A note on the contribution of dispersive fluxes to momentum transfer within canopies

• Published in: Boundary-layer meteorology 2004-06, Vol.111 (3), p.615-621
• Main Authors: POGGI, D, KATUL, G. G, ALBERTSON, J. D
• Format: Article
• Language: English
• Subjects: Boundary layer; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Momentum transfer; Turbulence; Wind tunnels
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1023/B:BOUN.0000016563.76874.47

Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are