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Keywords:

  • geo-referenced;
  • roughness;
  • urban

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

The parameterization of surface roughness characteristics is an essential element in models of atmospheric boundary processes. Key elements in deriving these characteristics involve analysis of comprehensive data bases of urban morphology. In this paper a procedure is described and tested for a part of Greater Manchester in northwest England, a complex urban area quite different from North American cities. The results are compared with previously published work. Although some differences are identified, the results of the analyses are generally consistent.


1. Introduction

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

The modelling of the urban atmospheric boundary layer in both process studies and operational meteorological models requires specific representations of the urban morphology.

In this paper information on urban morphology available for Greater Manchester in northwest England is analysed, and an approach to derive key surface roughness parameters used in modelling the urban atmospheric boundary layer, such as the surface sensible heat flux (Carraça and Collier, 2007) is described.

A surface morphological database for Greater Manchester was developed from analysis of high resolution digitized geo-referenced data of the height of the surface elements, aerial photography, maps and field surveys (Section '2. Data sets'). The data were analysed and mapped using the geographic information system ESRI ArcView 3.2 software package. After identification of the surface morphological parameters relevant for the modelling purposes (such as the surface elements height, zH, frontal area index, λF, and plan area index, λP) (Section '3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface'), the surface morphological database for the study area was developed through the following steps:

The authors consider that the publication of this data base and the methodology of its construction is important because of its extent (24 × 24 km2) and resolution (1 × 1 km2), and the fact that the roughness parameters developed here can be used for future urban studies.

2. Data sets

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

Digitized geo-referenced data of the surface elements, aerial photography, maps and field surveys allowed the mapping of buildings, car parks, rivers, roads, railways and vegetated areas, and were used for the surface characterization of Greater Manchester. The data were analysed and mapped using the geographic information system ESRI ArcView 3.2 software package. Both digitized data sets of surface elements, the commercial Cities Revealed (CR) data, supplied by The GeoInformation Group (2002), and that supplied by the Environment Agency (EA) (2002), were provided in an ArcView compatible file format.

The CR data, which were derived from high resolution aerial visible imagery, provide information on the spatial distribution of the buildings, their mean height and corresponding plan areas. These data are shown in Figure 1 for a sample area of Greater Manchester, where buildings are already delimited and appear in the image as polygons with a certain height and horizontal area. CR data are essential for a more expeditious calculation of the roughness parameters over built urban areas when using morphometric methods.

image

Figure 1. Distribution of buildings, obtained from CR data, covering a sample area of 2.5 × 5 km2 of Salford and Manchester. The axes labels represent the UK National Grid co-ordinates of the area of interest (Ordnance Survey national grid system), X: 380 000–385 000 m; Y: 398 000–400 500 m. The legend shows the values of the buildings mean height (in metres) associated with the different colours

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On the other hand, the EA data, measured using airborne lidar, provide information on the height of any surface elements (for example, buildings, trees) and permit the identification of water layers. These data files give the height value associated with each pixel, allowing recognition of the presence of any element above the surface, but they do not provide more specific information on buildings. Therefore, in the present work the different data products were taken into consideration together for a more complete characterization of the study area.

3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

The surface morphological parameters, mean surface elements height, zH, plan area index, λP, and frontal area index, λF, were estimated in order to derive the roughness parameters (such as the zero-plane displacement height, zD, and roughness height for momentum, z0M) needed to model for example the surface sensible heat flux.

In this study, for urbanized zones the surface morphological parameters were estimated mainly from the geometric dimensions of the buildings (Figure 2). For an homogeneous array of buildings, the mean surface elements height, zH, and plan area index, λP, are direct results from the CR data statistics obtained using ArcView 3.2. The fraction of the built up area, i.e. the quotient between the total horizontal plan built area over an urban array and the total area of the urban array, was taken as an estimate of the plan area index, λP. The frontal area index, λF, i.e., the mean area of the surface elements facing the wind, was calculated in the following way. Considering the equivalences:

  • equation image(1)
  • equation image(2)

it is possible to write:

  • equation image(3)
  • equation image(4)

Therefore, the frontal area index can be estimated from the expression:

  • equation image(5)

In this equation, AP, AP, and AT are direct results from the CR data statistics performed using ArcView. The mean frontal area AF is calculated using the approximation that buildings are rectangular parallelepipeds with a squared base, thus:

  • equation image(6)

Note that this calculation involves two important approximations. The buildings are approximated to rectangular parallelepipeds (see, for example, Stephenson, 1965, page 322) and the frontal area index, as ‘seen’ by the oncoming wind, is considered independent of the wind direction. On the other hand, in this first approach just the buildings were considered, instead of considering all surface roughness elements over the urban area. Although the buildings are the obstacles more relevant to airflow over cities, other elements such as trees will be considered later.

image

Figure 2. Definition of some surface dimensions used in morphometric analysis (adapted from Grimmond and Oke, 1999)

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The analysis of the building heights was checked for some of the buildings in Manchester and Salford using a hand-held inclinometer and a measuring tape to provide a baseline for a triangular estimation procedure. Also, the height of some buildings was checked by counting the number of floors and considering a height of 3.5 m per floor.

4. Establishment of the land-use categories for Greater Manchester

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

In order to establish land-use categories for the Greater Manchester urban area, first an exploratory study has been carried out for a sample area (5 × 2.5 km2) considered to be representative of Greater Manchester. The objective of this preliminary study was to find some typical values of surface morphological parameters for Greater Manchester and to test the mapping procedures before applying them to the entire study domain (24 × 24 m2). One reason for using this procedure was that CR data on the building height are available only for a small fraction (less than 5%) of the study domain. The distribution of the buildings over this sample area of Salford and Manchester is mapped (Figure 1), where buildings are projected on the terrain level horizontal plan as obtained from CR data.

Based on the CR data set layer, homogeneous tiles in terms of surface morphology and surface cover were delimited, forming a non regular polygon grid over the area of interest (Figure 3). Each tile was regarded as homogeneous with respect to the relevant characteristics at the local scale of order of magnitude ∼100 m. The aspects of the morphology used to define the tiles to be mapped were the buildings' height, plan area and density, and street width. Also identified were tiles as green zones, water surfaces, roads and railways. EA data, maps, aerial photographs and some field surveys at randomly chosen locations enabled checks to be made of the three-dimensional nature of the urban surface and the distribution of built, vegetative or water cover zones.

image

Figure 3. Map of the tiles delimited over the selected sample area of 2.5 × 5 km2 of Salford and Manchester (see Figure 1), forming a non regular polygon grid over the area of interest. Different tile patterns/colours refer to different categories, as shown on the legend. The number in each square cell (1 × 1 km2) gives the UK National Grid co-ordinates (in km) of the cell inferior left corner; the first three digits are the latitudinal co-ordinate and the last three are the longitudinal co-ordinate. This mapping procedure was later extended to the entire study domain

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Based on the analysis of the selected sample area, the tiles were grouped according to their similarity, and classified in terms of some land-use categories as shown in Figure 3. These categories have been adjusted considering the entire study area and a classification of 15 categories has been established for Greater Manchester where typical values of surface morphological parameters, such as surface elements height, zH, the plan area index, λP, and frontal area index, λF, were attributed to each category. These categories are presented in Tables 1 and 2 with some typical values for Greater Manchester. The values of the roughness parameters, such as mean surface elements height, zH, and frontal area index, λF, are summarized in Table 2, for each land-use category. These include built-up areas, vegetated areas and water surfaces.

Table 1. Roughness parameters for each land-use category of Greater Manchester. Mean buildings height, zH, plan area index, λP, frontal area index, λF, and mean horizontal plan area of the buildings, respectively
CategoryzH (m)λPλFĀP (m2)
Centre260.510.262401
CentreN140.170.08942
CentreP150.290.141581
Cl100.210.061718
RailS200.630.166584
Res80.190.13142
Res_mix80.180.06736
ResH220.120.10715
Uni150.160.061628
Trees8.7 (R)0.45 (R)0.18 (R)
Baren0.05*0.01*
Green0.15*0.01*
RailW0.05*0.01*
Road0.05*0.01*
Water0.001*0.001*
Table 2. Land-use categories defined for Greater Manchester (first column) and Urban Terrain Zones (UTZ) of Ellefsen (1990–1991)Thumbnail image of

Many of the surface properties are strongly related to, but not necessarily directly associated with, socio-economic activities. However, it was convenient to associate the tiles with some major land-use categories. The names of the categories were essentially selected on the basis of the research of Ellefsen (1990–1991) and are partly summarized in Table 2. The classifications were selected by comparing the aerial photographs and the land-use data set with the classifications by Ellefsen (1990–1991). For the classifications that matched the published definitions those classifications are used. For those classifications that were different, a new classification was defined as close to the published definitions as possible (see Table 2).

Comparisons with earlier published work revealed similarities to previous representations, but also some differences. These differences are probably due to the nature of the urban areas in the United Kingdom, for example the distribution and type of buildings and green spaces. In general the Urban Terrain Zones (UTZs) identified by Ellefsen (1990–1991) for U.S.A. cities can be significantly different from those found in urbanized areas of the United Kingdom. In the case of Salford and Manchester, for example, there are substantial green areas and the industrial and residential high rise buildings have important differences in their characteristics. However, some of the UTZs are comparable to some urban categories present in the study area.

Figures 1 and 3 show part of Manchester city centre (Centre, in dark blue) and its peripheral streets (CentreP, in purple), where the mean buildings height is relatively high. The Salford Precinct zone, with some tall blocks and relatively low residential houses, is also evident (CentreN, in pink). Residential areas of high rise buildings (ResH, in red) are also marked. The larger green areas are urban parks. Note that the residential areas (Res, in yellow), of typically low buildings, and the mix zones (Res_mix, in orange), of low residential and commercial/industrial buildings, occupy an extensive fraction of the urban area. The River Irwell (Canal, in bright blue) and a main railway (RailW, in black) are also marked.

The attributes zH, λP and λF for the first nine categories of Table 1, typically urban categories, were obtained from the CR data analysis discussed in Section '3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface'. The letter ‘R’ indicates the roughness parameters taken from the published literature, which has been used for the categories named as ‘Trees’ (Grimmond et al., 1998). For the rest of the categories, very low values (*) for zH and λF are assumed.

Note that, while for urbanized zones (where the first nine categories of Table 1. are predominant) zH and λF are estimated mainly from the geometric dimensions of the buildings, for permeable-rough surfaces with vegetation and water surfaces these values are extrapolated from reference tables in the published literature. In fact, although the definitions illustrated in Figure 2 are general and apply to permeable-rough surfaces covered with vegetation: in this case it is not easy from geometric analysis to assign values to zH and λF. Thus, in the present study the values of zH = 8.7 m and λF = 0.18 are assumed for the land-cover category ‘trees’, which are values presented by Grimmond et al. (1998) for urban parks with few trees and the values of zH = 0.15 m and λF = 0.01 are assigned to the category ‘green’ (extrapolated from the roughness values of Pasquill (1950) for long grass; in Brutsaert (1982) and Weiringa (1993)).

On the other hand, in the study domain the water surfaces are mainly small rivers, canals and a reservoir, often surrounded by rows of trees. The reservoir, sited some miles away from the city, is quite large, occupying about two domain cells, but its environmental impact is beyond the scope of this study. In the present work, over water the values of zH = 0.001 m and λF = 0.001 are assumed. It is expected that this will lead to results in agreement with parameterizations usually considered over water: zD = 0 and z0M = 2 × 10−4 m (Brutsaert, 1982; Wieringa, 1993; Grimmond and Oke, 1999). It is assumed that the great variation in the surface heterogeneity over our study domain is mainly due to differences between urbanized and rural surface cover, and that over urban zones is due to the different types of the constructions. The rural vegetated zones in the Manchester region are relatively homogeneous (typically grass with a few scattered bushes and clumps of trees).

5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

Based essentially on maps and aerial photographs (www.multimap.com, scale: 1/25 000) the identification and mapping of the different land-use tiles was extended to the entire Greater Manchester study domain (24 × 24 km2), according to the land-use classification established in Section '4. Establishment of the land-use categories for Greater Manchester'. Therefore, using ArcView software, homogeneous tiles of similar morphology and surface cover were delimited forming a non-regular polygonal grid over the entire study area. These tiles were classified according to the typical land-use categories previously defined in Section '4. Establishment of the land-use categories for Greater Manchester', and the respective roughness parameters were assigned according to Table 1.

Because, in general, a characterization of the study area using a regular base is more convenient for subsequent use with numerical models, a rectangular grid with square cells of 1 × 1 km2 was mapped over the entire Greater Manchester study area (as exemplified by Figure 3), and estimates were obtained for each domain cell also using ArcView software.

The intersection of the polygonal and the squared grids enables estimation of the area occupied by each land-use category in each domain cell of 1 × 1 km2: the sum of the areal percentages of the typically urban categories (Res, ResH, Res_mix, CI, Centre, CentreN, CentreP, RailS, Uni and Road) in each domain cell gives the fraction of urbanized area in each domain cell. These estimates are shown in Figure 4, which reveals the spatial variation of the degree of urbanization for the present study area.

image

Figure 4. Fraction of urbanized area for the Greater Manchester study domain. The co-ordinates X and Y are the UK National Grid co-ordinates. The total study area is 24 × 24 km2 and the area of each grid cell is 1 × 1 km2. The legend on the right-hand side refers to the values of the fraction of urbanized area

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The estimates of the mean height of the surface elements, zH, and frontal area index, λF, for each domain cell are area-weighted averages of the values attributed to each land-use category (Table 1), considering the percentage of each land-use present in the cell. Thus, the values of zH and λF for each domain cell depend on the percent of each land-use area present, and the results for a particular cell will reflect the characteristics of the predominant land-use category. The estimates of zH and λF obtained for Greater Manchester necessary to model the surface fluxes over the study domain are shown in Figure 5. These spatial distributions of the surface morphological parameters zH and λF over the study domain show the difference between the rural areas to the east and south compared with the urban areas. Also the area of high rise buildings is clearly evident.

image

Figure 5. Surface morphologic parameters for the Greater Manchester study domain. The co-ordinates X and Y are the UK National Grid co-ordinates. The total study area is 24 × 24 km2 and the area of each grid cell is 1 × 1 km2. (a) Mean building height, zH, for each cell in the study domain. The legend on the right-hand side refers to the values of zH expressed in metres. (b) Mean frontal area index, λF, for the same study area. The legend on the right-hand side refers to the values of λF

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6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

The zero-plane displacement height, zD, and the roughness length for momentum, z0M, (Figure 6) were calculated as a function of the height of the surface roughness elements, zH, and frontal area index, λF, (Figure 5) using Raupach's (1994, 1995) method, which is one of the morphometric approaches recommended by Grimmond and Oke (1999) for urban areas:

  • equation image(7)
  • equation image(8)

where,

  • equation image(9)

Typical values specified by Raupach (1994) used here are cS = 0.003, cR = 0.3, cdl = 7.5, ψh = 0.193 and (u*/u)max = 0.3. In these equations, u is the wind speed, u* the friction velocity, zH the surface elements height, k ( = 0.4) is the von Karman's constant, cS the drag co-efficient for the substrate surface at height zH in the absence of roughness elements, cR the drag co-efficient of an isolated roughness element mounted on the surface at height zH, cdl is a free parameter, ψh is the roughness sublayer influence function (Raupach, 1994, 1995).

image

Figure 6. Model estimates of surface roughness parameters for the Greater Manchester study domain shown in Figures 4 and 5. The total study area is 24 × 24 km2 and the area of each grid cell is 1 × 1 km2. (a) Zero-plane displacement height, zD, for each domain cell. The legend on the right-hand side refers to the values of zD expressed in metres. (b) Roughness length for momentum, z0M, for each domain cell. The legend on the right-hand side refers to the values of z0M expressed in metres

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This formulation scheme (Equations ((7)-(9))) was applied for all the cells of the study domain. Figure 6(a) and (b) shows the model estimates of the roughness parameters zD and z0M, over the study area, necessary to model the surface fluxes over the Greater Manchester study domain derived from the values of zH and λF shown in Figure 5.

Note that, as discussed by Wieringa (1993), the effective roughness of heterogeneous terrain exceeds the area-weighted arithmetical average of the z0M values of individual patches because relatively rough patches contribute more than their area fraction to the integral effective roughness (see also Garratt, 1992, for a discussion about the effective roughness). Thus the area-weighted average used in the present study might not be the most appropriate way to obtain mean roughness parameters representative of a certain area. This aspect deserves more investigation, and has to be taken into consideration in future work.

The 1 km2 resolution maps of zD, z0M, zH and λF for the study area (24 × 24 km2) clearly show the Manchester city centre and surrounding urban area (Figures 5 and 6). Rural areas, presenting low roughness values, are shown on the borders of the study area to the south and east. Suburban areas such as Salford and Sale to the west, and Stockport to the south-southeast of Manchester are also shown. Also evident is the Mersey River Valley which runs from the west perimeter of the study area approximately 4–5 km south of Manchester city centre. The Valley is evident in the data for a length of approximately 10 km as it runs initially east and then southeast.

7. Assessment of the roughness values obtained for Greater Manchester

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

Since it is possible to specify the height-normalized values of zero-plane displacement height, zD/zH, and roughness length for momentum, z0M/zH, as a function of frontal area index, λF, it is interesting to compare these relations to the equivalent conceptual curves of Grimmond and Oke (1999, Figure 1(b)). Figure 7 shows zD/zH and z0M/zH versus λF, for the range of values observed over the entire study domain of Greater Manchester. The continuous lines are reproductions of the conceptual curves given by Grimmond and Oke (1999). The vertical dashed line shows the lower limit of real-city frontal area index (0.05 < λF < 0.47) and envelopes contained by the curved dashed lines define the reasonable limits outlined by Grimmond and Oke (1999). The two sets of discrete marks (24 × 24 = 576, each) correspond to the values of zD/zH and z0M/zH, (Figures 5 and 6) estimated using the Raupach (1994, 1995) model, Equations ((7))–((9)).

image

Figure 7. Height-normalized values of zero-plane displacement height, zD/zH, and roughness length for momentum, z0M/zH, versus frontal area index, λF, for the range of values observed over the entire study domain. The two sets of discrete marks (24 × 24 = 576, each) correspond to the values of zD/zH and z0M/zH, estimated using Raupach (1994, 1995) model equations, for the entire study domain (see Figures 5 and 6). The continuous lines are reproductions of the conceptual curves given in figure 1(b) of Grimmond and Oke (1999). The vertical dashed line shows the lower limit of real-city frontal area index (0.05 < λF < 0.47) and envelopes contained by the curved dashed lines define the reasonable limits outlined by Grimmond and Oke (1999)

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As can be seen in the Figure 7, there is a significant difference between both relations, Raupach (1994, 1995) and Grimmond and Oke (1999). In fact the Grimmond and Oke (1999) curve is a conceptual representation, as designated by the authors. In Figure 1 of their article, Grimmond and Oke (1999) illustrated their heuristic arguments that provide some quasi-physical reasoning to explain what happens when extra roughness elements are added to a surface. However, these arguments are unable to give the exact shape of the curves, so they are only sketched as shaded zones, referred to by the authors as the ‘reasonable’ zones or envelopes.

Figure 7 shows that although the model values do not follow the Grimmond and Oke (1999) curve, they mostly lie inside the ‘reasonable’ zone defined by the authors. In the case of the representation of zD/zH versus λF, different colours were used to distinguish between three levels of urbanization, < 25, 25–50 and > 50% of urbanized area, in a similar way to that of Figure 4. This figure shows that the domain cells with higher percentage of the urbanized area tend to have higher values of zD/zH. The values of zD/zH for cells with a higher percentage of urbanized area tend to lie inside the reasonable limits defined by Grimmond and Oke (1999).

Figure 8 was also obtained from all the roughness values over the entire study domain. The graph of Figure 8(a) represents zD and z0M versus zH (values of Figure 6(a) and (b) versus values of Figure 5(a)) and the graph of Figure 8(b) represents zD versus z0M (values of Figure 6(a) versus values of Figure 6(b)). Note that taking all the roughness values over the entire study domain it was found that zD = 5z0M, zD = 0.4zH, z0M = 0.08zH (Figure 8(a) and (b)). These results are in agreement with published literature (see, for example, Grimmond and Oke, 1999; Grimmond, 2006).

image

Figure 8. (a) Zero-plane displacement height, zD, and roughness length for momentum, z0M, as functions of the surface roughness elements height, zH. (b) Zero-plane displacement height, zD, versus roughness length for momentum, z0M. Each line represents the fitted linear function between a pair of variables resulting from linear regression. R is the linear correlation co-efficient. The graphs were obtained from all the roughness values (24 × 24 = 576) over the entire study domain (Figures 5 and 6)

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8. Final remarks

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References

A classification of 15 land-use categories (9 urban terrain zones) has been established for Greater Manchester and reference morphological parameters, such as building mean height, zH, plan area index, λP, and frontal area index (λF), were attributed to each category. Comparisons with earlier published work (e.g., Ellefsen, 1990–1991, for U.S.A. cities) revealed similarities to previous representations, but also some differences. These differences are probably due to the nature of the urban areas in the United Kingdom, for example the distribution and type of buildings and green spaces.

The surface morphological parameters, surface elements height, zH, and frontal area index, λF, were estimated and mapped over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study domain. The estimates of zH and λF, for each grid cell were weighted averages of the values attributed for each land-use category, considering the percentage of each category present in the cell.

The model estimates of the zero-plane displacement height, zD, and roughness length for momentum, z0M, derived from the values of zH and λF, were also presented (in Figure 6) for the study domain over a rectangular grid of 1 × 1 km2 resolution. These estimates of the roughness parameters zD and z0M are comparable to previously published values. In addition, taking the roughness values obtained for the entire study domain it was found that zD = 5z0M, zD = 0.4zH, z0M = 0.08zH. These results are in agreement with published literature (see, for example, Grimmond and Oke, 1999).

Note that, although derived from different data sources, the patterns of the satellite surface temperature TR (see Carraça and Collier, 2007) and roughness expressed by the parameters zH, λF, zD, and z0M are similar; they reveal the presence of the city and the variations of the building density and urban morphology.

The surface roughness parameters characteristic of the Greater Manchester study area were considered to be the same for all study days, and have been used to model the spatial distribution of surface sensible heat flux, QH, over the Greater Manchester urban area (Carraça and Collier, 2007). In fact, for the construction of the present surface morphology database, it has been assumed that the roughness surface elements, such as buildings, are rigid parallelepipeds and that the roughness effects do not vary with the wind direction. A more realistic approach must take into consideration the orientation and shape of the buildings. Further improvements to the data base should involve a dynamic approach, where roughness parameters such as the frontal area index may assume different values depending on the wind direction. In the present study the study domain was described in a static sense. One description was used for all periods of measurements or modelling, which does not vary with wind direction or meteorological conditions (e.g., atmospheric stability). In reality, surface properties are spatially heterogeneous and there are preferred wind directions. Consequently, the properties of the surface area contributing to a turbulent flux at any point are constantly changing. This suggests a dynamic approach, where the surface characteristics and changing meteorological conditions are taken into account. This may be a more appropriate way to describe a site when measurements are being conducted, especially if modelled data are to be evaluated against measured data to ensure spatial consistency (Grimmond, 1992; Grimmond and Souch, 1994).

The development of the present database was slow and arduous because most of the roughness parameters of interest were not derived automatically from the digitized georeferenced dataset of surface elements height. Computational research should be carried out to improve the process of construction of this type of database, in order to construct them faster, allow for an easy update (for example, Manchester has been changing very quickly in the last few years and the database needs to be updated for the city centre and its neighbourhoods quite often) and calculate roughness parameters for different wind directions.

References

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Data sets
  5. 3. The mean surface elements height, zH, frontal area index, λF, and other morphological parameters of the urban surface
  6. 4. Establishment of the land-use categories for Greater Manchester
  7. 5. Mapping the surface roughness parameters over a rectangular grid of 1 × 1 km2 resolution, for the Greater Manchester study area (24 × 24 km2)
  8. 6. The surface morphologic database, and model estimates of the surface roughness parameters zero-plane displacement height, zD, and roughness length for momentum, z0M, for Greater Manchester
  9. 7. Assessment of the roughness values obtained for Greater Manchester
  10. 8. Final remarks
  11. References