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Chapter 6 that the increase in the nocturnal UHI intensity reached a threshold as the city approached its current extent. The results for the scenarios representing future urbanisation presented in Figure 7.4 show a slow increase for the first six simulations, and then a steeper increase as the urban land cover starts to dominate, covering over 40% of the domain area. A non linearity in the urban effects of expansion and fraction of vegetation within the urban area depending on the size of the urban area has been identified in some previous studies (e.g. Best et al. 2002; Trusilova 2006). UHI at 02:00 am (K) 4 3.5 3 2.5 2 1.5 1 0.5 0 0.20 0.25 0.30 0.35 Mean urban land cover fraction 0.40 0.45 0.50 Figure 7.4: Maximum UHI intensity (K) as a function of mean urban land cover fraction as computed by the simulations in the EXPANSION series at 02:00 of the second day of simulation 7.2.2 Diurnal temperature range (DTR) The effect of the results presented for the scenarios in the EXPANSION series is that the behaviour of the domain averaged DTR is different from that found in Chapter 6. As a result of the larger increase in the daytime temperature compared to the night time 220
temperature, an increase in the mean DTR is found (see Figure 7.5). This contradicts both past observational and modelling studies (e.g. Gallo et al. 1996; Easterling et al. 1997; Kalnay et al. 2003), which found a reduction in the DTR with increased urbanisation, due to the fact the daytime temperature increased less rapidly than the night time temperature. Whilst these results appear to contract experimental studies on past urbanisation, an increase in daytime temperatures than exceeds night time temperatures has been observed in a numerical study (Wichansky et al. 2008), although in this case a failure of the model RAMS was identified due to the treatment of anthropogenic heat storage and release. An increase in the maximum diurnal temperature is also observed in the DENSITY series in Chapter 6, although it was smaller than the increase in the minimum diurnal temperature (night time). The main way in which the DENSITY series differed from the other simulations in Chapter 6 was the fact that the other series removed all of the suburban areas outside of the critical radius which defined the city area. This implies that the more spread out, less dense, suburban areas are critical for determining the increase in the daytime temperature, when compared to a much smaller, more compact city of higher density at the centre of the domain. The series of urban expansion runs presented in this Chapter are also characterised by an increase in the lower density, suburban areas around the city, as well as a densification of the central urban area. The number of urbanised grid cells (defined as having an urban land cover fraction greater than 30%) increases sharply, as well as the mean urban land cover fraction within existing urban cells. This drives a sharper increase in daytime temperatures compared to night time, and the consequent increase in the DTR. 221
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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
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over the city centre, convergent fl
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Cold islands can also occur in urba
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of the atmosphere. For example, it
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amount of solar radiation absorbed
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(Peterson 1969; Changnon 1992; Coll
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2.3 Justification of modelling appr
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Trusilova (2006) carried out a stud
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constant flux surface layer in the
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the vertical distribution of vegeta
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using different building parameters
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Table 2.2: SWOT analysis for the ME
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Fraction of urban land cover [%] Fi
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The built up area of London can cur
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Figure 2.5(a-f): Builtup area of Lo
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• Mainly high density development
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climate of London, and currently on
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For mid-latitude cities, of which L
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warming of 0.04 °C per decade, in
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out differences in the urban heat i
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adopted in previous studies ranges
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y the implementation of a specific
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A prognostic equation is used to re
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At the surface the temperature is c
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The boundary conditions used for th
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are defined in the file m1tini_TAPE
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The terms representing the impact o
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3.2.1 Calculation of dynamical effe
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θ k ∆z / 2 H Siu (Equation 3.12)
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3.3 Implementation of BEP in METRAS
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difficulties in the linking of the
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Table 3.3: Description of how the 2
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3.5.2 Orography data Orography data
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epresenting suburban development a
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Table 3.7 shows a summary of the MI
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3.6 Summary In this PhD study the m
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temperature, air pressure and air d
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Potential temperature [K] Figure 4.
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Wind speed [ms -1 ] Figure 4.2: Ver
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4.2.1 Horizontal cross sections of
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affected, as the flow bends around
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Downwind of the city on the other h
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the city, which is expected since t
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Potential temperature [K] Figure 4.
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simulation this effect is verticall
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differences are expected during day
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tend to cool the surface, but can a
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Figure 4.15 shows the vertical prof
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appropriate value for these paramet
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0.61) and that of conventional grav
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Difference in potential temperature
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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
Page 183 and 184: 6.2 Effects of the current state of
Page 185 and 186: During daytime the shape of the are
Page 187 and 188: The calculation was performed for t
Page 189 and 190: ecause finding a rural reference po
Page 191 and 192: development (GLA 2006) and differen
Page 193 and 194: 6.2.4 Wind speed and direction Klai
Page 195 and 196: Wind speed and direction (ms -1 ) F
Page 197 and 198: “urban” cells to be affected by
Page 199 and 200: the urban surface. Many other studi
Page 201 and 202: eason the near surface potential te
Page 203 and 204: Mean potential temperature at 12:00
Page 205 and 206: where A(x) is the area affected by
Page 207 and 208: Table 6.8: REI for the RADIUS, DENS
Page 209 and 210: COMBINED series also show a reducti
Page 211 and 212: for all runs in the series, and cor
Page 213 and 214: The runs are then combined into one
Page 215 and 216: to be more linear than the smaller
Page 217 and 218: The total reduction in mean wind sp
Page 219 and 220: The REI showed that for all the var
Page 221 and 222: Higher temperatures in the urban ar
Page 223 and 224: Over the next ten years the populat
Page 225 and 226: 7.1.1 EXPANSION series The first sc
Page 227 and 228: Another cause of this form of expan
Page 229 and 230: 7.2 Effects of urbanisation for the
Page 231 and 232: Mean near surface potential tempera
Page 233: UHI intensity (K) 4 3.5 3 2.5 2 1.5
Page 237 and 238: 7.2.3 Wind speed The effect of the
Page 239 and 240: 7.3 Effects of urbanisation for the
Page 241 and 242: mean near surface potential tempera
Page 243 and 244: 7.3.2 Wind speed Changes in the mea
Page 245 and 246: expansion strategies such as the re
Page 247 and 248: should also be considered to avoid
Page 249 and 250: models, for example Martilli et al.
Page 251 and 252: cities (Baker et al. 2002). Already
Page 253 and 254: uses, painting the roofs white to r
Page 255 and 256: The full influence of different met
Page 257 and 258: References Abercrombie, P. (1945).
Page 259 and 260: Bottema, M. (1997). "Urban Roughnes
Page 261 and 262: Craig, K. J. and R. D. Bornstein (2
Page 263 and 264: Grimmond, C. S. B. and T. R. Oke (2
Page 265 and 266: 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
Page 271 and 272: 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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