HR: 16:00h
AN: H24B-01 INVITED    [Abstracts]
TI: Micro- and Macro- Dispersive Fluxes in Canopy Flows
AU: * Katul, G G
EM: gaby@duke.edu
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Box 90328, Durham, NC 27708-0328, United States
AU: Poggi, D
EM: davide.poggi@polito.it
AF: Dipartimento di Idraulica, Trasporti ed Infrastrutture Civili, Politecnico di Torino, Torino, 10137, Italy
AB: Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling high Reynolds number geophysical flows. How to best represent the effects of such complex morphological variability and any concomitant topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted. Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however, such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that remain "unclosed" or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances can be labeled as "micro- dispersive" fluxes. When averaging is intended to eliminate low-wavenumber topographic variations, we refer to these covariances as "macro-dispersive" terms. Two flume experiments were used to explore the magnitude and sign of both micro- and macro- dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (>30%). For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and flume experiments here suggest that the macro-dispersive fluxes cannot be neglected, and their modeled value may be some 20% of the typical Reynolds stresses.
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 3379 Turbulence (4490)
SC: Hydrology [H]
MN: 2007 Fall Meeting