Dispersive fluxes of momentum, sensible heat and carbon dioxide in a forest canopy

In vegetation canopies, exchange of energy, momentum, and mass are not solely controlled by turbulent motions (i.e. vertical turbulent flux densities) but also - at least theoretically - by the local flow around individual canopy elements (i.e. properties correlated to the mean vertical wind at a given location). In the framework of spatial averaging of Raupach and Shaw (1982) the latter process is referred to as ‘dispersive fluxes' - spatial correlations in the time-averaged flow.

Results from wind tunnel studies suggest that the dispersive flux of momentum is insignificant above and in the upper part of canopies (Raupach et al., 1986). However, in the bottom layer of canopies dispersive fluxes of momentum have been shown to reach the same magnitude as the turbulent fluxes (Böhm et al., 2000). Poggi et al. (2004) concluded that dispersive fluxes are important in sparse canopies. Christen and Vogt (2004) calculated small but consistent dispersive fluxes of momentum from field data measured in the trunk space of a sparse cork oak plantation. There is no evidence whether those results are transferable to sensible heat and trace-gas exchange. A priori, given the different source-sink distributions for momentum, heat and different trace-gases there is no justification for a similarity.

This contribution is evaluating the role of dispersive fluxes of momentum, sensible heat and carbon dioxide in a forest using field data. Sixteen 3d-ultrasonic anemometer-thermometers were simultaneously operated in the trunk space of a uniform Scots Pine forest at Hartheim Research Station (University of Freiburg, Germany, 47º 56' 04” N, 7º 36' 02” E, stand density 800 trees ha-1). All sensors were installed at a uniform height of z = 2 m above ground (z/h = 0.13, where h is canopy height of h = 14.5 m). Sensors were arrayed with spacings between 4 and 192 m. Ten sensors were additionally equipped with inlets to a gas-multiplexer system that was measuring average carbon dioxide concentrations. Six sensors were paired with open-path gas-analyzers to determine the in-canopy variability of turbulent carbon dioxide fluxes. The set-up was operated continuously for 30 days (April 13 to May 13, 2006). This extensive data set with a huge number of flow situations allows a statistical estimation of dispersive fluxes even with the relatively low number of point-measurements. Dispersive fluxes are discussed in relation to the magnitude of the quasi-spatially averaged turbulent fluxes measured as an average of all sensors at same heigh and are compared to above-canopy fluxes.

On average, the dispersive momentum flux points in the same direction as the Reynolds stress and lies in the same order of magnitude. However, there is a strong run-to-run variability and absolute values are small compared to the turbulent flux above the canopy. The dispersive flux of sensible heat is more significant and reaches on average 70% of the values of the turbulent flux at same height. The dispersive flux of sensible heat is important during daytime, and in particular during low wind conditions. Dispersive fluxes of carbon dioxide are less obviously correlated to the turbulent fluxes and will be discussed in the context of the in-canopy and above-canopy turbulent exchange.

References

Böhm M., Finnigan J. J., and Raupach M. R. (2000): ‘Dispersive fluxes and canopy flows: just how important are they?'. Proc. of 24th Conf. Agr. For. Meteorol., 14-18 August 2000, Davis, CA.

Christen A. and Vogt R. (2004): ‘Direct measurement of dispersive fluxes within a cork oak plantation'. Proc. of 26th AMS Conf. Agr. For. Meteorol. 23-27 August, Vancouver, Canada.

Poggi D., Katul G. G. and Albertson J. D. (2004): ‘A note on the contribution of dispersive fluxes to momentum transfer within canopies'. Bound.-Lay. Meteorol. 111 pp. 615-621.

Raupach M. R., Coppin P. A., and Legg B. J. (1986): 'Experiments on scalar dispersion within a model-plant canopy. I. The turbulence structure'. Bound.-Lay. Meteorol. (1986) 35 pp. 21-52.

Raupach M. R. and Shaw R. H. (1982): ‘Averaging procedures for flow within vegetation canopies'. Bound.-Lay. Meteorol. 22 pp. 79-90.