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(LCCP 2006). It is considered that such policies are vital to avoid urban green spaces being reduced further, with a consequent negative impact on temperatures and human comfort. Currently, high temperatures are rarely a problem for London (LCCP 2006). However, as average temperatures rise due to climate change, excessive urban temperatures could become a greater problem. Adverse health effects such as increased heat stress and increased mortality have already been observed in extreme cases, such as the heat wave in the summers of 1995 and 2003 (Rooney et al. 1998; Johnson et al. 2005), and higher urban temperatures due to the UHI effect are increasing the installation of air conditioning in homes (LCCP 2006). The results of this PhD study suggest that extensive future growth of the London urban area has the potential to increase temperatures, with significant increases for both daytime and night time. The area of increase coincides generally with the area of increasing urbanisation, although an effect is also identified downwind of the city. Mitigation strategies, such as reducing the albedo of the urban surface, are also considered for an idealised domain. Changes in albedo are found to have the potential to mitigate daytime urban temperatures by up to 0.4 K for a dense urban area, however retrofitting climate adaptation measures to existing buildings and infrastructure is a costly and challenging task and the benefits must be fully analysed. It is easier to use climate mitigation opportunities in new developments, and therefore their potential must be fully understood so that these might form part of a city-wide planning process for design and construction (LCCP 2006). So far there are no identified systematic policies to implement cool roofs (with lightly coloured coatings to reflect and emit heat, and reduce the UHI effect), however Transport for London have started implementing a similar strategy on 238
uses, painting the roofs white to reduce absorption of solar radiation (LCCP 2006). However when considering the mitigating potential of high albedo materials it is necessary to also take into account the effect on temperatures during winter months, to ensure the benefit of reducing summertime temperatures is not offset by increased heating demand during winter, and for this reason a modelling approach would require a proper evaluation of METRAS+BEP for winter conditions. 239
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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
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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
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 and 234: UHI intensity (K) 4 3.5 3 2.5 2 1.5
Page 235 and 236: temperature, an increase in the mea
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: cities (Baker et al. 2002). Already
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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