Abstract
Progress on practical problems such as quantifying gene flow and seed dispersal by wind or turbulent fluxes over nonflat terrain now demands fundamental understanding of how topography modulates the basic properties of turbulence. In particular, the modulation by hilly terrain of the ejection-sweep cycle, which is the main coherent motion responsible for much of the turbulent transport, remains a problem that has received surprisingly little theoretical and experimental attention. Here, we investigate how boundary conditions, including canopy and gentle topography, alter the properties of the ejection-sweep cycle and whether it is possible to quantify their combined impact using simplified models. Towards this goal, we conducted two new flume experiments that explore the higher-order turbulence statistics above a train of gentle hills. The first set of experiments was conducted over a bare surface while the second set of experiments was conducted over a modelled vegetated surface composed of densely arrayed rods. Using these data, the connections between the ejection-sweep cycle and the higher-order turbulence statistics across various positions above the hill surface were investigated. We showed that ejections dominate momentum transfer for both surface covers at the top of the inner layer. However, within the canopy and near the canopy top, sweeps dominate momentum transfer irrespective of the longitudinal position; ejections remain the dominant momentum transfer mode in the whole inner region over the bare surface. These findings were well reproduced using an incomplete cumulant expansion and the measured profiles of the second moments of the flow. This result was possible because the variability in the flux-transport terms, needed in the incomplete cumulant expansion, was shown to be well modelled using “local” gradient-diffusion principles. This result suggests that, in the inner layer, the higher-order turbulence statistics appear to be much more impacted by their relaxation history towards equilibrium rather than the advection-distortion history from the mean flow. Hence, we showed that it is possible to explore how various boundary conditions, including canopy and topography, alter the properties of the ejection-sweep cycle by quantifying their impact on the gradients of the second moments only. Implications for modelling turbulence using Reynolds-averaged Navier Stokes equations and plausible definitions for the canopy sublayer depth are briefly discussed.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Antonia R (1981) Conditional sampling in turbulence measurement. Ann Rev Fluid Mech 13:131–156
Aubinet M, Heinesch B, Yarneaux M (2003) Horizontal and vertical CO2 advection in a sloping forest. Boundary-Layer Meteorol 108:397–417
Aubinet M, Berbigier P, Bernhofer C, Cescatti A, Feigenwinter C, Granier A, Grunwald H, Havrankova K, Heinesch B, Longdoz B, Marcolla B, Montagnani L, Sedlak P (2005) Comparing CO2 storage fluxes and advection at night at different CARBOEUROFLUX sites. Boundary-Layer Meteorol 116:63–94
Baldocchi D, Finnigan J, Wilson K, Paw U, Falge E (2000) On measuring net ecosystem carbon exchange over tall vegetation on complex terrain. Boundary-Layer Meteorol 96:257–291
Baldocchi D, Falge E, Gu LH, Olson R, Hollinger D, Running S, Anthoni P, Bernhofer C, Davis K, Evans R, Fuentes J, Goldstein A, Katul G, Law B, Lee X, Malhi Y, Meyers T, Munger W, Oechel W, Paw K, Pilegaard K, Schmid H, Valentini R, Verma S, Vesala T, Wilson K, Wofsy S (2001) FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bull Amer Meteorol Soc 82(11):2415–2434
Belcher S, Hunt J (1993) Turbulent shear-flow over slowly moving waves. J Fluid Mech 251:109–148
Belcher S, Hunt J (1998) Turbulent flow over hills and waves. Annu Rev Fluid Mech 30:507–538
Bitsuamlak G, Stathopoulos T, Bedard C (2004) Numerical evaluation of wind flow over complex terrain: review. J Aero Eng 17:135–145
Cava D, Katul G, Scrimieri A, Poggi D, Cescatti A, Giostra U (2005) Buoyancy and the sensible heat flux budget within dense canopies. Boundary-Layer Meteorol 118:217–240
Feigenwinter C, Bernhofer C, Vogt R (2004) The influence of advection on short term 2 budget in and above a forest canopy. Boundary-Layer Meteorol 113:201–224
Fer I, McPhee M, Sirevaag A (2004) Conditional statistics of the Reynolds stress in the under-ice boundary layer. Geophys Res Lett 31–15:L15311
Finnigan J (1988) Air flow over complex terrain. In: Steffen WL, Denmead OT (eds) Flow and Transport in the Natural Environment: Advances and Applications. Springer-Verlag, New York, 384 pp
Finnigan J (2000) Turbulence in plant canopies. Ann Rev Fluid Mech 32:519–571
Finnigan J, Belcher S (2004) Flow over a hill covered with a plant canopy. Quart J Roy Meteorol Soc 130:1–29
Finnigan J, Brunet Y (1995) Turbulent airflow in forest on flat and hilly terrain. In Wind and Trees, vol. Wind and trees. Coutts MP, Grace J, Cambridge University Press, UK: 501 pp
Finnigan J, Raupach M, Bradley E, Aldis G (1990) A wind-tunnel study of turbulent-flow over a 2-dimensional ridge. Boundary-Layer Meteorol 50:277–317
Fokken T, Hassager C, Hipps L (2005) Editorial: surface fluxes over land in complex terrain. Theor Appl Climatol 80:79
Kader B, Yaglom A (1990) Mean fields and fluctuation moments in unstably stratified turbulent boundary layers. J Fluid Mech 212:637–662
Kaimal J, Finnigan J (1994) Atmospheric Boundary Layer Flows: Their Structure and Measurement. Oxford University Press, New York, 304 pp
Katul G, Chang W (1999) Principal length scales in second-order closure models for canopy turbulence. J Appl Meteorol 38:1631–1643
Katul G, Hsieh C, Kuhn G, Ellsworth D, Nie D (1997a) The turbulent eddy motion at the forest-atmosphere interface. J Geophys Res 102:13409–13421
Katul G, Kuhn G, Schielde J, Hsieh C (1997b) The ejection-sweep character of scalar fluxes in the unstable surface layer. Boundary-Layer Meteorol 83:1–26
Katul G, Mahrt L, Poggi D, Sanz C (2004) One and two equation models for canopy turbulence. Boundary-Layer Meteorol 113:81–109
Katul G, Porporato A, Nathan R, Siqueira M, Soons M, Poggi D, Horn H, Levin S (2005) Mechanistic analytical models for long-distance seed dispersal by wind. Amer Nat 166:367–381
Katul G, Williams C, Siqueira M, Poggi D, Porporato A, McCarthy H, Oren R (2006b) Spatial modeling of transgenic conifer pollen, Landscapes, genomics and transgenic conifers. Williams C Springer: 261 pp
Katul G, Finnigan J, Poggi D, Leuning R, Belcher S (2006a) The influence of hilly terrain on canopy-atmosphere carbon dioxide exchange. Boundary-Layer Meteorol 118:189–216
Monin A, Yaglom A (1971) Statistical fluid mechanics: mechanics of turbulence, vol 1. The MIT Press, Cambridge, pp. 769
Nakagawa H, Nezu I (1977) Prediction of the contributions to the Reynolds stress from bursting events in open channel flows. J Fluid Mech 80:99–128
Nathan R, Katul G (2005) Foliage shedding in deciduous forests lifts up long-distance seed dispersal by wind. Proc Nat Acad Sci 102:8251–8256
Nathan R, Katul G, Horn H, Thomas S, Oren R, Avissar R, Pacala S, Levin S (2002) Mechanisms of long-distance dispersal of seeds by wind. Nature 418:409–413
Poggi D, Katul G, Albertson J (2006) Scalar dispersion within a model canopy: measurements and three-dimensional Lagrangian models. Adv Water Res 29:326–335
Poggi D, Porporato A, Ridolfi L (2002) An experimental contribution to near-wall measurements by means of a special laser Doppler anemometry technique. Exp Fluids 32:366–375
Poggi D, Katul G, Albertson J (2004a) Moment transfer and turbulent kinetic energy budgets within a dense model canopy. Boundary-Layer Meteorol 111-3:589–614
Poggi, D., Porporato A, Ridolfi L, Katul G, Albertson J (2004b) The effect of vegetation density on canopy sublayer turbulence. Boundary-Layer Meteorol 111-3:565–587
Raupach M (1981) Conditional statistics of the Reynolds stress in rough-wall and smooth-wall turbulent boundary layers. J Fluid Mech 108:363—382
Raupach M, Shaw R (1982) Averaging procedures for flow within vegetation canopies. Boundary-Layer Meteorol 22-1:79–90
Ross A, Vosper S (2005) Neutral turbulent flow over forested hills. Quart J Roy Meteorol Soc 131:1841–1862
Shaw R, Tavangar J, Ward D (1983) Structure of the Reynolds stress in a canopy layer. J Clim Appl Meteorol 22:1922á1-1931
Soons M, Heil G, Nathan R, Katul G (2004) Determinants of long-distance dispersal by wind in grasslands. Ecology 85:3056–3068
Staebler R, Fitzjarrald D (2004) Observing subcanopy CO2 advection. Agr Forest Meteorol 122:139–156
Tennekes H, Lumley J (1972) A first course in turbulence, vol I. MIT Press, Cambridge MA, 300 pp
Wilson N, Shaw R (1977) A higher order closure model for canopy flow. J Appl Meteorol 16:1198–1205
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Poggi, D., Katul, G. The ejection-sweep cycle over bare and forested gentle hills: a laboratory experiment. Boundary-Layer Meteorol 122, 493–515 (2007). https://doi.org/10.1007/s10546-006-9117-x
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10546-006-9117-x