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defined for the horizontal grid in the scheme. Therefore four parameters in total, canyon width, building width, building height distribution and street direction (W, B, γ(h), ξ) uniquely define the urban morphology for the BEP scheme. At the mesoscale, a parameter λV can be defined to represent the percentage of vegetation coverage in the horizontal grid. This represents the land cover percentage of the vegetated land cover classes, such as ‘Meadows’. Figure 3.3: A schematic representation of the urban grid, in which W is the street width, B is the building width, IU is the centre of a vertical model level, Fiu H represents the flux of a quantity through the horizontal surfaces with the area Siu H , and FIU V represents the flux of a quantity through the vertical surfaces with the area of SIU V . (Taken from Martilli et al. 2002, page 267). BEP allows the city to be represented by up to ten urban classes, which are characterised by defining the averaged distribution of building heights, the average street direction, length and width and the radiation, roughness and building parameters. Whilst not all this data will be readily available for the London are, this approach allows to resolve some of the complexity of the urban surface, and to study how changes in urban form have affected regional climate. The data sources used for the implementation are described in Section 3.5. 68
3.2.1 Calculation of dynamical effects The dynamical effects due to buildings calculated by BEP are: • Loss of momentum due to horizontal surfaces (roofs and canyon floors) inducing a frictional force. For this term the classic surface layer (MOST) formulae of Louis (1979) are used to calculate the momentum flux for the horizontal surfaces. This term is however vertically distributed from ground to the highest building, and is H proportional to the fractional area of the horizontal surface in the grid cell S . The turbulent momentum flux at level iu due to the horizontal surfaces is: r Fu H iu k = −ρ ∆z [ln( z 2 IU oiu / 2 )] 2 f m ∆z ( z IU oiu / 2 , Ri B ) U hor IU r U IU S H IU iu (Equation 3.9) where k is the von Karmen constant (0.4), fm comes from Louis (1979), RiB is the hor bulk Richardson number calculated at level IU, and U is the horizontal wind component. • Exchange of momentum through the vertical surfaces (walls) due to pressure and viscous drag forces induced by the presence of buildings on the flow. The momentum flux due to the vertical surfaces is: r Fu V IU r ort ort V = −ρC d U IU U IU S IU (Equation 3.10) IU 69
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MODELLING THE IMPACT OF URBANISATIO
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Abstract Urban areas have well docu
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Abbreviations aLMo Meteo Swiss oper
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4.2.2 Impact of the urban area on t
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List of figures Figure 2.1: Scales
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Figure 4.21: Diurnal variation of t
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Figure 6.16: Mean horizontal wind s
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Chapter 1: Introduction Urban areas
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with trends derived from a statisti
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scheme, and therefore it was necess
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The effects of future urban expansi
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2.1 Urban climate modification Urba
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latitudes (Taha 1997), and this is
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100 - 200 m 1 - 2 km 10 - 20 km 100
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epresentatively sample the underlyi
Page 31 and 32: over the city centre, convergent fl
Page 33 and 34: Cold islands can also occur in urba
Page 35 and 36: of the atmosphere. For example, it
Page 37 and 38: amount of solar radiation absorbed
Page 39 and 40: (Peterson 1969; Changnon 1992; Coll
Page 41 and 42: 2.3 Justification of modelling appr
Page 43 and 44: Trusilova (2006) carried out a stud
Page 45 and 46: constant flux surface layer in the
Page 47 and 48: the vertical distribution of vegeta
Page 49 and 50: using different building parameters
Page 51 and 52: Table 2.2: SWOT analysis for the ME
Page 53 and 54: Fraction of urban land cover [%] Fi
Page 55 and 56: The built up area of London can cur
Page 57 and 58: Figure 2.5(a-f): Builtup area of Lo
Page 59 and 60: • Mainly high density development
Page 61 and 62: climate of London, and currently on
Page 63 and 64: For mid-latitude cities, of which L
Page 65 and 66: warming of 0.04 °C per decade, in
Page 67 and 68: out differences in the urban heat i
Page 69 and 70: adopted in previous studies ranges
Page 71 and 72: y the implementation of a specific
Page 73 and 74: A prognostic equation is used to re
Page 75 and 76: At the surface the temperature is c
Page 77 and 78: The boundary conditions used for th
Page 79 and 80: are defined in the file m1tini_TAPE
Page 81: The terms representing the impact o
Page 85 and 86: θ k ∆z / 2 H Siu (Equation 3.12)
Page 87 and 88: 3.3 Implementation of BEP in METRAS
Page 89 and 90: difficulties in the linking of the
Page 91 and 92: Table 3.3: Description of how the 2
Page 93 and 94: 3.5.2 Orography data Orography data
Page 95 and 96: epresenting suburban development a
Page 97 and 98: Table 3.7 shows a summary of the MI
Page 99 and 100: 3.6 Summary In this PhD study the m
Page 101 and 102: temperature, air pressure and air d
Page 103 and 104: Potential temperature [K] Figure 4.
Page 105 and 106: Wind speed [ms -1 ] Figure 4.2: Ver
Page 107 and 108: 4.2.1 Horizontal cross sections of
Page 109 and 110: affected, as the flow bends around
Page 111 and 112: Downwind of the city on the other h
Page 113 and 114: the city, which is expected since t
Page 115 and 116: Potential temperature [K] Figure 4.
Page 117 and 118: simulation this effect is verticall
Page 119 and 120: differences are expected during day
Page 121 and 122: tend to cool the surface, but can a
Page 123 and 124: Figure 4.15 shows the vertical prof
Page 125 and 126: appropriate value for these paramet
Page 127 and 128: 0.61) and that of conventional grav
Page 129 and 130: Difference in potential temperature
Page 131 and 132: educed near surface temperatures of
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fraction was increased in steps of
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works. In the control simulation th
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characterised by the albedo, therma
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is higher by a similar amount. The
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(a) Potential temperature [K] (c) T
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Figure 4.25 shows the vertical sect
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tests also reproduced well document
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Chapter 5: Evaluation of the urbani
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Fraction of urban land cover [%] Fi
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For the summers of 1995-2000 the an
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Table 5.2: Selected periods of simu
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commonly used as a rural reference
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newly urbanised version of the METR
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adiation trapping in the street can
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the second day of simulation (31 st
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Air temperature ( o C) 35 30 25 20
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the model. The smaller percentage o
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Wind speed (ms -1 ) Wind direction
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London Heathrow Airport (LHR) Wind
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A full set of hourly wind speed and
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contain both roofs exposed to direc
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Chapter 6: The effects of urban lan
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simulations with two hypothetical s
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dependent on the r(i,j), the radial
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centre of the domain was more than
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6.2 Effects of the current state of
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During daytime the shape of the are
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The calculation was performed for t
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ecause finding a rural reference po
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development (GLA 2006) and differen
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6.2.4 Wind speed and direction Klai
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Wind speed and direction (ms -1 ) F
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“urban” cells to be affected by
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the urban surface. Many other studi
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eason the near surface potential te
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Mean potential temperature at 12:00
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where A(x) is the area affected by
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Table 6.8: REI for the RADIUS, DENS
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COMBINED series also show a reducti
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for all runs in the series, and cor
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The runs are then combined into one
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to be more linear than the smaller
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The total reduction in mean wind sp
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The REI showed that for all the var
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Higher temperatures in the urban ar
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Over the next ten years the populat
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7.1.1 EXPANSION series The first sc
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Another cause of this form of expan
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7.2 Effects of urbanisation for the
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Mean near surface potential tempera
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UHI intensity (K) 4 3.5 3 2.5 2 1.5
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temperature, an increase in the mea
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7.2.3 Wind speed The effect of the
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7.3 Effects of urbanisation for the
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mean near surface potential tempera
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7.3.2 Wind speed Changes in the mea
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expansion strategies such as the re
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should also be considered to avoid
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models, for example Martilli et al.
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cities (Baker et al. 2002). Already
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uses, painting the roofs white to r
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The full influence of different met
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References Abercrombie, P. (1945).
Page 259 and 260:
Bottema, M. (1997). "Urban Roughnes
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Craig, K. J. and R. D. Bornstein (2
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Grimmond, C. S. B. and T. R. Oke (2
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Jin, M. and J. M. Shepherd (2005).
Page 267 and 268:
Lemonsu, A. and V. Masson (2002). "
Page 269 and 270:
Nitis, T., Z. B. Klaic and N. Mouss
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alance in urban areas - Recent adva
Page 273 and 274:
Seaman, N. L. (2000). "Meteorologic
Page 275 and 276:
Therry, G. and P. Lacarrere (1983).
Page 277:
Yague, C., E. Zurita and A. Martine
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