A NOTE ON THE CONTRIBUTION OF DISPERSIVE FLUXES TO MOMENTUM TRANSFER WITHIN CANOPIES Research Note D. POGGI Dipartimento di Idraulica, Trasporti ed Infrastrutture Civili, Politecnico di Torino, Torino, Italy & Department of Civil And Environmental Engineering, Duke University, Durham, North Carolina, U.S.A. G. G. KATUL Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, U.S.A. J. D. ALBERTSON Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, U.S.A. (Received in final form 1 September 2003) Abstract. 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 hori- zontal 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 interpreta- tions. 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). Keywords: Canopy density, Canopy turbulence, Dispersive fluxes, Mean momentum equation. 1. Introduction The need to understand and quantify momentum transfer close to roughness ele- ments, such as canopies or buildings within a city, has ignited substantial interest in the structure of turbulence within a matrix of roughness elements. Obvious applic- ations include carbon dioxide and water vapour exchange between the biosphere and the atmosphere, ozone dry deposition on canopies, pollen or seed dispersal within or among landscapes, and pollutant transport within the airspace of build- ings, to name a few. Progress on such complex problems can only be achieved after Boundary-Layer Meteorology 111: 615–621, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.