A Note on the Contribution of Dispersive Fluxes to Momentum Transfer within Canopies

Dispersive flux terms are formed when the time-averaged mean momentum equation is spatially averaged within the canopy volume. These fluxes represent a contribution to momentum transfer arising from spatial correlations of the time-averaged velocity components within a horizontal plane embedded in the canopy sublayer (CSL). Their relative importance to CSL momentum transfer is commonly neglected in model calculations and in nearly all field measurement interpretations. Recent wind-tunnel studies suggest that these fluxes may be important in the lower layers of the canopy; however, no one study considered their importance across all regions of the canopy and for a wide range of canopy roughness densities. Using detailed laser Doppler anemometry measurements conducted in a model canopy composed of cylinders within a large flume, we demonstrate that the dispersive fluxes are only significant (i.e., > 10%) for sparse canopies. These fluxes are in the same direction as the turbulent flux in the lower layers of the canopy but in the opposite direction near the canopy top. For dense canopies, we show that the dispersive fluxes are < 5% at all heights. These results appear to be insensitive to the Reynolds number (at high Reynolds numbers).