Climate of cities

Authored by: C.S.B. Grimmond

The Routledge Handbook of Urban Ecology

Print publication date:  December  2010
Online publication date:  December  2010

Print ISBN: 9780415498135
eBook ISBN: 9780203839263
Adobe ISBN: 9781136883415

10.4324/9780203839263.ch10

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Abstract

Urban areas create distinct local and micro-scale climates. Commonly cited effects are presented in Table 10.1. What this summary does not make clear, however, is that urban climates vary significantly both within and between cities. Urban climates result from changes in the nature of the urban surface (the materials, its morphology, the fraction of built and vegetated cover, etc.) and the activities of the cities’ inhabitants (generating heat, greenhouse gases, aerosols, etc.) as they move around, work and live in the city. Ultimately urban climates are due to the surface-atmosphere exchanges of energy, mass and momentum (represented conceptually in Figure 10.1). Understanding these exchanges, and the effects of a particular urban setting on their spatial and temporal dynamics, are key to understanding urban climates at the scale of the city, neighbourhood or individual street or property level, and to predicting and mitigating negative effects. This chapter describes these energy and mass exchanges and highlights key urban controls with data and examples of studies that document effects. Table 10.1 Controls on urban climate – general and urban specific effects with examples of studies of urban influences

Variable

General controls

Urban controls/effects

Examples of studies documenting urban effects

Incoming solar radiation (K↓)

Latitude; synoptic conditions/cloud cover

Air quality/industrial sources influence scattering

Gomes et al. (2008), Robaa (2009)

Outgoing solar radiation (K↑)

Incoming solar radiation, albedo

Surface materials; surface morphology/geometry

Aida (1982), Kanda et al. (2005), Fortuniak (2008)

Incoming long wave radiation (L↓)

Synoptic conditions/cloud cover

Air quality/industrial sources affect absorption

Nunez et al. (2000), Jonsson et al. (2006)

Outgoing long wave radiation (L↑)

Surface temperatures, emissivity, sky view factor

Thermal and radiative properties of materials; surface morphology/geometry

Kobayashi and Takamura (1994), Sugawara and Takamura (2006)

Net all-wave radiation (Q)

Latitude; synoptic conditions/cloud cover

Materials, morphology, air quality

Oke (1988, 1997), Offerle et al. (2003), Harman et al. (2004)

Sensible heat flux (QH)

Temperature gradient; atmospheric stability; synoptic conditions

Building volume; built fraction

Oke (1988), Grimmond and Oke (1995,2002), Christen and Vogt (2004), Offerle et al. (2005a, 2006a, 2006b), Roth (2007)

Latent heat flux (QE)

Moisture gradient; atmospheric stability; synoptic conditions

Fraction greenspace; irrigated surfaces; piped/channelled water systems; detention ponds etc

Grimmond and Oke (1991,1995, 1999a, 2002), Christen and Vogt (2004), Offerle et al. (2005a,2006a,b), Mitchell et al. (2007), Roth (2007)

Storage heat flux (ΔQS)

Air-ground temperature gradients; thermal properties surface materials

Materials and morphology urban surface; orientation walls; mass/volume urban surface

Grimmond and Oke (1999b), Offerle et al. (2005b), Roberts et al. (2006)

Anthropogenic heat flux (QF)

Latitude; continentality; regional setting; economic development status

Heating/cooling requirements; industrial activity; socio-economic conditions; population/building density; Transportation routes and methods

Oke (1988), Grimmond (1992), Sailor and Lu (2004), Klysik (1996), Offerle et al. (2005b), Pigeon et al. (2007), Moriwaki et al. (2008), Flanner (2009)

Air temperature

Latitude; continentality; regional setting

Materials and morphology of urban surface release of anthropogenic heat; air quality

Oke Chapter 11 this volume

Humidity

Latitude; continentality; regional setting – proximity to water bodies

Reduced vegetation; fewer moist surfaces localised releases (industrial sources) as by-product combustion; urban air temperature

Hage (1975), Holmer and Eliasson (1999), Fortuniak et al. (2006), Kuttler et al. (2007)

Wind field

Synoptic conditions

Building density; morphology of buildings and roofs affect roughness and displacement lengths; channelling through urban canyons

DePaul and Sheih (1986), Nakamura and Oke (1988), Rotach (1995), Grimmond and Oke (1999c), Roth (2000), Martilli et al. (2002), Eliasson et al. (2006), Offerle et al. (2007), Nelson et al. (2007)

Precipitation

Latitude (solid, liquid); synoptic conditions; topographic variations

Air quality/industrial-traffic sources → cloud condensation nuclei; roughness elements/surface heating → convection

Jauregui and Romales (1996), Lowry (1998) Bornstein and Lin (1998), Shepherd et al. (2002); Rosenfeld (2000); Shepherd et al. Chapter 12 this volume

Figure 10.1 Schematic figure to show the link between the energy and water exchanges at the urban surface (source: Grimmond and Oke 1991).

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