Turbulence structure above a vegetation canopy

Abstract

We compare the turbulence statistics of the canopy/roughness sublayer (RSL) and the inertial sublayer (ISL) above. In the RSL the turbulence is more coherent and more efficient at transporting momentum and scalars and in most ways resembles a turbulent mixing layer rather than a boundary layer. To understand these differences we analyse a large-eddy simulation of the flow above and within a vegetation canopy. The three-dimensional velocity and scalar structure of a characteristic eddy is educed by compositing, using local maxima of static pressure at the canopy top as a trigger. The characteristic eddy consists of an upstream head-down sweep-generating hairpin vortex superimposed on a downstream head-up ejection-generating hairpin. The conjunction of the sweep and ejection produces the pressure maximum between the hairpins, and this is also the location of a coherent scalar microfront. This eddy structure matches that observed in simulations of homogeneous-shear flows and channel flows by several workers and also fits with earlier field and wind-tunnel measurements in canopy flows. It is significantly different from the eddy structure educed over smooth walls by conditional sampling based only on ejections as a trigger. The characteristic eddy was also reconstructed by empirical orthogonal function (EOF) analysis, when only the dominant, sweep-generating head-down hairpin was recovered, prompting a re-evaluation of earlier results based on EOF analysis of wind-tunnel data. A phenomenological model is proposed to explain both the structure of the characteristic eddy and the key differences between turbulence in the canopy/RSL and the ISL above. This model suggests a new scaling length that can be used to collapse turbulence moments over vegetation canopies. IntroductionWithin the inertial sublayer (ISL) of high-Reynolds-Number rough-wall boundary layers, the mean wind profile is close to logarithmic and the log law relates the momentum flux to the mean wind gradient in a convenient form that is exploited in measurements and predictive models. When buoyancy forces are present, Monin-Obukhov similarity theory (MOST) introduces an extra length scale -the Obukhov length L -allowing the log law to be extended to diabatic conditions and providing the theoretical basis for surface-layer meteorology. It has been known for a considerable time, however, that sufficiently close to a rough surface, MOST formulae must be modified to reflect changes in the character of the turbulence (Thom et al.

Does the canopy mixing layer model apply to highly flexible aquatic vegetation? Insights from numerical modelling

“…Our current theoretical understanding of aquatic vegetated flows has been based on our understanding of terrestrial flows through crop fields or forest environments (as reviewed by Finnigan et al [15]). Terrestrial canopy research led to the development of a canonical theory for canopy mixing layers, based upon classical free shear layers, or mixing layers, which has been used to describe flow through and above terrestrial vegetation canopies [16,17] (see Sect. 2).…”

Analysis of coherent structures and atmosphere-canopy coupling strength during the CABINEX field campaign

“…Turbulent mixing is the fundamental driver in the exchange of mass, momentum and scalars between a forest canopy and the atmosphere (Finnigan et al, 2009;Harman and Finnigan, 2008). Quantifying these turbulent processes is necessary to understand the surface energy budget (Oncley et al, 2007), the global carbon budget (Law et al, 2002) and the fate of reactive trace gas species (Holzinger et al, 2005;Sörgel et al, 2011).…”

“…Coherent structures are defined as a distinct pattern of organized turbulence with length scales on the order of the canopy height. They typically result from hydrodynamic instabilities caused by large differences in horizontal wind speeds (wind shear) near the top of the canopy (Finnigan et al, 2009) and are thought to be the main driver of local-scale Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Momentum balance in flows over mobile granular beds: application of double-averaging methodology to DNS data

“…The DAM stems from the terrestrial canopy aerodynamics (e.g. Finnigan, Shaw, & Patton, 2009;Raupach & Shaw, 1982). In recent years, it has also been successfully employed for studies of open-channel flows over fixed rough beds, examples of which are given in Nikora and Rowiński (2008), Franca, Ferreira, and Lemmin (2008), Mignot, Barthelemy, andHurther (2009), Ferreira, Ferreira, Ricardo, andFranca (2010), Yuan and Piomelli (2014).…”