Frontal Area Index Research Articles

Urban morphology detection and computation for urban climate research

Urban morphology information is an important indicator in urban planning, information management, and urban climatic applications. However, the problem of urban morphological data harmonization, quality, and availability are well-known, especially in developing countries. In this study, a novel satellite-based approach was proposed to extract 3D urban morphology information, and then retrieve and validate typical urban morphological parameters for urban climatic applications. Some most widely used urban morphological parameters—building coverage ratio (BCR), building height (BH), building volume density (BVD), frontal area index (FAI), sky view factor (SVF), and roughness length (RL)—were calculated and validated. Experiments conducted throughout the entire urban environment of Kowloon Peninsula, the most complex and high-density urban area in Hong Kong, demonstrated that all of the retrieved parameters had a high prediction accuracy, compared with the actual data at spatial resolutions of hundreds of meters. In particular, the prediction accuracy of BCR, BH, BVD, and RL was 70–80% while the accuracy of SVF and FAI was 80–90%. This set of urban morphological data is employed to generate a Hong Kong Urban Climatic Map. As an extension of the World Urban Database and Access Portal Tools (WUDAPT) method to provide 2D urban form data for cities worldwide, this newly developed method can quickly and efficiently produce highly accurate 3D urban morphology data for cities where actual urban morphology data are not accessible. This study also expands spatial understanding of urban climatic conditions in a fast and efficient way, enabling urban climatic application in urban planning for sustainable urban living.

Evaluating Urban Forms for Comparison Studies in the Massing Design Stage

We introduce five performance indicators to facilitate the comparison of urban massing design in the early design stages. The five simple indicators are based on existing studies and cover three main performance areas that are sensitive to urban form changes: solar, ventilation, and connectivity potentials. The first three indicators—the non-solar heated façade to floor area index, daylight façade to floor area index, and photovoltaics envelope to floor area index—measure the solar potential. The frontal area index measures the ventilation potential and the route-directness index measures the connectivity potential. The indicators are simple to use, as they only require urban geometry data for their calculation. We demonstrate the indicators in two case studies; variations in the values of these indicators show that they are sensitive to urban form changes and can be used in comparative studies to identify better performing urban forms among massing designs. We implement the indicators as an open-source Python library, Pyliburo, that designers and researchers can readily access and integrate into their existing design workflows.

Assessing the role of mangrove forest in reducing coastal inundation during major hurricanes

A vegetation-resolving CH3D-SWAN surge–wave modeling system is used to examine the role of mangroves and salt marshes along the shore of Biscayne Bay in buffering surge, wave, and inundation in Southeast Florida during Hurricane Andrew (1992). First, the 3D vegetation-resolving model is validated by comparing the simulated and measured high water marks from post-hurricane field survey, debris lines, and time series of water level at the Haulover Pier. The simulated water levels and magnitude and extent of maximum inundation agree well with the observed data, whereas the removal of vegetation from the model leads to massive flooding with increased total inundation volume and total inundation area in the highly populated low-lying area behind the Biscayne Bay. Additional simulations show that the surge–wave–inundation buffering capacity of the mangrove forest depends on the vertical structure of the wetted leaf area index, A_{text{w}} and the frontal leaf area index, A_{text{f}}. The study demonstrates the capability of CH3D-SWAN in quantifying the role of mangroves in buffering storm surge, wave, and inundation, and demonstrates its potential application for assessing the effectiveness of coastal wetland restoration projects. Meanwhile, the accurate representation of vegetation’s vertical structure can enhance the numerical modeling of flow–vegetation interaction processes.

Modeling of urban wind ventilation using high resolution airborne LiDAR data

Accurate mapping of wind ventilation in an urban environment is challenging when large spatial coverage is required. This study has developed a GIS-based model for estimating the frontal area index (FAI) of buildings, infrastructure, and trees using very high resolution airborne light detection and ranging (LiDAR) data, which can also be used to investigate the “wall effect” caused by high-rise buildings at a finer spatial scale along the coasts in the Kowloon Peninsula of Hong Kong. New algorithms were created by improving previous algorithms utilizing airborne LiDAR data in raster unit, as well as considering the backward flow coefficient between windward and leeward buildings.The ventilation corridors estimated by FAI and least cost path (LCP) analysis were analyzed. The optimal ventilation corridors passing through the Kowloon peninsula were observed in the east-west and west-east directions. In addition, these ventilation paths were validated with a computer fluid dynamics (CFD) model i.e. Airflow Analysis in ESRI. The newly developed model calculates finer FAI with greater accuracy when compared with vector-based building polygons. This model further depicts buildings, infrastructure, and trees which are considered as obstacles to wind ventilation. The results can be used by environmental and planning authorities to identify ventilation corridors, and for scenario analysis in urban redevelopment.

Effect of flexible and rigid roughness elements on aeolian sand transport

ABSTRACTIn developing countermeasures to reduce the negative effects of strong dust events, the fundamental relationship between surface conditions and sand transport remains problematic. We conducted field observations and wind tunnel experiments to examine the effect on sand transport efficiency of actual frontal area when using flexible roughness elements (artificial grass) and rigid roughness elements (tufts of stiff wire). In the field observations, the sand trap ratio approached a limit as the frontal area of artificial grass, measured as the Actual Frontal Area Index (AFAI), approached 25%, equivalent to a vegetation cover of 20%. The wind tunnel experiments showed that the height of deposited sediment decreased downwind with both roughness elements, due to their reduction of both shear velocity and sand transport rates. The sediment flux decreased as the AFAI increased, and the rigid roughness element trapped more sand particles than the flexible one except at higher wind speeds and lower AFAI values. The sand trapping efficiency was greater for rigid roughness elements than for flexible ones, probably due to their high aerodynamic resistance, and thus wire tufts were more effective for trapping sand than artificial grass at higher AFAI values.

A quantitative method to detect the ventilation paths in a mountainous urban city for urban planning: A case study in Guizhou, China

In this paper, a new frontal area index (FAI) mapping method is presented to describe the surface roughness of Renhuai, as an example of typical medium high mountain gorge type Karst city. Comparing with the traditional calculating method of FAI, a local topography (hills) factor is added to the calculation of the FAI in each 100 m × 100 m grid cell. The results show that the modified FAI is more strongly related to the mountainous land-use type than traditional, and local topography regions with large podium structure had higher values than other urban land-use types in mountainous city. With the frontal area index mapping procedure and a self-compiled least cost path analysis method, the potential airflow corridors traversing through the study area can be located, and the total computation time is shorter and less than 1 s. Air volume, a significant measurement index of urban ventilation capacity, is then simulated in computational fluid dynamics model (CFD-PHOENICS) to confirm the significance and efficiency of these specific ventilation corridors. Based on our findings, the government and urban planners may use the descendent maps to understand the urban ventilation paths within a mountainous city for urban local renovation decisions.

Developing Street-Level PM2.5 and PM10 Land Use Regression Models in High-Density Hong Kong with Urban Morphological Factors.

Monitoring street-level particulates is essential to air quality management but challenging in high-density Hong Kong due to limitations in local monitoring network and the complexities of street environment. By employing vehicle-based mobile measurements, land use regression (LUR) models were developed to estimate the spatial variation of PM2.5 and PM10 in the downtown area of Hong Kong. Sampling runs were conducted along routes measuring a total of 30 km during a selected measurement period of total 14 days. In total, 321 independent variables were examined to develop LUR models by using stepwise regression with PM2.5 and PM10 as dependent variables. Approximately, 10% increases in the model adjusted R(2) were achieved by integrating urban/building morphology as independent variables into the LUR models. Resultant LUR models show that the most decisive factors on street-level air quality in Hong Kong are frontal area index, an urban/building morphological parameter, and road network line density and traffic volume, two parameters of road traffic. The adjusted R(2) of the final LUR models of PM2.5 and PM10 are 0.633 and 0.707, respectively. These results indicate that urban morphology is more decisive to the street-level air quality in high-density cities than other cities. Air pollution hotspots were also identified based on the LUR mapping.

HISTORICAL GIS DATA AND CHANGES IN URBAN MORPHOLOGICAL PARAMETERS FOR THE ANALYSIS OF URBAN HEAT ISLANDS IN HONG KONG

Abstract. Rapid urban development between the 1960 and 2010 decades have changed the urban landscape and pattern in the Kowloon Peninsula of Hong Kong. This paper aims to study the changes of urban morphological parameters between the 1985 and 2010 and explore their influences on the urban heat island (UHI) effect. This study applied a mono-window algorithm to retrieve the land surface temperature (LST) using Landsat Thematic Mapper (TM) images from 1987 to 2009. In order to estimate the effects of local urban morphological parameters to LST, the global surface temperature anomaly was analysed. Historical 3D building model was developed based on aerial photogrammetry technique using aerial photographs from 1964 to 2010, in which the urban digital surface models (DSMs) including elevations of infrastructures and buildings have been generated. Then, urban morphological parameters (i.e. frontal area index (FAI), sky view factor (SVF)), vegetation fractional cover (VFC), global solar radiation (GSR), Normalized Difference Built-Up Index (NDBI), wind speed were derived. Finally, a linear regression method in Waikato Environment for Knowledge Analysis (WEKA) was used to build prediction model for revealing LST spatial patterns. Results show that the final apparent surface temperature have uncertainties less than 1 degree Celsius. The comparison between the simulated and actual spatial pattern of LST in 2009 showed that the correlation coefficient is 0.65, mean absolute error (MAE) is 1.24 degree Celsius, and root mean square error (RMSE) is 1.51 degree Celsius of 22,429 pixels.

HISTORICAL GIS DATA AND CHANGES IN URBAN MORPHOLOGICAL PARAMETERS FOR THE ANALYSIS OF URBAN HEAT ISLANDS IN HONG KONG

Rapid urban development between the 1960 and 2010 decades have changed the urban landscape and pattern in the Kowloon Peninsula of Hong Kong. This paper aims to study the changes of urban morphological parameters between the 1985 and 2010 and explore their influences on the urban heat island (UHI) effect. This study applied a mono-window algorithm to retrieve the land surface temperature (LST) using Landsat Thematic Mapper (TM) images from 1987 to 2009. In order to estimate the effects of local urban morphological parameters to LST, the global surface temperature anomaly was analysed. Historical 3D building model was developed based on aerial photogrammetry technique using aerial photographs from 1964 to 2010, in which the urban digital surface models (DSMs) including elevations of infrastructures and buildings have been generated. Then, urban morphological parameters (i.e. frontal area index (FAI), sky view factor (SVF)), vegetation fractional cover (VFC), global solar radiation (GSR), Normalized Difference Built-Up Index (NDBI), wind speed were derived. Finally, a linear regression method in Waikato Environment for Knowledge Analysis (WEKA) was used to build prediction model for revealing LST spatial patterns. Results show that the final apparent surface temperature have uncertainties less than 1 degree Celsius. The comparison between the simulated and actual spatial pattern of LST in 2009 showed that the correlation coefficient is 0.65, mean absolute error (MAE) is 1.24 degree Celsius, and root mean square error (RMSE) is 1.51 degree Celsius of 22,429 pixels.

Mitigating urban heat islands: A method to identify potential wind corridor for cooling and ventilation

In summer, the urban heat island effect causes unbearable warmth in the cities of Taiwan. This study maps out ventilation paths that are expected to draw in breezes from suburban areas and the sea to mitigate the urban heat island effect. Wind corridors are estimated based on the concept that wind moves along paths of low urban roughness, as defined by the frontal area index (FAI) and the least cost path (LCP) methodology followed in this study. The primary wind corridor was proposed with a main function of identifying wind that minimizes net daytime heating through a redistribution of the total surface heat flux. Land use that affects urban temperature is discussed by indicating the areas of potential rises and drops in air temperature in order to describe the temperature influences in both windward and leeward areas. The horizontal resolution of the calculation unit was a hundred meters, which is called an urban block scale in this study. At that resolution, detailed analyses can be discussed in a detailed plan scheme. A slightly more sophisticated CFD model confirms the more simplistic GIS-based mode, and thus, these results have the potential to guide urban wind corridor planning efforts. The land use map of potential temperature influences is superimposed with the current wind corridors to determine the exact areas in which to execute the mitigation strategies. Four sites are recommended for urban surface improvement according to their land uses, respectively. Furthermore, a potential cooling route on the east side of the study area was suggested for the FAI reduction strategy.

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).

Developing a thermal atlas for climate-responsive urban design based on empirical modeling and urban morphological analysis

This paper aims at developing a thermal atlas system at the urban district scale to evaluate the urban thermal environment and identify thermally vulnerable areas. Shanghai Lujiazui Central Business District was selected as a test bed for the method. Six key morphological indicators, i.e., sky view factor, pavement cover ratio, vehicle traffic density, green plot ratio, frontal area index and proximity to heat sink were used to describe different aspects of the urban thermal environment including building density, land use, anthropogenic heat, greenery, ventilation potential and heat sink. Spatial analysis tools were developed in geographical information system to generate maps for these morphological indicators. In situ meteorological measurements were carried out on peak summer days to investigate the spatial variation of microclimatic parameters, based on which the physiological equivalent temperature (PET) index was calculated. Empirical regression models were built correlating PET values with local morphological indicators, based on which PET maps were generated. This thermal atlas system can rapidly analyze and visualize the spatial variations of urban microclimate from the thermal comfort aspects as affected by different urban design scenarios, thus can be a useful decision-support tool for urban design towards alleviating urban heat island intensity and improving outdoor thermal comfort.

Determination of aerodynamic parameters of urban surfaces: methods and results revisited

Estimates of aerodynamic parameters, in particular roughness length z0 and displacement height d, are important for the analysis of the roughness of an urban surface, which affects processes that occur within the urban boundary layer such as pollutant dispersion and urban ventilation. Findings regarding the aerodynamic effects of various configurations of urban arrays were compiled from various studies. Several experimental, numerical and semi-empirical studies to estimate z0 and d were reviewed and compared with each other. The results can be summarized as follows: (1) the influence of the frontal area index (λf) on z0 is significant and their relationship has been confirmed by both experimental and numerical data; (2) compared to one-parameter and two-parameter fitting methods, the three-parameter fitting method is the least accurate; (3) the physical meaning of d remains vague because its definition as the height where surface drag acts may not be accurate for sharp-edged roughness blocks and (4) the peak values of z0 for uniform and heterogeneous block heights indicate presence of skimming or wake-interference flow effects, which may influence surface roughness. Finally, the semi-empirical models were found to be limited to cases derived from available experimental data, which normally involve uniform arrays of cubes.

Influence of mesoscale urban morphology on the spatial noise attenuation of flyover aircrafts

The influence of urban morphology of low-density built-up areas on spatial noise level attenuation of flyover aircrafts is investigated at a mesoscale. Six urban morphological parameters, including Building Plan Area Fraction, Complete Aspect Ratio, Building Surface Area to Plan Area Ratio, Building Frontal Area Index, Height-to-Width Ratio, and Horizontal Distance of First-row Building to Flight Path, have been selected and developed. Effects of flight altitude and horizontal flight path distance to site, on spatial aircraft noise attenuation, are examined, considering open areas and façades. Twenty sampled sites, each of 250m*250m, are considered. The results show that within 1000m horizontal distance of flight path to a site, urban morphology plays an important role in open areas, especially for the buildings with high sound absorption façades, where the variance of average noise level attenuation among different sites is about 4.6dB at 3150Hz. The effect of flight altitude of 200–400ft on average noise level attenuation is not significant, within about 2dB at both 630Hz and1600Hz in open areas. Urban morphological parameters influence the noise attenuation more in open areas than that on façades. Spatial noise attenuation of flyover aircrafts is mainly correlated to Building Frontal Area Index and Horizontal Distance of First-row Building to Flight Path.

Wind tunnel study on the morphological parameterization of building non-uniformity

Because there is a lack of systematic studies on non-uniform buildings and most of them are qualitative research, this study implemented a morphological parameterization scheme to determine building morphology, which includes the frontal area index (λf), planar area index (λp), shape index (SBC), and integrated non-linear coefficient (R). This scheme can systematically describe the non-uniformity of buildings with high precision. In addition, wind tunnel experiments were carried out to study the effects of the morphologic parameters of buildings on the drag force. A floating experimental platform was designed to measure the drag force of the whole area. The results showed that different levels of terrain roughness have little effect on the drag coefficient. In contrast, the frontal area index is the main factor that affects the drag coefficient when the wind direction changes. The drag coefficient was found to increase with increases in the frontal area index, density index, and shape index as well as decreases in the integrated non-linear coefficient.

A New Aerodynamic Parametrization for Real Urban Surfaces

This study conducted large-eddy simulations (LES) of fully developed turbulent flow within and above explicitly resolved buildings in Tokyo and Nagoya, Japan. The more than 100 LES results, each covering a 1,000 $$\times $$ 1,000 m $$^{2}$$ area with 2-m resolution, provide a database of the horizontally-averaged turbulent statistics and surface drag corresponding to various urban morphologies. The vertical profiles of horizontally-averaged wind velocity mostly follow a logarithmic law even for districts with high-rise buildings, allowing estimates of aerodynamic parameters such as displacement height and roughness length using the von Karman constant $$=$$ 0.4. As an alternative derivation of the aerodynamic parameters, a regression of roughness length and variable Karman constant was also attempted, using a displacement height physically determined as the central height of drag action. Although both the regression methods worked, the former gives larger (smaller) values of displacement height (roughness length) by 20–25 % than the latter. The LES database clearly illustrates the essential difference in bulk flow properties between real urban surfaces and simplified arrays. The vertical profiles of horizontally-averaged momentum flux were influenced by the maximum building height and the standard deviation of building height, as well as conventional geometric parameters such as the average building height, frontal area index, and plane area index. On the basis of these investigations, a new aerodynamic parametrization of roughness length and displacement height in terms of the five geometric parameters described above was empirically proposed. The new parametrizations work well for both real urban morphologies and simplified model geometries.

Spatial variability of frontal area index and its relationship with urban heat island intensity

In densely urbanized regions, the local climate is greatly influenced by the urban morphology, including interactions between buildings, space, and human activities. The Kowloon Peninsula in Hong Kong, with some of the greatest urban population densities in the world, represents an extreme case of the influence of the built environment on climate, with high-rise buildings, narrow street canyons, and little green space. In this study, the building frontal area index (FAI), a parameter for estimating aerodynamic resistance of the urban surface as a predictor of wind ventilation, was derived from a three-dimensional building database. The relationship between FAI and urban heat island intensity (UHII) from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite image was calculated at different scales. The highest correlation (r = 0.574, n = 4900) was obtained at 100 m resolution, which suggests that the optimum operational scale of FAI is 100 m resolution for the study area, i.e. the scale and size at which FAI impacts on the urban climate. The presence/trend of any higher correlation at different resolutions was tested using the nonparametric Mann–Kendall test, and the results show that the statistic Z-value generated from the test is smaller than the hypothesis significance levels of 90%, 95%, and 99%; thus, the hypothesis of having a higher correlation at any scale other than 100 m resolution is rejected. Planning authorities may use the FAI generated at 100 m resolution for designing wind ventilation corridors across Hong Kong at this scale, especially when temperature and air quality in the inner city are of major concern. However, for applications to other cities with different standard morphologies, the FAI–UHII relationship should be re-evaluated.

Equations for the Drag Force and Aerodynamic Roughness Length of Urban Areas with Random Building Heights

We use a conceptual model to investigate how randomly varying building heights within a city affect the atmospheric drag forces and the aerodynamic roughness length of the city. The model is based on the assumptions regarding wake spreading and mutual sheltering effects proposed by Raupach (Boundary-Layer Meteorol 60:375–395, 1992). It is applied both to canopies having uniform building heights and to those having the same building density and mean height, but with variability about the mean. For each simulated urban area, a correction is determined, due to height variability, to the shear stress predicted for the uniform building height case. It is found that u*/u*R, where u* is the friction velocity and u*R is the friction velocity from the uniform building height case, is expressed well as an algebraic function of λ and σh/hm, where λ is the frontal area index, σh is the standard deviation of the building height, and hm is the mean building height. The simulations also resulted in a simple algebraic relation for z0/z0R as a function of λ and σh/hm, where z0 is the aerodynamic roughness length and z0R is z0 found from the original Raupach formulation for a uniform canopy. Model results are in keeping with those of several previous studies.

Large-Eddy Simulation of Plume Dispersion within a Regular Array of Cubic Buildings

*There is a potential problem that the intentional release of radioactive materials by terror attacks and the accidental spillage from the transportation or storage of flammable and toxic gas occur within populated urban area. For the assessment of human health hazard or the safety analysis of the hazardous gas, not only mean but also fluctuating concentrations should be estimated. Therefore, we perform Large-Eddy Simulation (LES) of turbulent flow and plume dispersion within a regular array of cubic buildings as idealized urban canopy and investigate the characteristics of mean and fluctuating concentrations. The geometry of the building array is 25×7 with λf=0.25. Here, λf is the building frontal area index and defined as the ratio of the total frontal area of buildings to the plan area of the study site. The plume source is located at the ground surface behind the building in the 7th row of the building array. We can capture the unsteady behaviors of turbulent flow and plume dispersion within the building array and show the spatial distribution of mean and fluctuating concentrations.

A study of the “wall effect” caused by proliferation of high-rise buildings using GIS techniques

This paper describes a novel method using Geographic Information Systems (GIS) to investigate the “wall effect” caused by proliferation of high-rise buildings along the coast in Kowloon Peninsula of Hong Kong. The research utilises the concept of building frontal area index which is calculated based on three dimensional buildings in 100 m grid cells. The main ventilation pathways across the urban area are located using Least Cost Path analysis in a raster GIS and validated by field measurements. Field measurements were also taken in front of windward and leeward buildings. Results show that winds are forced by high frontal area values, to deviate around coastlines with blocks of “wall effect” buildings parallel to the coast. Average wind speeds of 10.5 m s −1 were observed on the windward side of “wall effect” buildings defined according to a southeasterly wind direction, while average wind speeds immediately to the lee side of “wall effect” buildings as well as further inland, were approximately 2.5 m s −1 (four times lower). To confirm the “wall effect” hypothesis, scenario analysis was performed by removing these buildings from the model and re-running it. This revealed a 5% increase of air ventilation to urban areas inland, since more fresh onshore air is able to penetrate from the coast. This improvement is significant since only 0.05% of buildings in the study area were removed. Overlay of the ventilation pathways over a thermal satellite image representing Heat Island Intensity (HII) indicated significantly lower HII values, and reduced extent of the core HII areas, around the ventilation paths.