Zero-plane Displacement Height Research Articles

Effects of canopy morphology and thermal stability on mean flow and turbulence statistics observed inside a mixed hardwood forest

The influences of thermal stability and seasonal changes in canopy morphology on mean flow and turbulence statistics in a mixed hardwood forest are presented from a year long field experiment at the University of Michigan Biological Station AmeriFlux site. A secondary wind speed maximum at z/ h = 0.07 ( z is height above ground and h is mean canopy height) below the level of peak vegetation area density (VAD) in the understory (young white pines) is observed more frequently and is more pronounced in fully leafed (closed) canopy than defoliated (open) canopy, and in stable than near-neutral and unstable conditions. A secondary wind speed maximum at z/ h = 0.58 is observed only in the closed canopy below the level of peak VAD in the upper canopy (crowns of mature aspen trees), which occurs less frequently and is less pronounced than that at z/ h = 0.07. Horizontal mean winds in the forest are observed to flow to the left (counter-clockwise) of that at the canopy top. The degrees of turning of the mean winds increase with increasing depth into the forest except a reversal (clockwise) near the forest floor in the closed canopy. The degrees of turning are greater in the closed canopy than the open canopy but smaller in near-neutral than unstable and stable conditions. The attenuations of Reynolds stress, correlation coefficient and velocity variances with increasing depth into the forest are more rapid in the closed canopy and in stable conditions. But the relative turbulence intensities are greater in the closed canopy than in the open canopy and decrease with increasing stability. In near-neutral stability, the zero-plane displacement height ( d) for the closed canopy decreases with increasing wind speed (∼0.81 h on average), while d for the open canopy does not show a clear dependence on wind speed (∼0.65 h on average). The bulk drag coefficient ( C D h ) measured at the canopy top is much greater over the closed canopy than the open canopy, contrary to earlier observations over a deciduous forest. But C D h * = C D h / VAI (VAI is vegetation area index) is about the same over the closed and open canopies (∼0.03 in near-neutral stability). The drag coefficient ( C d) for the parameterization of drag force in mean momentum budget equations in closure models increases with decreasing wind speed and varies with height. The drag coefficient ( C d LES ) for the parameterization of drag force in prognostic momentum equations in large-eddy simulations of airflow in plant canopies is smaller than C d, and the ratio C d LES / C d is greater in the open canopy than closed canopy and in stable than near-neutral and unstable conditions due to smaller relative turbulence intensities. All drag coefficients decrease and the displacement height increases with increasing stability, which indicates that these estimated aerodynamic parameters are not entirely the properties of vegetation elements, but are influenced by vertical turbulent mixing of momentum. Both eddy-diffusivity and mixing-lengths for momentum transfer decrease with increasing stability. An evidence of non-local transport is shown by peak values in estimated eddy-diffusivity and mixing-lengths below the crowns of mature aspen trees in the closed canopy. Otherwise, the eddy-diffusivity decreases with increasing depth into the forest, while the mixing-lengths above the level of the peaks are greater in the open canopy and the opposite is true below the level of the peaks.

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.

Time series vegetation aerodynamic roughness fields estimated from modis observations

Most land surface models used today require estimates of aerodynamic roughness length in order to characterize momentum transfer between the surface and atmosphere. The most common method of prescribing roughness is through the use of empirical look-up tables based solely on land cover class. Theoretical approaches that employ satellite-based estimates of canopy density present an attractive alternative to current look-up table approaches based on vegetation cover type that do not account for within-class variability and are oftentimes simplistic with respect to temporal variability. The current research applies Raupach's formulation of momentum aerodynamic roughness to MODIS data on a regional scale in order to estimate seasonally variable roughness and zero-plane displacement height fields using bulk land cover parameters estimated by [Jasinski, M.F., Borak, J., Crago, R., 2005. Bulk surface momentum parameters for satellite-derived vegetation fields. Agric. For. Meteorol. 133, 55–68]. Results indicate promising advances over look-up approaches with respect to characterization of vegetation roughness variability in land surface and atmospheric circulation models.

Bulk surface momentum parameters for satellite-derived vegetation fields

The bulk aerodynamic parameters associated with the absorption of surface momentum by vegetated landscapes are theoretically estimated within the context of Raupach's roughness sublayer formulation. The parameters include the bulk plant drag coefficient, maximum u */ U h , sheltering coefficient, and canopy area density at onset of sheltering. Parameters are estimated for the four principal IGBP land cover classes within the U.S. Southern Great Plains: evergreen needleleaf forests, grasslands, croplands, and open shrublands. The estimation approach applies the Method of Moments to roughness data from several international field experiments and other published sources. The results provide the necessary land surface parameters for satellite-based estimation of momentum aerodynamic roughness length and zero-plane displacement height for seasonally variable vegetation fields employed in most terrestrial and atmospheric simulation models used today. Construction of sample displacement and roughness maps over the Southern United States using MODIS land products demonstrates the potential of this approach for regional to global applications.

A scheme for drag partition over rough surfaces

We present a drag partition scheme for surfaces of various roughness densities. The total drag is partitioned into three components: a pressure drag, a skin drag due to momentum transfer to the surfaces of roughness elements and a surface drag due to momentum transfer to the ground surface. We introduce an effective frontal area index to characterize the mutual sheltering effect of roughness elements on drag partition and propose a method for the calculation of the drag components. The drag partition scheme is then applied to the calculation of bulk quantities such as roughness length and zero-plane displacement height. The results are compared with wind-tunnel measurements. Roughness length is found to increase with frontal area index λ to a maximum at about λ = 0.2 , and then decreases with λ . This finding is in agreement with observations. The drag partition scheme has applications to studying scalar exchange in boundary layers over rough surfaces, such as urban boundary layers.

Aerodynamic roughness over an urban area and over two farmlands in a populated area as determined by wind profiles and surface energy flux measurements

Aerodynamic roughness lengths ( z 0) were determined for an urban area and two rice paddies mixed with buildings over a complex terrain in central Taiwan. At each of the three sites, surface net radiation, ground heat flux and vertical profiles of wind speed, temperature and humidity within the atmospheric surface layer (ASL) were measured. The similarity theory of the ASL in conjunction with the energy budget equation of land surface under unstable atmospheric conditions was used to determine the roughness values. Over the urban area, the roughness was determined to be 1.2 m with a zero-plane displacement height ( d 0) of 11 m. Over the two mixed farmlands, the roughness values were determined to be 0.2–0.5 m, 20 times higher than the values (0.008–0.02 m) for homogeneous rice paddies reported in the literature. A preliminary relationship for estimating roughness value as a function of residential fraction, farmland fraction and building height (or population density) is derived. The observations show that during the daytime, the mean height range of the ASL was 30 (±6) ≤ ( z − d 0)/ z 0 ≤ 106 (±38) and the fingerprint areas extended 1.5 km upwind from the three profile sites.

Surface roughness around a 325-m meteorological tower and its effect on urban turbulence

Based on slow- and fast-response measurements under neutral stratification conditions from a 325-m meteorological tower located in a built-up area of north-central Beijing as well as a descriptive survey of surface roughness elements (i.e., buildings and trees) around the tower site, urban roughness lengths,z 0, with zero-plane displacement height are estimated using logarithmic wind profile and morphometric methods in eight 45° directional sectors. When comparing their results with each other, the slow-response method tends to give smallerz 0 values. At a given location, considerable directional variations in values are observed. The effect of surface roughness on urban turbulence characteristics in terms of non-dimensional standard deviations of three-component velocity, σi/u*1 (wherei = u,v,w andu *1 is local friction velocity), is investigated.

Air Temperature Measurement Errors in Naturally Ventilated Radiation Shields

AbstractTwo sources of systematic errors are considered for estimating air temperature. The first source is ambiguity of the definition of the standardized measurement height over vegetated surfaces of varying heights. Without such a standardization, evaluation of the horizontal air temperature gradient is contaminated by the vertical variation of air temperature. This error is generally small in daytime unstable conditions, but increases with increasing stability at night. In an attempt to reduce such error, the use of the zero-plane displacement height for standardizing the measurement height is proposed.The second source of systematic errors is radiative forcing on the sensor–shield systems. A series of experiments is performed over a grass field to investigate the radiatively induced error in the air temperature estimate by the Onset HOBO Pro thermistor in a naturally ventilated multiplate shield. The magnitude of this error is estimated by comparing air temperature measurements by a platinum resistance temperature detector (RTD) sensor in a mechanically aspirated shield. In contrast to the errors resulting from the first source, the radiatively induced error increases with increasing instability. An empirical model is developed for correcting the radiatively induced temperature error using information on wind speed and net or shortwave radiation. The robustness of the model is examined with independent data.

Determination of Roughness Lengths for Heat and Momentum Over Boreal Forests

The roughness length for momentum (z0m), zero-plane displacementheight (d), and roughness length for heat (z0h) are importantparameters used to estimate land-atmosphere energy exchange. Although many different approaches have been developed to parameterizemomentum and heat transfer, existing parameterizations generally utilizehighly simplified representations of vegetation structure. Further, a mismatch exists between the treatments used for momentum and heat exchange and those used for radiative energy exchanges. In this paper, parameterizations are developed to estimate z0m, d, and z0h for forested regimes using information related to tree crown density and structure. The parameterizations provide realistic representationfor the vertical distribution of foliage within canopies, and include explicit treatment for the effects of the canopy roughness sublayer and leaf drag on momentum exchange. The proposed parameterizationsare able to realistically account for site-to-site differences in roughness lengths that arise from canopy structural properties.Comparisons between model predictions and field measurements show good agreement, suggesting that the proposed parameterizations capture the most important factors influencing turbulent exchange of momentumand heat over forests.

Area-Averaged Sensible Heat Flux and a New Method to Determine Zero-Plane Displacement Length over an Urban Surface using Scintillometry

Field observations of area-averagedturbulence characteristics were conducted in a densely built-up residential neighbourhood in Tokyo, Japan. In addition to eddy-correlation (EC) sensors a scintillometer was used for the first time in a city. Significant results include: (1) Scintillometer-derived sensible heat fluxes, QH, obtained at a height 3.5 times the building height agree well with those using the EC technique; (2) source areas for the scintillometer fluxes are larger than for the EC sensors, so that at low heights over inhomogeneous terrain scintillometry offers advantages; (3) new similarity relationships for dissipation rates are proposed for urban areas; (4) a new technique that uses simultaneous scintillation measurements at two heights to directly estimate area-averaged zero-plane displacement height, zd, is proposed. zd estimated in this way depends slightly on atmospheric stability (lower zd under more unstable conditions).

Momentum Transfer By A Mountain Meadow Canopy: A Simulation Analysis Based On Massman's (1997) Model

Using a mountain meadow as a case study it is the objective of the present paper todevelop a simple parameterisation for the within-canopy variation of the phytoelementdrag (Cd) and sheltering (Pm) coefficients required for Massman's model of momentum transfer by vegetation. A constant ratio between Cd and Pm is found to overestimate wind speed in the upper canopy and underestimate it in the lower canopy.Two simple parameterisations of Cd/Pm as a function of the plant area density and the cumulative plant area index are developed, using values optimised by least-squares regression between measured and predicted within-canopy wind speeds. A validation with independently measured data indicates that both parameterisations work reliably for simulating wind speed in the investigated meadow. Model predictions of the normalised zero-plane displacement height and the momentum roughness length fall only partly within the range of values given in literature, which may be explained by the accumulation of plantmatter close to the soil surface specific for the investigated canopies. The seasonal course of the normalised zero-plane displacement height and the momentum roughness length are discussed in terms of the seasonal variation of the amount and density of plant matter.

Drag coefficients, roughness length and zero-plane displacement height as disturbed by artificial standing vegetation

Standing vegetation, for its disturbance to the near-surface airflow, is widely used in the arid and some semi-arid lands over the world to control wind erosion. Knowledge of drag coefficient (CD), roughness length (Z0) and displacement height (D) is essential to completely define the state of wind and the protective role of standing vegetation in wind erosion. Using standing sticks as model standing vegetation, detailed wind velocity distributions were measured above the vegetated surface in wind tunnel, a mass conservation method to estimate the zero-plane displacement height was tried, and the drag coefficients and roughness length were derived by a curve-fit method. Due to the disturbance of standing vegetation, the velocity distributions deviated from the logarithmic profile. The deviation increased with increasing vegetation density and height. The wind velocity profile disturbed by standing vegetation can be divided into three sections, and each section can be expressed by a logarithmic function. Displacement height is a significant parameter for tall and dense vegetation; choice of the value for displacement height greatly influences other parameters such as roughness length and drag coefficient, but its accurate value is not available yet. In the authors’ philosophy, drag coefficient and roughness length should be recommended. However, caution should also be taken in selecting the wind velocity measurements used to derive drag coefficient and roughness length. Both the height and density of standing vegetation influenced drag coefficient and roughness length, but their relative importance was different. A new parameter, effective lateral cover (Lec) was introduced to characterize the structure of standing vegetation. It was found that effective lateral cover and height/spacing ratio were better structural parameters than the others when the effects of standing vegetation on drag coefficient and roughness length were assessed. Good correlation existed between the derived roughness length and drag coefficient, implying that obtaining drag coefficient and roughness length of vegetated surface by curve fit method is reliable so long as caution is taken in selecting the appropriate measurements. Of the three parameters roughness length is the most sensitive to characterize the effects of roughness length on near-surface airflow.

An approach to deriving roughness length and zero-plane displacement height from satellite data, prototyped with BOREAS data

Global climate models typically use relatively crude estimates of roughness length, often based on constant values for land cover category. However, large variations may exist within any of the vegetation categories. Global large-scale estimates of roughness length, such as required by climate models, cannot rely on field experiments for practical reasons and must, therefore, be derived from satellite observations. This paper develops a satellite retrieval method for the roughness parameters, momentum roughness length, z o, and displacement height, d. The formulas, based on the observational data of Raupach [Raupach, M.R., 1992. Bound.-Layer Meteor. 60, 375–395; Raupach, M.R., 1994. Bound.-Layer Meteor. 71, 211–216] and Lindroth [Lindroth, A., 1993. Bound.-Layer Meteor. 66, 265–272], are applicable to most types of vegetation. Raupach relates the momentum roughness length and zero-plane displacement height to the frontal area index, λ, and to the height of the vegetation, h. The frontal area index is related to the shape of an average tree or plant crown and to the average density of the canopy elements. The crown shape factor includes the aspect (height-to-width) ratio of the crown and the geometric shape of the crown. Lindroth relates the momentum roughness length to the leaf area index (LAI) and to the height of the trees. The functional form of z o/ h is peaked in both frontal area index, with the maximum occurring at λ=0.152, and average plant LAI ( L p), with the maximum occurring at L p=0.878. As λ and L p increase, d/ h also increases and approaches an upper limit asymptotically at high values of λ and L p. As the vegetation height is difficult to obtain from present satellite data, retrievals are restricted to fractional values (i.e. z o/ h and d/ h) for global application. This retrieval then requires data for the crown aspect ratio, the canopy density (fractional overstory canopy cover) and the LAI. Data from BOREAS ( BOReal Ecosystem Atmosphere Study) are used to illustrate this satellite retrieval method.

Estimation of vegetation aerodynamic roughness of natural regions using frontal area density determined from satellite imagery

Parameterizations of the frontal area index and canopy area index of natural or randomly distributed plants are developed and applied to the estimation of local aerodynamic roughness using satellite imagery. The formulas are expressed in terms of the subpixel fractional vegetation cover and one non-dimensional geometric parameter that characterizes the plant's shape. Geometrically similar plants and Poisson distributed plant centers are assumed. An appropriate averaging technique to extend satellite pixel-scale estimates to larger scales is provided. The parameterization is applied to the estimation of aerodynamic roughness using satellite imagery for a 2.3 km 2 coniferous portion of the Landes Forest near Lubbon, France, during the 1986 HAPEX-Mobilhy Experiment. The canopy area index is estimated first for each pixel in the scene based on previous estimates of fractional cover obtained using Landsat Thematic Mapper imagery. Next, the results are incorporated into Raupach's [Raupach, M.R., 1994. Boundary-Layer Meteorol. 71, 211–216] analytical formulas for momentum roughness and zero-plane displacement height. The estimates compare reasonably well to reference values determined from measurements taken during the experiment and to published literature values. The approach offers the potential for estimating regionally variable vegetation aerodynamic roughness lengths over natural regions using satellite imagery when there exists only limited knowledge of the vegetated surface.

A determination of the Dalton and Stanton numbers over a saturated barley field

In order to determine the values of the Dalton and the Stanton numbers, an observational study was made over a saturated barley field in the Hachirogata. flat reclaimed land located in the northern part of Japan. The mean height of the barley was about 11 cm; the roughness height for momentum was determined from the mean wind profiles as 4.24 ± 0.18 cm and the zero-plane displacement height as 0 cm. Under neutral stratification and a saturated surface, the reciprocals of the Dalton and Stanton numbers, Da-1 and St-1, were estimated. The results show that the mean value of St-1 is 12.1, and the ratio of the bulk transfer coefficient for heat to that for momentum. CH/CM, is 0.53, which are not entirely incompatible with the few results available from past observations. The mean value of Da- is 38.2. This value is larger than St-1, and fits reasonably well with the semi-empirical equation proposed by Brutsaert (1975a,b).

Estimates of roughness length and zero plane displacement for a foliated and non-foliated oak canopy

Measurements of wind speed were made above the canopy of an oak forest in foliated and non-foliated conditions. Zero plane displacement height was estimated at 7.2 and 5.5 m, while roughness length was estimated at 1.0 and 0.9 m, for foliated and nonfoliated conditions respectively. Mean tree height was 9.6 m. The effect of both parameters on evaporation of intercepted rainfall is evaluated.

Wind profile constants in a neutral atmospheric boundary layer over complex terrain

The roughness height z 0 and the zero-plane displacement height d 0 were determined for a region of complex terrain in the Pre-Alps of Switzerland. This region is characterized by hills of the order of 100 m above the valley elevations, and by distances between ridges of the order of 1 km; it lies about 20 to 30 km north from the Alps. The experimental data were obtained from radiosonde observations under near neutral conditions. The analysis was based on the assumption of a logarithmic profile for the mean horizontal wind existing over one half of the boundary layer. The resulting (z 0/h) and (d 0/h) (where h is the mean height of the obstacles) were found to be in reasonable agreement with available relationships in terms of placement density and shape factor of the obstacles, which were obtained in previous experiments with h-scales 2 to 4 orders of magnitude smaller than the present ones.

Evaporation and Humidity Profiles for Neutral Conditions over Rugged Hilly Terrain

Abstract An analysis was made of vertical profiles of mean specific humidity in the hilly pre-Alpine region in Switzerland under conditions of new neutral atmospheric stability. The profile data were derived from radiosonde observations during the summer season of 1982. A logarithmic layer was found to extend from approximately 2h to 6h [where h(=95 m) is the mean height of the hills]. The zero-plane displacement height for water vapor d0ν appeared to be close to that for momentum, i.e., about 50 m; on the other hand, the water vapor roughness height z0ν for those ascents with a wet surface was found to be around 10 to 15 times smaller than the momentum roughness z0(=3.8 m). The values of evaporation, deduced from the humidity profiles with the friction velocity of the corresponding velocity profiles, were very close to those measured with a lysimeter.

Actual evapotranspiration over a summer period in the “Hupsel catchment”

Actual evapotranspiration was determined in a rural catchment, “Hupselse Beek”, situated in a sandy region in the eastern part of the province of Gelderland, in The Netherlands, the experiment covered a period of nearly 80 days in the summer of 1976, during which a severe drought was experienced in western Europe. Actual evapotranspiration was obtained indirectly by the energy budget on an hourly basis, in which the sensible heat flux was calculated from the mean temperature profile and the wind speed. The sensitivity of the results was analyzed for the choice of the roughness and the zero-plane displacement height in the wind profile, and for errors in the measured temperatures. The calculated results were quite insensitive to the exact formulation of the Monin-Obukhov functions for the mean wind and temperature profiles. However, it was found that the effect of non-neutral atmospheric conditions cannot be neglected; the effect of the buoyancy due to water-vapor stratification on the overall stability of the atmosphere was also noticeable. A comparison with a measure of potential evapotranspiration showed that the actual evapotranspiration was far from the potential.

Turbulence and Wind Speed Spectra within a Japanese Larch Plantation

Abstract Mean horizontal wind speed profiles within and above a plantation of Japanese larch were obtained. A log-profile analysis of above-vegetation wind speeds yielded a wide range of values for the roughness length parameter z0 and the zero-plane displacement height D. The computed Eulerian space scale of turbulence within the vegetation showed deeper penetration of large eddies after needle fall and during high winds. Power spectra showed that at the base of the plantation most of the variation in wind speed was associated with gusts of about 100 m wavelength. Power spectra at the most dense portion of the plantation canopy showed considerable modification due to the tree spacing.