Urban Roughness Research Articles

Local synergies and antagonisms between meteorological factors and air pollution: A 15-year comprehensive study in the Sydney region

Associated with rapid urbanization and escalation of bushfire events, Sydney has experienced significant air quality degradation in the XXI century. In this study, we present a 15-year retrospective analysis on the influence of individual meteorological factors on major air pollutants (NO2, O3, PM10 and PM2.5) at 14 different sites in Greater Sydney and Illawarra. By applying a newly developed “zooming in” approach to long-term ground-based data, we disclose general, seasonal, daily and hourly patterns while increasing the level of spatial associativity. We provide evidence on the pivotal role played by urbanization, sprawling dynamics, global warming and bushfires on local meteorology and air pollution. We strike associations between temperature and O3, both as average trends and extremes, on account of increasing heat island effects. The role of wind in a coastal-basin environment, influenced by a vast desert biome inland, is investigated. A steady trend towards stagnation is outlined, boosted by enhanced urban roughness and intensified heat island circulation. Relative humidity is also crucial in the modulation between NO2 and O3. With a sharp tendency towards drier and hotter microclimates, NO2 levels dropped by approximately 50% over the years at all locations, while O3's median levels almost doubled in the last 10 years. Further, O3 and PMs shifted towards more frequent extreme events, strongly associated with the exacerbation of bushfire events. Such results suggest an urgent need to prioritize emission control, building air tightness improvement and urban heat mitigation, towards a future-proof governance in Sydney and similar regions in the world.

A theoretical study of the influence of urban surfaces on the convective rolls dynamics

Convective rolls are two-dimensional convective quasi-lines that can strongly influence phenomena associated with severe weather. They can also influence the urban micro-climate by increasing the surface wind shear. In this study, we aim to theoretically estimate the influence of the urban heat island and of the urban roughness on the dynamics of the convective rolls. To this purpose, we first derive analytical expressions for the vertical velocity of the rolls in the framework of the Monin–Obukhov similarity theory, which depend on the surface heat flux and of the friction velocity. By using the urban surface roughness, parameterized by its drag coefficient, related directly to the friction velocity, we use the obtained expressions of the vertical velocity of the rolls to visually analyze the contribution of surface heat flux and of the urban surface roughness on the rolls’ velocity. We thus prove that the increased heat fluxes associated with urban surfaces have a strong impact on the convective motion, and that the urban surface roughness can influence the rolls’ dynamics over urban heat islands. Finally, we argue that the studies investigating the urban heat island-induced sub-cloud convective flows would be much improved if, besides the surface temperature distributions and the surface heat fluxes, they would include the urban surface roughness.

Impact of Urban Roughness Representation on Regional Hydrometeorology: An Idealized Study

AbstractMesoscale climate models provide indispensable tools to understand land‐atmosphere interactions over urban regions. However, uncertainties in urban canopy parameters (UCPs) and parameterization schemes lead to degraded representation of the drag effect in complex built terrains. In particular, for the widely applied single‐layer urban canopy model (SLUCM) coupled with the Weather Research and Forecasting (WRF) model, near‐surface horizontal wind speed is known to be overestimated systematically. In this study, idealized large eddy simulations (LES) and WRF‐SLUCM simulations are conducted to study the separate effect of UCPs and aerodynamic parameterization on atmospheric boundary layer processes and rainfall variabilities in Phoenix, Arizona. For LES that explicitly resolves surface geometry, significant differences between three‐dimensional (3D) versus two‐dimensional (2D) representation of urban morphology are found in the surface layer and above. When surface drag is parameterized following SLUCM, surface morphologies have little impacts on the mean momentum transfer. WRF‐SLUCM simulation results, incorporated with 3D urban morphology data, indicate that simply refining the frontal area index will reduce the surface drag, which further amplifies the systematic positive bias of SLUCM in predicting horizontal wind speed. Replacing the drag parameterization in SLUCM by LES‐based aerodynamic parameters has evident impacts on near‐surface wind speed. The impact of urban roughness representation becomes the most evident during rainfall periods, due to the important role of surface drag in dictating moisture convergence. Our study underlines that apart from intensive efforts in obtaining detailed UCPs, it is also critical to enhance the urban momentum exchange parameterization schemes.

Turbulent flows over real heterogeneous urban surfaces: Wind tunnel experiments and Reynolds-averaged Navier-Stokes simulations

Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes (RANS) approaches are used to examine the urban boundary layer (UBL) development above Kowloon Peninsula, Hong Kong Special Administrative Region (HKSAR). The detailed urban morphology is resolved by computational fluid dynamics (CFD) and is fabricated by 3D-printing (reduced scale) for wind tunnel experiments. Different from the majority existing results based on idealized, homogeneous urban geometries, it was found that the wind and turbulence in the UBL over downtown Kowloon are characterized by the wake behind several high-rise buildings. In particular, local maxima of turbulence kinetic energy (TKE) and shear stress are found at the roof level of those high-rise buildings. In the downstream region where the flows are already adjusted to the urban surfaces, the urban roughness sublayer (URSL) can be further divided into two layers based on the structures of the mixing length lm, effective drag Dx and dispersive stress. In the lower URSL (z ≤ 100 m), lm is rather uniform, and the Reynolds stress and dispersive stress are comparable. In the upper URSL (100 m ≤ z ≤ 300 m), on the contrary, lm is peaked at the mid-height and the magnitude of dispersive stress is smaller than that of the Reynolds stress (< 30%). The effective drag Dx is negligible in the upper URSL.

Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures.Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.

PHYSICAL AND NUMERICAL MODELING OF TSUNAMI INUNDATION IN COASTAL URBAN AREA

This coastal hazards emphasize the need for engineers to understand the fundamental processes causing damage and the potential of maximum damage in order to design coastal communities with increased resilience to tsunami events. Common methods used to evaluate local conditions caused by tsunamis include post-disaster reconnaissance field surveys, numerical modelling, and laboratory experiments. Behavior of land side tsunami, inundation, is not well known as well as the fluid forcing, fragility characteristics and accuracy of tsunami hazard mapping. This study targets to understand local tsunami behavior in a city scale including complex buildings and improve modelling of tsunami inundation in an urban area. Laboratory experiments are an essential starting point in the investigation of urban roughness effects on wave propagation and maximum pressures in coastal communities. Physical modelling usually uses solitary wave, bore wave, and wave imitating Nankai Trough Earthquake as a tsunami wave.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/-4niVrzXviE

Observational Practices for Urban Microclimates Using Meteorologically Instrumented Unmanned Aircraft Systems

The urban boundary layer (UBL) is one of the most important and least understood atmospheric domains and, consequently, warrants deep understanding and rigorous analysis via sophisticated experimental and numerical tools. When field experiments have been undertaken, they have primarily been accomplished with either a coarse network of in-situ sensors or slow response sensors based on timing or Doppler shifts, resulting in low resolution and decreasing performance with height. Small unmanned aircraft systems (UASs) offer an opportunity to improve on traditional UBL observational strategies that may require substantive infrastructure or prove impractical in a vibrant city, prohibitively expensive, or coarse in resolution. Multirotor UASs are compact, have the ability to take-off and land vertically, hover for long periods of time, and maneuver easily in all three spatial dimensions, making them advantageous for probing an obstacle-laden environment. Fixed-wing UASs offer an opportunity to cover vast horizontal and vertical distances, at low altitudes, in a continuous manner with high spatial resolution. Hence, fixed-wing UASs are advantageous for observing the roughness sublayer above the highest building height where traditional manned aircraft cannot safely fly. This work presents a methodology for UBL investigations using meteorologically instrumented UASs and discusses lessons learned and best practices garnered from a proof of concept field campaign that focused on the urban canopy layer and roughness sublayer of a large modern city with a high-rise urban canopy.

Identification of pedestrian-level ventilation corridors in downtown Beijing using large-eddy simulations

Ventilation corridor, which is defined as air path that is composed of urban open spaces with low aerodynamic roughness and airflow resistance, has been discussed in Beijing previously. This study links up with different scales of ventilation characteristics in Beijing using multi-scale numerical simulations. Validated Weather Research and Forecasting simulations demonstrate the spatial wind patterns in two seasons and the impact of urban roughness on wind speed. Combined with a Lagrangian backward trajectory model, boundary layer ventilation corridors of Beijing are identified. To provide more fundamental and precise supporting information to tackle the complexity of ventilation environment in the capital city, a set of building-resolved large-eddy simulations (LES) was conducted. Each simulation covers a neighbourhood of 3 km by 3 km and the combined model domain covers the major part of downtown Beijing. Urban canopy ventilation and the influences of urban parameters are investigated. A multiple linear regression model between site-averaged velocity ratios and morphological parameters including building coverage ratio, frontal area index, and building volume is established to identify the importance of different urban factors. Open spaces in urban areas play an important role in urban ventilation. Pedestrian-level ventilation corridors are identified by combining all LES-computed velocity ratios in downtown Beijing. For both annual prevailing (South-south-west) and winter prevailing (North-north-west) winds, three pedestrian-level ventilation corridors were identified. Land use on the upstream and along these corridors should be protected for climatic and environmental enhancement.

A one-dimensional model of turbulent flow through “urban” canopies (MLUCM v2.0): updates based on large-eddy simulation

Abstract. In mesoscale climate models, urban canopy flow is typically parameterized in terms of the horizontally averaged (1-D) flow and scalar transport, and these parameterizations can be informed by computational fluid dynamics (CFD) simulations of the urban climate at the microscale. Reynolds averaged Navier–Stokes simulation (RANS) models have previously been employed to derive vertical profiles of turbulent length scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as large-eddy simulations (LES), we observed that vertical profiles of turbulent kinetic energy and associated turbulent length scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of turbulent flow through urban areas and updated the parameterization of turbulent length scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density (λp) is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). In order to ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and direct numerical simulation (DNS) studies. After implementing the updated drag coefficients and turbulent length scales in the 1-D model of urban canopy flow, we evaluated the results by (a) testing the 1-D model against the original LES results and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and (b) testing the 1-D model against LES results for a test case with realistic geometries. Results suggest a more accurate prediction of vertical turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoscale.

Multiscale modeling of the atmospheric environment over a forest canopy

Vegetation constitutes one of the fundamental types of land use on Earth. The presence of trees in urban areas can decrease local winds and exchange sensible and latent heat with the surrounding environments, thus exerting notable influences on the urban microenvironment. A better understanding of the turbulent transfer of momentum and scalars around vegetation canopy could significantly contribute to improvement of the urban environment. This work develops a large-eddy simulation (LES) method that is applicable to model the flow and scalar transport over the forest canopy. We study the atmospheric flow over complex forested areas under typical weather conditions by coupling LES to the mesoscale model. Models of radiation and energy balance have been developed with explicit treatment of the vegetation canopy. By examining the flow over a forest canopy under a range of stability conditions, we found that buoyancy enhances or suppresses turbulent mixing in unstable or stable atmosphere respectively, with decreasing or increasing wind shear, respectively. From the multiscale modeling of the Beijing Olympic Forest Park, the present coupling scheme proves to better resolve the diurnal variations in wind speed, temperature, and relative humidity over complex urban terrains. The coupling scheme is superior to the traditional mesoscale model in terms of wind field simulation. This is mainly because the coupling scheme not only takes the influences of external mesoscale flow into consideration, but also resolves the heterogeneous urban surface at a fine scale by downscaling, thus better reproducing the complex flow and turbulent transport in the urban roughness sublayer.

MULTISCALE ENVIRONMENTAL MONITORING IN URBAN TOPOCLIMATE

Climate is a complex system subject to much natural and anthropogenic interference. Urban microclimates and topoclimates need effective methodologies to localize coexisting phenomena, including multiscale approaches that demonstrate their relational character. Air temperature, air relative humidity, and wind are microclimatic variables that can be evaluated with respect to their qualitative energy flow characteristics. The technical performance of this monitoring depends on a good estimate of the statistics of the digital signal assessed, which in turn represents greater care in the management and implementation of structural change indicators derived from frequency-domain modeling. Therefore, this study aimed to analyze the power spectra of the air temperature, air relative humidity and wind data series using the traditional Morlet Continuous Wavelet Transform (CWT) and the Cross Wavelet Transform (XWT), with Wavelet Coherence (WC) analysis. The daily, monthly and annual scales were studied, and the results demonstrated significant interrelationships between the variables. The WC values found were validated by the Monte Carlo method, scalograms and spectrograms, representing seasonal and daily cycle frequencies. The urban fractions presented behavior in accordance with the seasonality of tropical climates of the Cerrado, as well as of interference of the urban roughness. The CWT, XWT and WC tools are adequate for urban monitoring and planning analyses, qualitatively reflecting the urban dynamics of microenvironments

Parametrizing Horizontally-Averaged Wind and Temperature Profiles in the Urban Roughness Sublayer

Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin–Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment.

Numerical modeling of tsunami inundation using upscaled urban roughness parameterization

This study develops and validates a numerical model of tsunami inundation using upscaled urban roughness parameterization and a Drag Force Model (DFM) to simulate the effect of structures as a drag force acting on flow. The DFM can deal with subgrid scale effects of structures using three upscaled parameters: the characteristic height of the structures, and the projected areas of the structures in the x− and y− directions. The validation is based on experimental results of the physical modeling of tsunami inundation conducted in the HyTOFU flume targeting the 2011 Tohoku Earthquake Tsunami; the physical model was a 1:250 scale idealization of the town of Onagawa in Japan. This study numerically simulated tsunami inundation by comparing the experimental results with numerical results reproduced by the DFM using roughness parameterizations. The validation of the DFM reveals that it can express the effect of the flow direction and inundation ratio, and it improves the estimation of inundated extent, maximum inundation depth, and arrival time when compared with other roughness parameterizations. This technique requires careful validation of inundation characteristics and mesh size dependency, and future work to improve estimates of the drag coefficient for the prototype scale and incident wave conditions is recommended.

Transport mechanism of urban plume dispersion

Winds (advection) and turbulence (diffusion) are the driving forces for urban plume dispersion whose partitioning is complicated by urban morphology and the conditions of atmospheric surface layer (ASL). Wind tunnel parametric tests are conducted to examine the plume behavior in the turbulent boundary layers (TBLs) over hypothetical buildings (roughness elements of ribs and bricks) with drag coefficient Cd (= 2uτ2/U∞2; where uτ and U∞ are, respectively, the friction velocity and the freestream velocity) varying from 4.7 × 10−3 to 10 × 10−3. Vehicular emission is modeled by atomizing water vapor via a ground-level line source in crossflows. The laboratory measurements show that the mean tracer concentrations ψ‾ exhibit the theoretical Gaussian form, supporting the classic plume dispersion framework. The horizontal advection tracer flux u‾ψ‾ dominates the transport processes compared with the vertical turbulent tracer flux w''ψ''‾. Increasing Cd leads to a decrease in u‾ψ‾ and an increase in w''ψ''‾ simultaneously, and vice versa. The changes in partitioning of transport processes in response to urban morphology imply that the conventional parameterization of dispersion coefficient σz (determined by atmospheric stability only) should be applied cautiously for urban setting. A new simple scheme of σz parameterization, which specifically accounts for the influence from urban roughness on the plume dispersion, is proposed.

Geomorphic Responses to Extreme Rainfall, Catastrophic Flooding, and Dam Failures across an Urban to Rural Landscape

The extreme rainfall of October 2015 in South Carolina generated numerous dam failures and spawned the flood of record at most U.S. Geological Survey stream gauges. Detailed field sampling and systematic image analysis are used to document the immediate and sustained geomorphic adjustments at four failed dams within the urbanized Gills Creek watershed. That urban focus is augmented with a similar analysis at five failed dams in more rural settings where less urban infrastructure exists. We also document the magnitude and type of geomorphic adjustments throughout the Gills Creek watershed unrelated to dam failures. Despite the extreme rainfall and associated flooding, the geomorphic effects were limited and localized, manifesting primarily at dam failures but not progressing significantly downstream, especially along Gills Creek, where the combination of intact dams, frequent urban roughness elements, thick floodplain vegetation, and numerous wetlands explains the lack of significant morphologic adjustments or major deposition. Dam failures in rural settings showed greater overbank deposition but the geomorphic effects were still limited to dam proximal locations because of the thick vegetal cover below the dam. Because these dams in rural and urban settings were constructed to maximize waterfront property, low gradient sites were selected; therefore, the geomorphic effects were unlike planned dam removals where massive headcutting, knickpoint migration, and sediment evacuation tend to occur. The most significant effect of the dam failures was the shift from an initial braided channel to a single thread channel within the emptied reservoirs, which occurred quickly, often within the first few months.

NUMERICAL MODELING OF TSUNAMI INUNDATION USING SUBGRID SCALE URBAN ROUGHNESS PARAMETERIZATION

The importance of accurate numerical modeling of tsunami inundation in an urban area has clearly realized due to the devastating damage from 2011 Tohoku Earthquake Tsunami. Although, numerical inundation simulations using high resolution topography data (O(1m)), the medium resolution tsunami inundation model (O(10m)-O(100m)) needs and useful for tsunami hazard assessment. This study develops and validates a numerical model of tsunami inundation using upscaled urban roughness parameterization: Drag Force Model (DFM) which deals with the effect of structures as drag force acting on flow based on physical modeling. The validation of the DFM reveals that the DFM can express the effect of the flow direction and inundation ratio.

Effects of a Fence on Pollutant Dispersion in a Boundary Layer Exposed to a Rural-to-Urban Transition

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.

Flow and temperature dynamics in an urban canyon under a comprehensive set of wind directions, wind speeds, and thermal stability conditions

Atmospheric flow and temperature dynamics in the urban roughness sublayer exhibit numerous complexities that cannot all be investigated using models or scaled-down experiments, thus these complexities necessitate careful field observations. Such dynamics were studied under a comprehensive set of wind directions, wind speeds, and thermal stability conditions in a field campaign held in Guelph, Ontario, Canada, from 13th to 25th of August 2017. The urban site was a quasi-two-dimensional canyon with unit canyon aspect ratio. Beside characterizing thermal stability, inertial effects, urban heat island intensity, and mean properties of the atmosphere, the turbulence statistics were studied carefully as functions of roof-level wind angle, diurnal time, the building Reynolds number, or the bulk Richardson number. Turbulence statistics in the vertical direction were observed to be influenced by local conditions, such as flow properties and nearby surface temperatures. These statistics also indicated presence of small integral lengthscales and short integral timescales. On the other hand, turbulence statistics in the horizontal directions were influenced by non-local conditions, such as horizontal heterogeneity in heating or urban morphology. These statistics indicated presence of large integral lengthscales and long integral timescales. In addition, a rigorous scaling analysis was performed to seek significant correlations between turbulence statistics and other known flow variables both locally or as measured at a nearby non-urban rural station. Variances scaled more successfully to mean quantities than covariances (Reynolds stresses and turbulence kinematic heat fluxes). In addition, the statistics in the vertical direction scaled more successfully compared to horizontal directions. Statistics in the horizontal directions, particularly in the along-canyon axis direction, were poorly scaled, suggesting presence of unorganized and irregular turbulence structures influenced by non-local conditions. Temperature difference between the atmosphere and nearby surfaces as well as measured velocity scales showed to scale vertical heat fluxes successfully.

Large eddy simulation of flow through a periodic array of urban-like obstacles using a canopy stress method

Large-eddy simulation (LES) of a turbulent flow through an array of building-like obstacles is an idealized model to study transport of contaminants in the urban atmospheric boundary layer (urban ABL). A reasonably accurate LES prediction of turbulence in such an urban ABL must resolve a significant proportion of the small but energetic eddies in the roughness sublayer, which remains prohibitive even though computational power has been increased significantly. Recently, some researchers also reported a high level of inaccuracy in turbulence prediction if LES were coupled with an adaptive mesh refinement technique in order to optimize the cost of resolving the roughness sublayer. In this article, we present a turbulence closure methodology for LES of urban ABLs in which the roughness elements are represented through the canopy stress method, and the subgrid scale stress is modeled through a dynamically adaptive eddy viscosity method. Unlike the classical Smagorinsky model that considers only the ‘strain portion’ of the velocity gradient tensor, we consider both the ‘strain tensor’ and the ‘rotation tensor’ to compute the eddy viscosity. This allows us to dynamically adapt the rate of energy dissipation to the scale of the energetic eddies in the roughness sublayer. Without employing a mesh conforming to the urban roughness elements, the effect of such solid bodies are represented in the LES model through a canopy stress method in which the loss of pressure and the sink of momentum due to the interaction between eddies and roughness elements are parameterized using the instantaneous velocity field. Simulation results of the proposed canopy stress method is compared with that of a conventional Computational Fluid Dynamics (CFD) method employing a block-structured mesh conforming around the roughness elements. For urban flow simulations, the results demonstrate that the proposed canopy stress model is accurate in predicting vertical profiles of mean and variance, as well as the temporal intermittency of coherent structures.

A wind tunnel study of ventilation mechanism over hypothetical urban roughness: The role of intermittent motion scales

Urban morphology is a major factor governing the dynamics in atmospheric surface layers (ASLs) of which our understanding is rather limited. In this paper, wind tunnel experiments are conducted to characterize the flows over different types of urban roughness in attempt to demystify the mechanism of street-level ventilation in isothermal conditions. Hypothetical urban areas are assembled by idealized street canyons using aluminum square tubes (ribs) and plastic LEGO® bricks (cubes). The velocity components are sampled by hot-wire anemometry (HWA) with X-wire probes. The drag coefficient Cd (= 2uτ2/U∞2; where uτ is the friction velocity and U∞ the freestream wind speed) is used to measure the aerodynamic resistance (3.588 × 10−3 ≤ Cd ≤ 10.799 × 10−3) and parameterize the street-level ventilation of urban areas. The results show that the air exchange rate ACH, as a measure of the aged air removal, is proportional to the root of drag coefficient (ACH ∝ Cd1/2), implying that rougher urban surfaces favor street-level ventilation. Quadrant analyses illustrate that ejection (Q2) and sweep (Q4) are enhanced by aerodynamic resistance so are the transport processes. Frequency spectra further demonstrate that the dynamics is dominated by large-scale motions (f × δ/uτ ≤ 10; where f is the spatial frequency and δ the thickness of turbulent boundary layer) which are more energetic with increasing drag coefficient. The above findings collectively suggest the importance of ASL large scales to street-level ventilation. In addition to promoting ground-level mean wind speed, increasing urban roughness could be a solution to the air quality problems nowadays.