Direct measurement of dispersive fluxes within a cork oak plantation

Flow within plant canopies is strongly influenced by individual roughness elements, with the consequence that turbulence statistics and flux densities are vertically and horizontally inhomogeneous. In contrast, most models treat the exchange processes one-dimensional and rely on horizontally (spatially) averaged variables. By averaging over time and space, contributions that result from local inhomogeneities in the mean flow have to be taken into account. These dispersive fluxes result from spatial correlations of the local departure of the temporal mean from the spatial-temporal mean averaged over the whole canopy (Raupach and Shaw, 1982). Usually, dispersive fluxes are considered irrelevant and neglected. It is important to know the magnitude of dispersive fluxes in order to quantify associated errors of this simplification.

Recently, two physical scale model studies investigated dispersive fluxes in simplified canopies. The results point out, that dispersive fluxes are insignificant above and in the upper part of the canopies, while they seem to be important in the bottom layers. There they can be in the same magnitude as the turbulent fluxes (Böhm et al., 2000). Poggi et al. (2004) concluded that dispersive fluxes are only important in sparse canopies. Up to now, no experimental results are available from real canopies.

To estimate the magnitude of the dispersive fluxes of momentum and sensible heat in a real canopy, 8 ultrasonic anemometers were deployed in an irregular array of 70 by 70 m within a cork oak plantation near Rio Frio, Portugal. Instruments were installed in the trunk space, all at a height of 1.8 m (z/h~ 0.18), hence the region where substantial dispersive fluxes are suggested by wind tunnel studies.

The dispersive flux of momentum was found to be in the order of 0-20% of the turbulent momentum fluxes (average 14%), thus supporting the above findings, that dispersive fluxes of momentum exist in the lower canopy. The dispersive flux always transports momentum in the same direction as the temporally and spatially averaged turbulent flux does. The dispersive flux of sensible heat is negligible, typically in the range between -5 and +5% of the turbulent flux.

References

Böhm M, Finnigan JJ, Raupach MR (2000): ‘Dispersive fluxes and canopy flows: just how important are they?’. 24th Conference on Agricultural and Forest Meteorology, 14-18 August 2000, Davis, CA.

Poggi D, Katul GG, Albertson JD (2004): ‘A note on the contribution of dispersive fluxes to momentum transfer within canopies’, Boundary-Layer Meteorol. 111: 615-621.

Raupach MR, Shaw RH (1982): ‘Averaging procedures for flow within vegetation canopies’, Boundary-Layer Meteorol. 22: 79-90.