Zero-plane Displacement Height Research Articles

Reynolds number dependence of turbulent flows over a highly permeable wall

Direct numerical simulations of turbulent flows over highly permeable porous walls were performed at various Reynolds numbers to examine the effects of the Reynolds number on permeable wall turbulence. The porous medium consisted of Kelvin cell arrays with porosity $0.95$ , and the permeability Reynolds number $Re_K$ ranged from approximately 7 to 50. Simulations with thin and thick porous walls were performed to investigate the effects of spanwise roller vortices associated with the Kelvin–Helmholtz instability. The results show that the effect of the Kelvin–Helmholtz instability becomes more significant with increasing the permeability Reynolds number, and spanwise rollers, for which length scale is an order of channel height, dominate turbulence when $Re_K \gtrsim 30$ . Spanwise rollers reinforce the negative correlation between the wall-normal and streamwise velocity fluctuations close to the porous/fluid interface, and intensify the turbulent velocity fluctuations away from the porous walls, leading to increased frictional resistance. An investigation of the Reynolds number dependence of the modified logarithmic law indicates that the zero-plane displacement and equivalent roughness height are proportional to the square root of permeability, whereas the von Kármán constant increases with the permeability Reynolds number because of the increased mixing length resulting from the relatively large-scale velocity fluctuations induced by spanwise rollers. We developed a model for the modified log law for permeable wall turbulence based on permeability, and confirmed that the skin friction coefficient obtained from the model reasonably predicts the skin friction coefficient for several types of high-porosity porous media. Hence, permeability is a key parameter that characterizes the logarithmic mean velocity profiles over a variety of porous media with high porosity.

Examination of outer-layer similarity in wall turbulence over obstructing surfaces

Turbulent flows over canopies of rigid filaments with different densities are studied using direct numerical simulations at moderate Reynolds numbers $Re_\tau \approx 550\unicode{x2013}1000$ . The canopies have heights $h^+\approx 110\unicode{x2013}220$ , and are used as an instance of obstructing substrate for the assessment of outer-layer similarity. We show that conventional methods used to determine the zero-plane displacement $\Delta y$ can be at odds with proper outer-layer similarity and may not be applicable for flows at moderate $Re_\tau$ . Instead, we determine $\Delta y$ and the length and velocity scales that recover outer-layer similarity by minimising the difference between the smooth-wall and canopy diagnostic function everywhere above the roughness sublayer, not just in the logarithmic layer. In addition, we explore the possibility of the zero-plane displacement and the friction velocity being set independently, but find that outer-layer similarity is recovered more consistently when they are coupled. We observe that although the Kármán constant, $\kappa$ may not have smooth-wall-like values, the flow statistics are smooth-wall-like in the logarithmic layer and above if the surface effect is limited within the near-wall region. This suggests a modified outer-layer similarity, where $\kappa$ is not 0.39, but turbulence is otherwise smooth-wall-like. When the canopy is dense, the flow above the tips is essentially smooth-wall-like, with smooth-wall-like $\kappa \approx 0.39$ and origin essentially at the tip plane. For canopies with intermediate density, the overlying flow perceives a deeper zero-plane displacement into the canopy, which is consistent with observations reported by previous studies, but exhibits a lower Kármán constant, $\kappa \approx 0.34\unicode{x2013}0.36$ . For sparse canopies, $\kappa$ tends back to its smooth-wall value, and the zero-plane displacement height is at the canopy bed. For all canopies studied, the decrease in $\kappa$ never exceeds $15\,\%$ , which is significantly less than that obtained in some previous works using conventional methods to assess outer-layer similarity.

Coupled estimation of global 500m daily aerodynamic roughness length, zero-plane displacement height and canopy height

Aerodynamic roughness length (z0m) and zero-plane displacement height (d), highly related to canopy height (h), are pivotal parameters to characterize the exchange of momentum, heat and water between the land and atmosphere and widely used in many remotely sensed evapotranspiration models, hydrological models and land surface models. In this study, we developed a practical method for coupled estimates of global 500 m daily z0m, d and h by combining the measurements over 2000–2019 at 271 sites worldwide, collocated remote sensing and reanalysis datasets, an optimized scheme, balanced random forest (BRF) and wind profile equation. The h estimated by the optimization (Opt) scheme, in comparison to that by five classical schemes, was in the highest agreement with the dynamic observations at 109 of 271 sites, with a root mean square error (RMSE) of 1.14 m (varying between 0.17 m and 2.58 m over different vegetated surfaces) and a coefficient of determination (R2) of 0.99. Meanwhile, the validation of h estimated by the BRF model against the inferred h at 271 sites and the Lidar-based observations showed an RMSE of 0.13 m - 4.84 m and an R2 of 0.79 - 0.95 over different vegetated land cover types. The z0m estimated by combining the BRF model and the Opt scheme presented a similar global spatial distribution compared to the CFSv2 and ERA5 products in 2019, with the estimated z0m exhibiting the greatest temporal-spatial variability. Moreover, the estimated z0m was more similar in magnitude to the ERA5 product but lower than the CFSv2 product. The generated d product showed similar spatial distribution and temporal-spatial variability to the z0m estimates, but was about 4 to 15 times larger than z0m in magnitude. This study could benefit the improvement of simulation of turbulent flux transfer between the land and atmosphere.

Velocity Distribution in Channels with Submerged Vegetation

An experimental study is presented for investigating the effect of vegetation element geometry on the velocity distribution within and above the canopy. Three types of artificial submerged vegetation elements with common parts and different foliage are used, with two density patterns each. Detailed velocity profiles are obtained and compared at nine locations in the vegetation array. The velocity distribution above the canopy is found to closely follow a logarithmic law and its parameters, namely the shear velocity u*, zero-plane displacement height d and roughness height z0 are determined. These depend on the vegetation density and type of element, but also on the particular position in the array. A unified velocity distribution over the water column is found appropriate for the case of stems with no foliage and also for the cases of compound elements at certain positions in the vegetation array but not at those where locally minimum velocity values occur at the foliage level. Moreover, the logarithmic profile obtained for the upper layer is seen to also fit well the measured velocities to a certain extent below the top of the canopy.

Quantifying and adjusting the impact of urbanization on the observed surface wind speed over China from 1985 to 2017

The observed surface wind speed (SWS) has declined across China over the last four decades, but the mechanisms responsible for this decline have been explored without reaching a consensus. In this study, we develop a physical method to quantify and adjust for the impact of urbanization around weather stations on the observed SWS over China from 1985 to 2017. The urbanization impact factor on the SWS is calculated based on Monin–Obukhov similarity theory, and the aerodynamic roughness length and zero-plane displacement height at each weather station are calculated yearly based on a 30-meter resolution satellite land cover product. The results show that urbanization around weather stations reduces the observed SWS by 11% on average over China. The urbanization impact on the observed SWS is the highest in southeastern China at 19% and the lowest over the Tibetan Plateau at 4%. Urbanization decreases the observed SWS by 9% over northwestern China and by 12% over northeastern China, northern China and southern China. More importantly, the proposed method can easily adjust for the urbanization impact on the observed SWS. After adjustment, the SWS appears to have started recovering during the 1990s, and the decreasing trend of SWS during the study period is nearly zero. The results shown here indicate that the observed decreasing trend of SWS from 1985 to 2017 over China is an observational local bias and does not reflect large-scale climatic variation. This inference is also consistent with geostrophic wind theory predictions; i.e., SWS exhibits strong decadal variability, but its long-term trend is negligible.

Predicting Urban Surface Roughness Aerodynamic Parameters Using Random Forest

AbstractThe surface roughness aerodynamic parameters z0 (roughness length) and d (zero-plane displacement height) are vital to the accuracy of the Monin–Obukhov similarity theory. Deriving improved urban canopy parameterization (UCP) schemes within the conventional framework remains mathematically challenging. The current study explores the potential of a machine-learning (ML) algorithm, a random forest (RF), as a complement to the traditional UCP schemes. Using large-eddy simulation and ensemble sampling, in combination with nonlinear least squares regression of the logarithmic-layer wind profiles, a dataset of approximately 4.5 × 103 samples is established for the aerodynamic parameters and the morphometric statistics, enabling the training of the ML model. While the prediction for d is not as good as the UCP after Kanda et al., the performance for z0 is notable. The RF algorithm also categorizes z0 and d with an exceptional performance score: the overall bell-shaped distributions are well predicted, and the ±0.5σ category (i.e., the 38% percentile) is competently captured (37.8% for z0 and 36.5% for d). Among the morphometric features, the mean and maximum building heights (Have and Hmax, respectively) are found to be of predominant influence on the prediction of z0 and d. A perhaps counterintuitive result is the considerably less striking importance of the building-height variability. Possible reasons are discussed. The feature importance scores could be useful for identifying the contributing factors to the surface aerodynamic characteristics. The results may shed some light on the development of ML-based UCP for mesoscale modeling.

Estimating global aerodynamic parameters in 1982–2017 using remote-sensing data and a turbulent transfer model

The bulk surface properties, including canopy height (h), zero-plane displacement height (d), roughness length (z0), and aerodynamic resistance (rb and rd), are crucial biophysical parameters that influence momentum, energy and mass exchanges at the land-atmosphere interface. The variabilities of these parameters are important for understanding possible impacts of the ecosystem on climate change, yet they have not been systematically evaluated due to the lack of large-scale, long-term observations. Here we provide global estimates of these bulk aerodynamic parameters, including d, z0, rb, and rd, for the period 1982–2017 based on remote-sensed leaf area index (LAI), h, and plant functional type dependent canopy morphological characteristics. The global h estimate is acquired from LAI using a semi-empirical relation in which the coefficients have been optimized based on the canopy height product from the Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat). Two LAI products, Global Land Surface Satellite (GLASS) and Global Inventory Modeling and Mapping Studies (GIMMS), are used to calculate canopy height and parameters separately. The products derived from the above two LAI datasets agree very well (spatial correlation coefficient, SCC = 0.99, relative root-mean-square-error, rRMSE = 8.28% for h, SCC = 0.99, rRMSE = 12.15% for d, and SCC = 0.98, rRMSE = 13.78% for z0). Verification of the products against in-situ canopy height records from the FLUXNET and eddy-covariance (EC)-based d and z0 estimates shows that the estimates can reproduce the annual means and seasonal variations of the bulk surface properties. We found significant positive trends in global h (0.04–0.05% year−1), d (0.07% year−1) and z0 (0.07% year−1) associated with the Earth greening, but they are different from the trend in LAI (0.14–0.25% year−1). The differences between the trends in LAI and the trends in derived parameters are also found in different latitudes. For instance, in the Northern Hemisphere Polar region, the GLASS LAI increases by 0.48% year−1, leading to positive trends of 0.03% year−1 in h, 0.18% in d, and 0.20% year−1 in z0. The above results highlight the importance of comprehensively considering changes in LAI, canopy height, and aerodynamic parameters when evaluating the effects of ecosystem change on climate. This long-term dataset can be used in climate models to assess the response of bulk aerodynamic parameters to climate changes and the impact of vegetation dynamics on regional and global climate.

Automated terrain generation for precise atmospheric boundary layer simulation in the wind tunnel

This study presents a two-stage framework to characterize boundary layer wind tunnel (BLWT) approach flows naturally developed over grid roughness for partial atmospheric boundary layer (ABL) simulation. The first stage applies curve fitting techniques to a comprehensive series of high-resolution spatially-averaged velocity profile measurements to estimate aerodynamic roughness parameters (ARPs) for a wide range of homogeneous (i.e., equal height) roughness element configurations. For this study, an automated (i.e., computer-controlled) 62 ​× ​18 roughness element array called the Terraformer was used to generate 33 unique roughness element fields. The mean flow structure was captured downwind to the Terraformer, where key ARPs—i.e., the urban canopy attenuation coefficient, zero-plane displacement height, shear (friction) velocity, roughness length, and Coles’ wake strength coefficient—were estimated. In contrast to previous ABL modeling methods that primarily focused on curve fitting of the inertial sublayer (ISL), the proposed approach applies the urban canopy exponential profile within the roughness sublayer (RSL), the log law in the ISL, and the law of the wake in the outer wake layer to model full-depth (i.e., floor to freestream) rough-wall turbulent boundary layers. Further, the method explicitly captures potential variability of Reynolds shear stress in the ISL and the wake strength in the outer layer to generalize characterization of naturally-developed BLs produced by traditional tunnel designs. The second stage applies a morphometric model for each ARP—calibrated with estimates from Stage 1—to predict flow characteristics for a wide range of roughness element configurations, with the goal of producing a deterministic solution for selecting an element configuration to satisfy user-specified aerodynamic objectives for the approach flow. The calibrated models effectively interpolate between estimates, e.g., ARPs estimated for open and suburban terrains can be applied in the second stage model calibration to predict ARPs for a “rough-open” condition without further experimentation. The findings of this study demonstrate that coupling the proposed framework with a mechanized roughness element grid can significantly reduce the trial-and-error required to commission a BLWT, while improving the quality of flow characterization.

PIV-based pressure estimation in the canopy of urban-like roughness

In-plane velocity measurements from PIV are used to estimate the pressure field above and within the canopy of two staggered arrays of cuboids, with distinct height distributions, via 2D-RANS and 2D-TH. The viability of this approach is examined by first comparing the mean drag profiles against reported wind-tunnel measurements that were carried out under similar test conditions and numerical simulations (LES and DNS). The surface drag is extrapolated from the nearest data point surrounding the roughness elements. Second, estimates of the friction velocity U_{tau }^p and the zero-plane displacement height d^p are obtained by integrating the axial pressure difference across each individual obstacle, assuming it is spanwise uniform. These are compared against direct measurements of the wall-shear stress from a floating-element balance and a pressure-tapped cube, as well as against estimates from indirect methods. In addition to mean pressure maps, snapshots of the pressure field are obtained via 2D-TH, based on Taylor’s Hypothesis, which are used to compute the RMS of the pressure fluctuations on the surface of a cube. The results indicate that 2D-RANS and 2D-TH perform adequately, providing reasonable estimates of the mean pressure distribution and of the boundary-layer flow parameters, outperforming indirect methods which rely on equilibrium assumptions that are often not verified.Graphic abstract

Influence of wind direction on the surface roughness of vineyards

Remote sensing-based models are the most viable means of collecting the high-resolution spatially distributed estimates of evaporative water loss needed to manage irrigation and ensure the effective use of limited water resources. However, due to the unique canopy structure and configuration of vineyards, these models may not be able to adequately describe the physical processes driving evapotranspiration from vineyards. Using data collected from 2014 to 2016 as a part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration Experiment (GRAPEX), the twofold objective of this study was to (1) identify the relationship between the roughness parameters, zero-plane displacement height (do) and roughness length for momentum (zo), and local environmental conditions, specifically wind direction and vegetation density and (2) determine the effect of using these relationships on the ability of the remote sensing-based Two-Source Energy Balance (TSEB) model to estimate the sensible (H) and latent (λE) heat fluxes. Although little variation in do was identified during the growing season, a well-defined sigmoidal relationship was observed between zo and wind direction. When the output from a version of the TSEB model incorporating these relationships (TSEBVIN) was compared to output from the standard model (TSEBSTD), there were large changes to the roughness parameters, particularly zo, but only modest changes in the turbulent fluxes. When the output from TSEBVIN was compared to that of a version using a parameterization scheme representing open canopies (TSEBOPN), the mean absolute difference between the estimates of do and zo were 0.44 m and 0.25 m, respectively. While these values represent differences in excess of 45%, the turbulent fluxes differed by just 13 W m−2 or 10%, on average. The results suggest that the TSEB model is largely insensitive to changes in the roughness parameters for the range in roughness values evaluated in this study. This also suggests that the requirement for highly accurate roughness values has limited utility in the application of the TSEB model in vineyard systems. Since there is no significant advantage to using the more complex TSEBOPN and TSEBVIN models, it is recommended that the standard model be used.

The impact of urbanization on wind speed and surface aerodynamic characteristics in Beijing during 1991–2011

Urbanization has a significant influence on climate and meteorological conditions through altering surface aerodynamic characteristics. Based on observational data collected at 15 levels on a 325 m meteorological tower in Beijing during 1991–2011, changes in wind speed, vertical profile, aerodynamic roughness length (\(z_{0}\)), and zero-plane displacement height (\(z_{\text{d}}\)) were analyzed. Decreasing trends were observed predominantly during this period, especially for levels between 65 and 140 m where the largest decreasing rates often occur. The annual and seasonal (spring, summer, autumn, and winter) mean wind speeds at 15 levels all present decreasing trends with average rates of 0.029, 0.024, 0.023, 0.040, and 0.019 m s−1 a−1, respectively. The decreases in strong wind categories contribute most to the reduction of mean wind speed. Furthermore, in 2005–2011, the diurnal maximum wind speeds at lower levels tend to appear earlier as compared to those in 1991–1997, while the patterns of diurnal cycle between different levels become more similar in these periods. Besides, the phenomena of “kink” in wind profiles are visible in various atmospheric stabilities, and the average height of a kink has increased from about 40 m to nearly 80 m associated with urbanization during 1991–2011. In addition, the results of \(z_{0}\) and \(z_{\text{d}}\) calculated using the wind profile method vary with wind directions due to surface heterogeneity and that larger values often occur along with southerly winds. Both \(z_{0}\) and \(z_{\text{d}}\) show increasing trends in different sectors during 1991–2011, and the annual mean \(z_{0}\) and \(z_{\text{d}}\) have increased from less than 1 m to greater than 2 m, and from less than 10 m to greater than 20 m, respectively.

Detection of ventilation corridors using a spatio-temporal approach aided by remote sensing data

ABSTRACTThe aim of this study is to detect ventilation corridors in three epochs (1992, 2002 and 2011) by mapping rough urban areas. The roughness parameters calculations are based on a 3D building database obtained from satellite imagery (SPOT) for thee epochs and are conducted accordingly to morphometric methods. Several ventilation paths are clearly recognisable for all three decades and based on the proximity analysis and roughness parameters (by calculating roughness length (z0) and zero-plane displacement height (zd)), we established their extent and analysed the structural and developmental changes. We conducted that changes occurred more rapidly in the second period – almost 90% of the study area was gradually built up, what obstructs the air flow. In addition, winds are blocked in the western part of the city – where the best wind conditions occur. The results urge to re-evaluate the current shape of ventilation corridors and suggest some new areas that should be included in the air ventilation system to improve citizens’ comfort and urban climate.

Observed turbulence characteristics in unstable conditions over the city of Tehran based on similarity theory

In a previous study, applicability of Monin–Obukhov similarity theory (MOST) was established for stable and relatively neutral conditions for Tehran, based on local scaling approach. This paper extends the application of MOST to unstable conditions for Tehran using SODAR data and five 2-D sonic anemometers. Normalized standard deviations of three components of wind velocity ( $$\sigma_{u, v, w} /u_{*}$$ , $$u_{*}$$ is friction velocity) as functions of $$\zeta = (z - z_{d} )/\varLambda$$ stability parameter (where $$\Lambda$$ is the local Obukhov length, $$z$$ is the height of measurement and $$z_d$$ is the zero-plane displacement height) have been computed and analyzed at various levels. Only the parameters measured at 105 m height provided sufficient accuracy for MOST evaluation and also reliable universal similarity relations, while those at the lower heights (45 and 75 m) did not match MOST. Near zero dependence of $$\sigma_{ w} /u_{*}$$ on $$\zeta$$ motivated further investigation using convective velocity scale and also another stability parameter (defined using mixed-layer height). Applying the second scaling parameters leads to more suitable similarity functions. For the present research, we demonstrate that under unstable conditions, normalized values of $$\sigma_{u, v, w}$$ could be collapsed into the similarity expressions, but are highly dependent on proper selection of scale and vertical position. So for the case of Tehran, transformation from one kind of similarity scale to another one is possible, as suggested by Hu and Zhang (Sci Atmos Sin 17:10–20, 1993), with the provision that it only works for certain focused variables and vertical position. In addition, we show significant differences in empirical constants between Tehran and the values reported for the other cities in the literature (from the other surface types). This is most likely due to the local effects of the underlying surface (both urban area and topographical features) in Tehran.

Simulation of Forest Evapotranspiration Using Time-Series Parameterization of the Surface Energy Balance System (SEBS) over the Qilian Mountains

We propose a long-term parameterization scheme for two critical parameters, zero-plane displacement height (d) and aerodynamic roughness length (z0m), that we further use in the Surface Energy Balance System (SEBS). A sensitivity analysis of SEBS indicated that these two parameters largely impact the estimated sensible heat and latent heat fluxes. First, we calibrated regression relationships between measured forest vertical parameters (Lorey’s height and the frontal area index (FAI)) and forest aboveground biomass (AGB). Next, we derived the interannual Lorey’s height and FAI values from our calibrated regression models and corresponding forest AGB dynamics that were converted from interannual carbon fluxes, as simulated from two incorporated ecological models and a 2009 forest basis map These dynamic forest vertical parameters, combined with refined eight-day Global LAnd Surface Satellite (GLASS) LAI products, were applied to estimate the eight-day d, z0m, and, thus, the heat roughness length (z0h). The obtained d, z0m and z0h were then used as forcing for the SEBS model in order to simulate long-term forest evapotranspiration (ET) from 2000 to 2012 within the Qilian Mountains (QMs). As compared with MODIS, MOD16 products at the eddy covariance (EC) site, ET estimates from the SEBS agreed much better with EC measurements (R2 = 0.80 and RMSE = 0.21 mm·day−1).

Modelling of atmospheric boundary-layer flow in complex terrain with different forest parameterizations

This work explores the accuracy of two approaches that account for the effects of the forest canopy on the wind flow by using a RANS-based model. The first approach implements additional terms in the RANS equations (canopy model), whilst the second one uses large values of roughness length and a zero-plane displacement height. The model uses a limited-length-scale k-ϵ turbulence closure that considers processes occurring in the Atmospheric Boundary-Layer (ABL) such as the Coriolis effects. Both the forest and the ABL implementations are compared with experimental data obtained from 118 m high met masts installed in a large mountain- range site with mixed forest characteristics for neutral stability cases. In order to perform a meaningful comparison at multiple mast locations, a novel methodology is presented which allows the selection of a velocity bin for a given wind direction and a stability class that minimizes the error of using short-term measurement periods at some masts compared to long-term wind statistics from a reference mast. Based on the outcome of the model validation it is possible to conclude that more consistent results are obtained by the canopy model since it reduces the uncertainty in the selection of correct input parameters in the large-roughness approach. The errors in the vertical profiles of velocity and turbulence intensity are reduced by the forest model by almost 63% and 11%, respectively, compared to the standard configuration (no forest). The large-roughness method reduces the error in the velocity profiles by 54% while the predictions of turbulence intensity are barely improved.

Evaluation of Turbulent Surface Flux Parameterizations over Tall Grass in a Beijing Suburb

Abstract Numerical weather and climate prediction systems necessitate accurate land surface–atmosphere fluxes, whose determination typically replies on a suite of parameterization schemes. The authors present a field investigation over tall grass in a Beijing suburb, where the aerodynamic roughness length (z0) and zero-plane displacement height (d) are found to be 0.02 and 0.44 m, respectively (the value of d is close to two-thirds the average grass height during this field experiment). Both z0 and d values are then used as input parameters of an analytic model of flux footprint; the footprint model reveals that eddy-covariance flux measurements are mainly representative of the tall grass surface concerned herein, potential influences from anthropogenic sources in this suburban area notwithstanding. Based on the “fair weather” data (with an energy balance ratio of 0.89), the authors evaluate four parameterizations of turbulent surface fluxes, namely, a total of three traditional “iterative” schemes and one “noniterative” scheme developed recently to reduce computational time. Their performances are intercompared in terms of the estimations of the sensible heat flux and two turbulence components (the friction velocity and temperature scale). In weakly stable to unstable conditions, two schemes are recommended here for their good performance overall; the first scheme stems jointly from the work of Högström and Beljaars and Holtslag, and the second stems from that of Li et al.. For this tall grass surface, the choice of z0/z0h = 100 (z0h is the thermal roughness length) is more appropriate than another choice of 10, because the former produces comparatively accurate sensible heat flux estimations.

Estimating zero-plane displacement height and aerodynamic roughness length using synthesis of LiDAR and SPOT-5 data

In this study, a combination of low and high density airborne LiDAR and satellite SPOT-5 HRG data were used in conjunction with ground measurements of forest structure to parameterize four models for zero-plane displacement height d(m) and aerodynamic roughness length z 0 m (m), over cool-temperate forests in Heihe River basin, an arid region of Northwest China. For the whole study area, forest structural parameters including tree height (Ht) (m), first branch height (FBH) (m), crown width (CW) (m) and stand density (SD)(trees ha − 1 ) were derived by stepwise multiple linear regressions of ground-based forest measurements and height quantiles and fractional canopy cover ( f c ) derived from the low density LiDAR data. The high density LiDAR data, which covered a much smaller area than the low density LiDAR data, were used to relate SPOT-5's reflectance to the effective plant area index (PAIe) of the forest. This was done by linear spectrum decomposition and Li–Strahler geometric–optical models. The result of the SPOT-5 spectrum decomposition was applied to the whole area to calculate PAIe (and leaf area index LAI). Then, four roughness models were applied to the study area with these vegetation data derived from the LiDAR and SPOT-5 as input. For validation, measurements at an eddy covariance site in the study area were used. Finally, the four models were compared by plotting histograms of the accumulative distribution of modeled d and z 0 m in the study area. The results showed that the model using by frontal area index (FAI) produced best d estimate, and the model using both LAI and FAI generated the best z 0 m . Furthermore, all models performed much better when the representative tree height was Lorey's mean height instead of using an arithmetic mean.

Zero-Plane Displacement Height in a Highly Built-Up Area of Tokyo

The zero-plane displacement height d was evaluated in downtown Tokyo by the two independent methods of temperature variance and scintillometer heat flux. Regardless of the method, d exceeds the area-weighted average building height. This may be because d, which represents the point of effect for wind drag, is elevated by some tall buildings that jut above others. The areaweighted average building height would then be unsuitable as a geometrical index of urban canopies with large height variations. Thus, the height of the upper envelope of the canopy was proposed as the representative canyon height.

Impact of surface variations on the momentum flux above the urban canopy

Based on the momentum flux–wind profile relationship of the Monin–Obukhov Similarity (MOS) theory, the observational data from the urban boundary layer field campaign in Nanjing are used to calculate the friction velocity ( $$ {u_*} $$ ) at the top of the urban canopy and the calculated results are evaluated. The urban surface roughness parameters (the roughness length z 0 and zero-plane displacement height z d) are estimated with the Ba method (Bottema’s morphological method). Two different regimes are employed for the calculations. In the homogeneous approach, z 0 and z d are averagely derived from the surface elements in the whole study area; while in the heterogeneous approach, z 0 and z d are locally derived from the surface elements in the corresponding upwind fetches (or source areas). The calculated friction velocities are compared to the measurement data. The results show that the calculated friction velocities from the heterogeneous approach are in better agreement with the observed values than those from the homogeneous approach are. This study implies that the local roughness parameters can properly represent the dynamical heterogeneity of urban surface, and its application can significantly improve the performance of parameterizations based on the MOS theory in the urban roughness sublayer.

Urban morphology and air quality in dense residential environments in Hong Kong. Part I: District-level analysis

Abstract The link between urban morphology and air quality in various dense residential environments of Hong Kong are investigated through field measurements and statistical analysis. With regard to the myriads of parameters available in the literature, this study aims to identify the most important urban morphological parameters in the street-level and district levels that affect air quality dispersion in Hong Kong. The study postulates that there is an association between urban morphology and urban air quality and a statistical model is developed to explain the relationship amongst urban air quality, micro-meteorology and urban morphological dimension. The study considered 20 different urban residential areas in five major districts of Hong Kong and real-time street-level air pollutant and microclimatic data are collected. 21 morphological variables are identified and calculated based on the geometry of the urban fabric. This study is the first part of a series which focuses to identify whether district demarcation is correlated with air quality. According to the findings of statistical analysis, only 6 out of 21 morphological variables (complete aspect ratio, occlusivity, roughness height, zero-plane displacement height, total building volume/number of buildings, and standard deviation of height) report distinguishable variability at the district level. Their R2 (correlation coefficient) values vary from 0.2937 (plan area index) to 0.5457 (occlusivity) which is considered moderate to high. All these parameters describe the overall feature of the site rather than the specific geometric features of the street. It translates to the fact that in spite of the large in-district variations in morphological variables amongst the five districts, air pollution variations are insignificant for most of the variables and it means that air quality in Hong Kong are distinguishable not by district-level variable but rather in-site fabrics.