion correction is also a function of the atmospheric stratification. In addition, it is discussed that the sonic anemometers have temperature dependent off-sets. These features of the investigated sonic anemometers make them unsuitable for measuring vertical velocities over highly turbulent forested terrain. By comparing the sonic anemometer results to that of a conically scanning Doppler lidar (Dellwik et al., 2010b), sonic anemometer accuracy for measuring mean flow tilt angles was estimated to between 2° and 3°. Use of planar fit algorithms, where the mean vertical velocity is calculated as the difference between the neutral and non-neutral flow, does not solve this problem of low accuracy and is not recommended. Because of the large uncertainties caused by flow distortion and vertical alignment, it was only possible to a limited extent to relate sonic anemometer flow tilt angles to upwind forest edges, but the results by the lidar indicated that an internal boundary layer affect flow tilt angles at 21m above the forest. This is in accordance with earlier studies at the site. Since the mean flow tilt angles do not follow the terrain, an estimate of the vertical advection term for near-neutral conditions was calculated using profile measurements of carbon dioxide. The estimated advection term is large, but it is not recommended to include it in the surface carbon balance, unless all terms in the carbon dioxide conservation equation can be precisely estimated.
Highlights
Eddy covariance measurements, which provide a direct method of measuring turbulent fluxes of carbon to and from the land surface, are the main component in the FluxNet (Baldocchi et al, 2001) network of towers for assessing the terrestrial carbon balance
In order to assess the importance of the flow distortion correction, the attack angle probability density function (“pdf”) on the Solent R2 sonic was calculated as arctan √ w, u2 +v 2 where v denotes the transversal wind component (Fig. 3, left)
Given the reported effects of flow distortion and the temperature depen- in the vertical, the flow distortion correction applied for the dence of transducers in Sect. 2, one can not expect the sonic Solent R2 was based on measurements of only a [0◦, 60◦] azanemometers to perfectly measure mean flow tilt angles and imuthal interval, and symmetry arguments were used to make vertical velocities
Summary
Eddy covariance measurements, which provide a direct method of measuring turbulent fluxes of carbon to and from the land surface, are the main component in the FluxNet (Baldocchi et al, 2001) network of towers for assessing the terrestrial carbon balance. In a recent study, Leuning et al (2008) stressed this point by stating a necessary accuracy for the mean vertical velocity of 1 mm s−1 in order to include the term at the short timescale. Such precision poses an extreme challenge, especially over forests, where the natural variations due to the turbulent nature of the flow are 100–1000 times greater
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