Davide Poggi

THE EFFECT OF VEGETATION DENSITY ON CANOPY SUB-LAYER TURBULENCE

The canonical form of atmospheric flows near theland surface, in the absence of a canopy, resembles a rough-wallboundary layer. However, in the presence of an extensive and densecanopy, the flow within and just above the foliage behaves as aperturbed mixing layer. To date, no analogous formulation existsfor intermediate canopy densities. Using detailed laser Dopplervelocity measurements conducted in an open channel over a widerange of canopy densities, a phenomenological model that describesthe structure of turbulence within the canopy sublayer (CSL) isdeveloped. The model decomposes the space within the CSL intothree distinct zones: the deep zone in which the flow field isshown to be dominated by vortices connected with vonKármán vortex streets, butperiodically interrupted by strong sweep events whose features areinfluenced by canopy density. The second zone, which is near thecanopy top, is a superposition of attached eddies andKelvin–Helmholtz waves produced by inflectional instability in themean longitudinal velocity profile. Here, the relative importanceof the mixing layer and attached eddies are shown to vary withcanopy density through a coefficient α. We show that therelative enhancement of turbulent diffusivity over its surface-layer value near the canopy top depends on the magnitude ofα. In the uppermost zone, the flow follows the classicalsurface-layer similarity theory. Finally, we demonstrate that thecombination of this newly proposed length scale and first-orderclosure models can accurately reproduce measured mean velocity andReynolds stresses for a wide range of roughness densities. Withrecent advancement in remote sensing of canopy morphology, thismodel offers a promising physically based approach to connect theland surface and the atmosphere without resorting to empiricalmomentum roughness lengths.

Mechanistic Analytical Models for Long‐Distance Seed Dispersal by Wind

We introduce an analytical model, the Wald analytical long‐distance dispersal (WALD) model, for estimating dispersal kernels of wind‐dispersed seeds and their escape probability from the canopy. The model is based on simplifications to well‐established three‐dimensional Lagrangian stochastic approaches for turbulent scalar transport resulting in a two‐parameter Wald (or inverse Gaussian) distribution. Unlike commonly used phenomenological models, WALD’s parameters can be estimated from the key factors affecting wind dispersal—wind statistics, seed release height, and seed terminal velocity—determined independently of dispersal data. WALD’s asymptotic power‐law tail has an exponent of −3/2, a limiting value verified by a meta‐analysis for a wide variety of measured dispersal kernels and larger than the exponent of the bivariate Student t‐test (2Dt). We tested WALD using three dispersal data sets on forest trees, heathland shrubs, and grassland forbs and compared WALD’s performance with that of other analytical mechanistic models (revised versions of the tilted Gaussian Plume model and the advection‐diffusion equation), revealing fairest agreement between WALD predictions and measurements. Analytical mechanistic models, such as WALD, combine the advantages of simplicity and mechanistic understanding and are valuable tools for modeling large‐scale, long‐term plant population dynamics.

A Note On The Contribution Of Dispersive Fluxes To Momentum Transfer Within Canopies

Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers).

Analysis of the small-scale structure of turbulence on smooth and rough walls

Energy spectra and structure functions of the streamwise and wall-normal turbulent velocity components in an open channel flow are experimentally investigated as a function of the distance from the wall. Both smooth- and rough-wall conditions are considered, with special attention to the zone close to the wall. In the core region, the small-scale turbulent flow field is always characterized by a high level of isotropy, while a strong anisotropy is found at small scales in the near-wall region. In the smooth-wall case, the extent of the scaling range increases as the wall is approached, but with exponents which are different from the classical ones. In the rough-wall case, the roughness strongly interacts with turbulence, destroying the scaling regions at small scales through the imposition of its characteristic scales. A lower level of intermittency and anisotropy is also observed at the small scales for rough-wall conditions. Energy spectra and structure functions suggest a connection of these behaviors ...

An experimental investigation of turbulent flows over a hilly surface

Gentle topographic variations significantly alter the mass and momentum exchange rates between the land surface and the atmosphere from their flat-world state. This recognition is now motivating basic studies on how a wavy surface impacts the flow dynamics near the ground for high bulk Reynolds numbers (Reh). Using detailed flume experiments on a train of gentle hills, we explore the spatial structure of the mean longitudinal (u) and vertical (w) velocities at high Reh. We show that classical analytical theories proposed by Jackson and Hunt (JH75) for isolated hills can be extended to a train of gentle hills if the background velocity is appropriately defined. We also show that these theories can reproduce the essential 2D structure of the Reynolds stresses. The basic assumptions in the derivation of the JH75 model are also experimentally investigated. We found that the linearization of the advective acceleration term and the mixing length proposed in JH75 are reasonable within the inner layer. We also sh...

Stationarity, Homogeneity, and Ergodicity in Canopy Turbulence

One of the defining syndromes of turbulence is nonlinear stochasticity. This view of turbulence motivated the development of statistical mechanics theories that have served to connect the basic Navier-Stokes (NS) equations of motion to the statistical results of numerous field experiments. In general, the proper averaging operator for stochastic processes is ensemble averaging. Given the transient nature of flow boundary conditions in natural systems, field experiments are typically unable to capture a suitable ensemble, in a strict sense. Instead, field experiments typically focus on time averaged statistics. Stationarity and ergodicity are two central concepts (required conditions) used to link field measurements and the NS equations or field measurements to “boundary conditions” at the land-atmosphere interface. In this Chapter, we present an elementary review of these two concepts for the atmospheric surface layer (ASL) and canopy sublayer (CSL) and proceed to show why the stable CSL tends to violate both conditions. A weaker form of these two conditions may be applicable to CSL flows that are only moderately stably stratified. Practical implications for night time CO2 flux corrections are also discussed.

Mean Flow Near Edges and Within Cavities Situated Inside Dense Canopies

A streamfunction-vorticity formulation is used to explore the extent to which turbulent and turbulently inviscid solutions to the mean momentum balance explain the mean flow across forest edges and within cavities situated inside dense forested canopies. The turbulent solution is based on the mean momentum balance where first-order closure principles are used to model turbulent stresses. The turbulently inviscid solution retains all the key terms in the mean momentum balance but for the turbulent stress gradients. Both exit and entry versions of the forest edge problem are explored. The turbulent solution is found to describe sufficiently the bulk spatial patterns of the mean flow near the edge including signatures of different length scales reported in canopy transition studies. Next, the ‘clearing inside canopy’ or the so-called ‘cavity’ problem is solved for the inviscid and turbulent solutions and then compared against flume experiments. The inviscid solution describes the bulk flow dynamics in much of the zones within the cavity. In particular, the solution can capture the correct position of the bulk recirculation zone within the cavity, although with a weaker magnitude. The inviscid solution cannot capture the large vertical heterogeneity in the mean velocity above the canopy, as expected. These features are better captured via the first-order closure representation of the turbulent solution. Given the ability of this vorticity formulation to capture the mean pressure variations and the mean advective acceleration terms, it is ideal for exploring the distributions of scalars and roughness-induced flow adjustments on complex topography.

Dispersal of Transgenic Conifer Pollen

Long-distance dispersal (LDD) of pollen in conifers presents a risk for transgenic escape into unmanaged forests. Here, we report simulations of transgenic pollen dispersal and LDD from genetically modified forests using a mechanistic turbulent dispersal model. The dispersal model is based on coupled Eulerian-Lagrangrian closure (CELC) principles that model turbulent velocity excursions within the canopy. Contrary to recent studies and measurements from annual crop canopies, which reported maximum pollen dispersal distances ranging from 6 m to 800 m, conifer pollen LDD can readily exceed 8 km in less than 1 hour without escaping the atmospheric boundary layer. These LDD estimates were conducted using a conservative terminal velocity (Vt) of 0.07 m s. When using a Vt of 0.03 m s ± 0.02 m s, which is characteristic of pine species pollen, LDD increased by almost a factor of 3, from about 8 to 21 km for a stand at its reproductive onset and from about 13 km to 33 km for a stand at nearharvesting age. The fact that pollen can travel such distances without escaping the ABL has important consequences about viability and ecological risk assessment and gene flow.

Productivity analysis of the full scale inertial sea wave energy converter prototype: A test case in Pantelleria Island

Wave power is one of the most rich and promising sources of renewable energy for the future. Approximately 2000 TWh/year can be produced through the exploitation of the wave energy potential. In the past four decades, hundreds of Wave Energy Converters have been proposed and studied, but so far a conclusive architecture to harvest wave power has not been identified. Many engineering problems are still to be solved; these include survivability, durability, and effective power capture in a variable wave climate. Reacting body devices use the inertia of a large mass to generate the reaction needed from the power take off (PTO). Heretically, in the case of a simple inertial mass, optimal control adjusts the dynamic parameters of the PTO, such as the spring constant and energy absorbing damping, to maximize energy absorption. The ISWEC (Inertial Sea Wave Energy Converter) uses a gyroscope to create an internal inertial reaction that is able to harvest wave power without exposing mechanical parts to the harsh o...

Flume Experiments on Turbulent Flows Across Gaps of Permeable and Impermeable Boundaries

Laser Doppler anemometery and laser-induced fluorescence techniques were used to explore the spatial structure of the flow within and above finite cavities created within porous and solid media. The cavities within these two configurations were identical in size and were intended to mimic flow disturbances created by finite gaps and forest clearing. Because flows over permeable boundaries differ from their solid counterparts, the study here addresses how these differences in boundary conditions produce differences in, (i) bulk flow properties including the mean vorticity within and adjacent to the gaps, (ii) second-order statistics such as the standard deviations and turbulent stresses, (iii) the relative importance of advective to turbulent stress terms across various regions within and above the gaps, and (iv) the local imbalance between ejections and sweeps and momentum transport efficiencies of updrafts and downdrafts. Both configurations exhibited a primary recirculation zone of comparable dimensions inside the gap. The mean vorticity spawned at the upstream corner of the gap was more intense for the solid configuration when compared to its porous counterpart. The free-shear layer spawned from the upstream corner-edge deeper into the gap for the porous configuration. The momentum flux at the interface within and above the gap was enhanced by a factor of 1.5–2.0 over its upstream value, and this enhancement zone was much broader in size for the porous configuration. For the turbulent transport terms in the longitudinal and vertical mean momentum balances, these transport terms were significant inside the gap for both boundary configurations when compared to their upstream counterpart. The effectiveness of using incomplete cumulant expansion methods to describe the momentum transport efficiencies, and the relative contributions of ejections and sweeps to turbulent stresses, especially in this zone, were also demonstrated. The flatness factor for both velocity components, often used as a measure of intermittency, was highest in the vicinity of the upstream corner in both configurations. However, immediately following the downstream corner, the flatness factor remained large for the porous configuration, in contrast to its solid configuration counterpart.