Urban climates

Chapter 13

Urban climates H. CLEUGH

Introduction Overview

Within the next decade, and probably by 2000, almost half of the global population will live in cities (HAMMOND, 1992) (Fig. 1). Any discussion of future climates should therefore include a description of the local climate for most of the world's population: the urban climate. As this chapter will show, cities do influence the local and regional climate. The magnitude of urban effects on precipitation and temperature were recently described (CHANGNON, 1992) as being similar to, and maybe even greater than, those changes predicted from global climate models (GCMs) to develop over the next 100 years as a result of greenhouse warming. This chapter describes present-day urban climates and urban climatic processes which provides the basis for a discussion of future urban climates. The exploding urbanisation in Fig. 1 is reflected in increased proportions of agricultural or "natural" landscapes being converted into urban land use. To some degree this has already happened in the highly urbanised developed nations (Fig. 1) where most urban expansion comprises suburban sprawl. The rapid growth of cities in developing and less developed nations, especially those in the tropics, is a much more recent phenomenon that began after World War II (JAUREGUI, 1986) and will continue into the 21st century. There has been a ninefold increase in the number of tropical cities with more than one million inhabitants from 1940 to 1970 (JAUREGUI, 1986). In 1992, the United Nations listed 21 cities that could be classed as megacities - urban agglomerations with current projected populations of 10 million, or more, by 2000. All but three of these were located in developing and less developed nations. Urbanisation initiates one of humanity's most dramatic land use changes: a natural landscape, often containing transpiring vegetation and a pervious surface, is converted to a built, largely impervious landscape made-up of rigid, sharp-edged roughness elements. Exchanges of heat, water vapour and momentum between the new urban "surface" and the atmosphere are modified by the different radiative, thermal, aerodynamic and moisture properties of the urban landscape. The air quality is altered by the input of wastes from urban activities. The natural topography and drainage networks are radically modified during urban development, as well illustrated by the straightened, concrete-lined channels that form the urban drainage system. Fig. 2 provides a useful overview of urban activities and their effects on physical climate processes.

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Urban climates IO0 0

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In this way cities indirectly modify those atmospheric variables, such as temperature, humidity, airflow and rainfall, whose ensemble averages define the climate

(OKE,

1980). This

happens at local (<1 km horizontally; <100 m vertically) and meso (<100 km horizontally; <~ 1 km vertically) spatial scales. The magnitude and extent of these weather and climate effects cannot be generalised for all cities because they depend on city-specific factors such as the pre-urban land use and the regional climate. Interest in urban climates began in 1833 when Luke Howard (HOWARD, 1833) first documented that cities appeared to be warmer than the surrounding countryside. Many studies in the 160 years since have well established this urban heat island effect; a phenomenon linked to differences in urban/rural cooling rates and best seen at night under clear skies and light winds. Longer term studies show that cities modify climates, at least at the local spatial scale, e.g. KARL et al. (1988) found urban effects on air temperatures in towns with only 10,000 inhabitants. Much research in the last 20 years has been directed towards not only describing the nature of urban climates, but understanding the processes that lead to the observed urban climate. In extrapolating this current knowledge to predict future urban climates, the following questions must be considered: 9

What will be the nature of future cities? Urban climates in the future will be determined by many factors, including: height and spacing of buildings; nature of construction materials; location of city; type and pattern of energy use; transport mode; type and location of industries; amount of greenspace; city layout etc.

478

Introduction I Concentration of commercial and industrial activities 9 ...... growth of transport sector

Concentration of population EFFECTS OF URBANISATION

I Increased metabohc ........ activity

consumption

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consumption .....

THE URBAN LANDSCAPE

URBAN EMISSIONS

MODIFIED PHYSICAL PARAMETERS AND PROCESSES

Surface hydrology

Surface morphology

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replaced by pipes

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URBAN CLIMATE

Fig. 2. Conceptual flowchart of the effects of urbanisation on the radiation, energy balance and local climate (modified from YAMASHITA and SEKn~, 1990/91). Note that feedbacks between the modified physical processes are not included.

9

9

Will there be an urban influence on the regional and global climate, and how do we separate this effect from that due to global climate changes, such as result from the greenhouse effect?. What will be the nature of the future global climate? Urban effects are superimposed on the regional and global climate, thus the climate of cities in the future has to be considered in light of global climate changes.

The last, and to a lesser extent the second, question can really only be answered using GCMs with an ability to include local scale urban effects, a task beyond the reach of current GCMs. Recent analyses of the US temperature record (e.g. KARL et al., 1988; KARL and JONES, 1989; JONES et al., 1989) have provided observational evidence of the contribution of urban warming to the continental and global temperature signal. The variation between estimates from different studies illustrates that they are by no means definitive. Numerical models potentially have an important role in answering questions about urban effects, comprehensive reviews of urban climate models can be found in OKE (1974, 1979a) and BORNSTEIN (1986, 1989). Many mesoscale numerical models that have been specifically applied to urban meteorology are limited by their crude land surface schemes (ROss and OKE, 1988). Two such examples are the modelling studies of HJELMFELT (1982) and SEAMAN et al. (1989) which are of great interest because they separate urban and topographic effects on mesoscale airflow and thermal features using a series of model "experiments". However, the results are limited by the use of a simple moisture availability factor to partition sensible and latent heat fluxes. Thus far models have helped to elucidate urban processes but at present they are of limited use in predicting future climate change in

479

Urban climates cities or the impact of urban effects at the larger scale. The focus of the chapter is therefore upon explanation of processes that lead to current urban climates which gives a basis for the discussion of future climates. This chapter does not include air pollution. This does not relegate atmospheric pollution to being unimportant in future cities as this is probably the major regional-scale impact of urban/industrial areas, but it is beyond the scope of the chapter. Future cities

Any survey of current urbanisation statistics reveals two facts about future cities. Firstly, much of the current and future urban growth, including the rise of the megacities, will be in the poor and developing nations (Fig. 1). An increase of 750 million people in cities in the developing nations is projected from 1986 to 2000 (UNITEDNATIONS, 1986 cited in OKE et al., 1990). Secondly, in terms of geography, much of the urban growth will be in the tropical latitudes. Of the 23 nations with the highest rates of urbanisation in the period 1980-1985, 18 were in Africa and the remaining five in Asia (UNITED NATIONS, 1989). These nations currently have the lowest level of urbanisation, but their urban population is predicted to grow by 40% per decade from 1990 to 2020 (UNITED NATIONS, 1989). Apart from these rapid urban growth rates in lower latitudes is the projection that, by 2000, 17 of the 26 cities with populations over ten million will be in the tropical latitudes (UNITED NATIONS, 1986, cited in OKE et al., 1990). Of the 20 megacities identified in a 1992 UNEP Report, ten were located between the Equator and 30 ~ and another six between 30 ~ and 35 ~ (South and North) (UNEP, 1992). Quite clearly a discussion of future urban climates must address urban effects in cities in the tropical latitudes, often located in poor nations (such as Africa) or rapidly industrialising nations such as Mexico or Thailand where economic growth and industrialisation will have a major influence on energy use, energy type, city growth and morphology. The dilemma is that most process-oriented knowledge arises from research conducted in mid-latitude cities located in the developed nations of North America, Europe and Oceania (Australia and New Zealand). As noted earlier, most urban growth in the tropical latitudes is post World War II. JAUREGUI (1986) argued that this is why studies of urban climates in tropical cities really only began in the 1970s. Rapid industrialisation in many tropical cities over the same period has led to deteriorating air quality which has also stimulated a demand for meteorological information. Limited financial resources has meant that often the only data available are from standard weather stations. This has prevented detailed process-oriented studies in tropical cities and led to many descriptive studies. This dearth of information on urban climate processes means that our models of urban climate processes have not been evaluated in tropical cites. Often, we can only make some broad generalisations based on a limited number of studies. The larger scale climate in which tropical cities are embedded, by comparison to midlatitude cities, is characterised by much smaller temperature ranges and seasons are marked by variations in humidity, rainfall and cloud rather than temperature (JAUREGUI, 1986). The inter-annual variation in radiation climate in principle will be small, but marked seasonality in cloud cover can modify this. Solar elevation angles are higher than in mid-latitudes.

480

Introduction

Building heights, spacings and construction materials will differ from typical mid-latitude urban morphologies. JAUREGUI (1986) suggested the following differences between tropical and mid-latitude city structure: lower buildings; larger proportion of unpaved streets; small proportion of greenspace and large slum areas. The climate regimes characterising tropical cities (JAUREGUI,1986) fall into the following classes, excluding topographical and continental effects: 9

equatorial humid climate: either wet all year (e.g. Singapore) or alternating dry/wet sea-

son (e.g. Jakarta); prevailing trade winds (easterly) all year; uniformly warm temperatures and high humidity; high radiation only during short dry season; 9

9

tropical/subtropical dry: with a short wet season (may be warm or cold); seasonal tem-

perature variation depends on whether the location is coastal or continental; high radiation all year; seasonal variation in air mass, wind speed and direction; wet and dry season subtropical (humid~sub humid): seasonal changes in air mass and variation in radiation climate (e.g. Mexico City).

It is of interest to note that of the ten megacities identified by UNEP (1992) that are located at latitudes less than 30 ~ (South and North), seven have tropical (either savanna, rainforest or steppe) climate regimes. Annual mean temperatures are greater than 25~

high humidity

and annual rainfall totals exceed 1000 mm (mostly occurring in heavy falls in the wet season) for all seven. The remaining three cities are Karachi (desert or subtropical dry); Mexico City and Sao Paulo. Future urban growth will thus be in the poor or developing nations; located in the humid tropics, wet/dry subtropics or deserts. There will be a continued large influx of rural populations into not only megacities but also small to medium-sized cities. A large portion of this immigrant population will be housed in makeshift shelters in unplanned, densely populated urban slums.

Methodological considerations Interpreting and quantifying the processes that lead to urban climates is complicated by two factors. Firstly, there are few pre-urban measurements, so it is difficult to establish the direction and magnitude of urban-induced changes. Secondly the complex nature of the urban landscape limits the applicability of conventional meteorological theory. Fortunately several methodological frameworks have enabled progress; these are discussed next. LOWRY (1977) proposed a methodology which he believes is necessary to adopt if we are to prove an urban influence on the observed climate. His methodology, in brief, states that an urban influence on a climate element can only be isolated if it is based on observations of that element both prior to and following urbanisation. Furthermore, any effects due to largescale climate change or landscape effects (e.g. topography) must be removed from the measurements and the data should be stratified by weather type. This methodology is all but impossible to adhere to, hence most empirical studies only approximate this ideal. In particular weekend/weekday comparisons and urban/rural comparisons will be flawed. A recent study by GRIMMOND et al. (1993) illustrates the importance of the "rural" land-cover in comparisons of urban and rural climate processes. They compared energy balances and microclimates between suburbia and two rural sites, one of which was irrigated and the other unirrigated. The size and direction of the urban effect depended on whether the comparison 481

Urban climates

was made between suburban and "dry" rural, or suburban and "wet" rural. Their study is a good reminder that "rural" land use is often just as likely to be influenced by human activities as urban land use and is not an appropriate surrogate for pre-urban land use and climates. CLEUGH and OKE

(1986) avoided this dilemma by using energy balances measured

simultaneously at a suburban site and a "control" site. This control site was flat with short grass which eliminated the complexity introduced by buildings and surface types that characterise suburban land use and enabled urban effects on atmospheric processes to be clarified. Unless such measurements are conducted over a long term, this methodology does not allow for an assessment of urban effects on climate, only on atmospheric processes. That this approach is not a substitute for Lowry's methodology illustrates the difficulty in proving urban climate effects - a point that should be borne in mind throughout this chapter. This methodological difficulty also illustrates the important role that physically based models have in understanding and detecting urban effects. There is an urgent need for development and validation of appropriate urban surface parameterisation schemes that can be linked to current generation mesoscale models and embedded into GCMs. Scales

A key to interpreting and/or modelling urban effects on atmospheric processes is to develop a methodology or conceptual framework that accounts for the heterogeneity of the urban "surface". As with any natural landscape, this heterogeneity is scale-dependent and OKE (1976) developed such a framework that took advantage of the natural range of scales present in urban morphology. Table I shows these scales and their spatial dimensions. Oke divided the urban surface and atmosphere into urban canopy and urban boundary-layers as shown in Fig. 3. The urban canopy layer (UCL), which lies below the mean roof level, is made up of an array of vertical, horizontal, sloping and multi-level surface elements. These surface elements each have varying thermal, moisture-holding, radiative and aerodynamic properties which, in turn, lead to a range of microclimates within the UCL. The urban

TABLE I SPATIAL SCALES IN THE URBAN ENVIRONMENT

Urban units

Urban features

Spatial dimensions (m) Width

Building Canyon Block Land use zone

City

Single building, 10 tree or garden Urban street and 30 bordering buildings City block, park, 500 factory complex Residential, 5000 commercial, industrial etc. Urban area 25,000+

Adapted from Oga~(1984).

482

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Micro

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Local

Surface, or constant flux, layer

Length 10 300 500 5000 UBL including surface layer 25,000+

Meso

Introduction

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boundary-layer (UBL) extends from mean roof level upwards. The UBL is derived from the concept of an internal boundary-layer growing with distance downwind from the leading edge, which in this case is the surface transition from upwind "rural" to urban land use (Fig. 3). The air flowing over the city becomes modified by, and adjusted to, the new surface, starting with the lowest air layers. If the horizontal extent of the city is sufficiently large, this urban-influenced boundary-layer (the UBL) will occupy the entire PBL. The constant flux layer (also referred to as the surface layer) lies at the base of the UBL and above the roughness layer (also known as the wake layer) where flow is influenced by individual buildings. Areally averaged turbulent fluxes from the underlying canopy are assumed to be constant with height in the surface layer, whose depth may reach ca. 100 m by day.

Nature of the urban surface Although we can identify many individual urban surface elements, Oke's approach treats these as simply facets of an integrated urban surface, which is actually a volume. Fig. 4 is a conceptualisation of this urban canopy volume, containing roughness elements such as trees and buildings; horizontal surfaces such as roads, pavements and rooftops; and the air contained within the canopy layer. Turbulent exchanges between the urban surface and atmosphere are represented by areally averaged fluxes flowing across the plane ABCD in Fig. 4a, into/out of the urban canopy volume. Provided our perspective is from some location at a distance above the top of the UCL, the urban canopy volume can be conceptualised as a two-dimensional "surface" representative of a larger spatial scale, e.g. a land use zone (Table I). We can then begin to note some characteristic features of this urban surface.

483

Urban climates

T

B

i

Boundary layer

(iBL) Canopy (UiL) layer

Fig. 4. (a) The urban canopy volume (after Or~, 1987). (b) The urban canyon with height (H) and width (W).

In suburban land use the dominant roughness elements are buildings (mostly houses and commercial buildings) and trees whose heights are typically of order 10 m. These roughness elements are sparsely distributed, spacings of ca. 20-50 m were identified for a suburban, North American city (Vancouver, SCHMID, 1988; SCHMID and OKE, 1992). This roughness element spacing is strongly influenced by the street pattern and hence can be anisotropic. The surface is also aerodynamically rough, buildings act as bluff bodies which extract momentum from airflow via skin friction and form drag. Roughness parameters vary with roughness element height and spacing. Suburban land use has typical roughness lengths of 0.5-1 m while urban roughness lengths may be as large as 4 m (OKE, 1980). The open canopy structure and the "patchy" array of horizontal surfaces with contrasting properties (e.g. dry roads and wet lawns) leads to active horizontal exchanges of heat and mass via microscale advection. It is important to realise that the atmospheric feedbacks and interactions that occur within the UCL mean that we cannot simply sum individual surface elements to determine the ef-

484

Urban climate processes fect of the entire urban surface on atmospheric processes and climates. Likewise, horizontal pavements or lawns cannot be used as surrogates for the urban surface. Rather, we have to understand the way that the entire UCL functions. Thus, we recognise the urban canyon (Fig. 4b) as a fundamental unit of the built landscape that is replicated throughout the UCL. The urban canyon is simply two building walls, with height H, separated by a road of width, W. The aspect ratio (H/W) describes building spacing and is an important descriptor of the UCL. Radiative exchanges and airflow characteristics depend on the size of the aspect ratio.

Urban climate processes

Fig. 2 shows the connections between urban activities and climatic processes, this section explains these connections and so addresses the question of "how do urban areas modify these physical processes?" The magnitude and direction of these urban-induced changes are quantified by reviewing the current state of knowledge, most of which has derived from a combination of observational and modelling studies. Radiative effects

The incoming shortwave radiation (K,[,) exerts a first order forcing on the climate of any location, as has been shown in earlier chapters. When discussing the radiation balance for the UCL, K,[, refers to the radiation impinging on a plane extending across the top of the UCL (plane ABCD in Fig. 4a). Shortwave radiation comprises both direct beam and diffuse components, the latter refers to radiation reflected and scattered by atmospheric particulates and gases. The amount of shortwave radiation absorbed within the UCL volume depends on the latter's albedo (a). Cities also affect the longwave radiation budget; downwelling longwave radiation from the urban atmosphere impinging on the plane ABCD in Fig. 4a is defined as L$. We begin by discussing atmospheric effects on downward shortwave and longwave radiation and then discuss the influence of the urban canopy on shortwave and longwave absorption. Readers are referred to Chapter 1 by HENDERSON-SELLERS which describes the pathways for radiative transfer in the atmosphere. A depletion in K,], would be one of the most obvious examples of the effect of cities on radiation. This reduction is expected because the polluted urban atmosphere will reduce the atmospheric transmissivity for shortwave radiation. Unfortunately observational studies find this hard to prove (OKE, 1979a) because nearby rural measurements may well be contaminated by downwind transport of these urban pollutants. Table II (from OKE, 1988) shows typical reductions in incoming shortwave radiation. These values tend to fall into two groups: (a) large reductions for those cities with a sizeable industrial base where SPM (suspended particulate matter) is common; here reductions of up to 30% in K,[, on individual days have been observed with typical annual reductions of around 10%; (b) other cities where SPM levels are low, but photochemical smog exists, reveal much smaller reductions in incoming shortwave radiation, typically less than 5%. Observations show that the reduction in K,[, is variable both spatially (i.e. from city to city) and temporally (i.e. diurnally and seasonally). Causes of these variations include (OKE, 1979a):

485

Urban climates

TABLE II ATTENUATION OF INCOMINGSHORTWAVERADIATION(FROM OKE, 1988)

Author (Year)

City

Data Period

Attenuation

(%) PASZYNSKI(1972) PROBALD(1974) HESS et al. (1978) HAY (1984) SANDERSONet al. ( 1 9 7 3 ) ROUSEet al. (1973) YAMASHITA(1979) SEKIHARA(1973) PETERSONand STOFFEL(1980) METHODand CARLSON(1982) CHOWand CHANG( 1 9 8 1 ) WANG and LIU (1982) ESTOURNELet al. (1983) PETERSONet al. (1978)

Upper Silesian industrial district Budapest, Hungary Cracow, Poland Vancouver, Canada Detroit-Windsor, USA Hamilton, Canada Tokyo, Japan Tokyo, Japan St. Louis, USA St. Louis, USA Shanghai,China Hangchow, China Toulouse, France Los Angeles, USA

1955-1968 1976-1978 1979-1981 1970-1971 1970-1972 1967-1968 1961-1971 1975-1977 1975-1976

1979 1973

12 9 12 1-2 9 12 12-14 10-15 3 7 15 9 3 6-8

Note: based on studies since 1970 only; includes data for studies with very different periods of observation (3 months-13 years) and cloud conditions (some only use clear skies).

9

geographical location and time of day/year (determine the optical pathlength);

9 9

averaging period; industrial and manufacturing activities and mode of transport;

9

wind direction in relation to the location of industries, major highways, etc.;

9

sky cover (viz. cloudy or clear).

Examples of such variability are found in the studies presented in Table II. A recent study by STANHILL and KALMA (1993) reported an annual decline in KS of 1.08% over the 35 years of measurements at Kowloon (Hong Kong). They attributed most of this to reduced direct beam radiation resulting from increased emissions of aerosols from fossil fuel combustion. This follows the results of STANHILL and MORESHET (1992) who showed the influence of vehicle pollution on depletion of KS in Israel. Cities with a large, old and inefficient vehicle fleet are likely to find similar reductions in KS. Such variability means that it is impossible to generalise about the size of the reduction in shortwave radiation in cities. In terms of future climates, cities in rapidly industrialising nations will likely have the greatest reductions in K,I, if fossil fuels remain the major primary energy source. Warmer UBL temperatures, resulting from greenhouse warming, have the potential to increase photochemical smog formation which may further reduce KS. These studies refer to the receipt of shortwave radiation at UCL level. Atmospheric constituents (including water vapour, aerosols and SPM) will selectively absorb radiation which further alters the radiation budget both of the UCL and the UBL, as described for the globe in Chapter 10 by ANDREAE. Some studies suggest that radiative heating from absorption within the UBL is important. Others agree with LYONS and FORGAN (1975) who noted that atmospheric pollutants tend to scatter, rather than absorb, the solar beam. Diffuse radiation, rather than radiative heating of the UBL, is then increased and accounts for 60-80% of the attenuated direct beam radiation (OKE, 1988). METHOD and CARLSON (1982) found that

486

Urban climate processes

maximum heating rates due to absorption by aerosols were more than 3~

near the sur-

face, but averaged over the day were only 1~ near the ground and closer to zero at the top of the urban convective boundary layer. They concluded that heating effects due to aerosols were not large and any heating was slightly less than the cooling that resulted from increased longwave emissions (see below). Numerical simulations (YOSHIDA, 1991) showed that the UBL temperature structure was influenced by aerosols during the daytime only. Under calm conditions, where radiation scattering is dominant, urban temperatures were lowered and the urban-induced thermal circulation (see below) weakened. This had the effect of enhancing the aerosol concentrations even further. Urban-induced circulations were not affected when radiation absorption, rather than scattering, dominates. In these conditions near surface temperatures decreased and upper temperatures either increased or were unchanged. Ultraviolet (UV) radiation is readily absorbed by ozone which is created in the UBL as a part of the photochemical smog cycle. Thus we would expect to find reduced levels of UV radiation beneath urban atmospheres, but estimates are highly variable in space and time; 10-15% reductions in Tokyo (Japan) and up to 50% on individual days in Los Angeles (USA) in the autumn (OKE, 1976, 1979b). While aerosols may reduce K,[,, they can increase the emission of longwave radiation because of their effect on UBL temperature and emissivity; an urban-induced "greenhouse effect" is created. As with the depletion of shortwave radiation, this effect varies from city to city, e.g. WHITE et al. (1978) found no difference in L,[, between urban St. Louis (USA) and the surrounding rural area whereas ROUSE et al. (1973) observed large daytime differences in the industrial city of Hamilton (Canada). There is evidence from many of these studies that a reduction in KS due to aerosol absorption yields an increase in L,],, hence there is little change to the total incoming radiation (OKE, 1988). Increases in L$ are not always due to aerosol effects on atmospheric emissivity, however, but result from a warmer urban atmosphere arising from the heat island effect (e.g. ESTOURNEL et al., 1983; OKE and FUGGLE, 1972; SUCKLING, 1981). The general consensus is that urban values of L,[, are larger, primarily because of the warmer atmosphere (OKE, 1988) but atmospheric aerosols may also play a role. Large downwelling longwave fluxes are often offset by increases in emitted longwave radiation from the UCL (i.e. LI") hence differences between urban and rural net longwave radiation will be small. Cities therefore influence the receipt of radiation because of the composition and temperature of the urban atmosphere. The factors discussed, plus the variability of pollution emissions (amount, type and timing) both between cities and within individual cities, means that it is difficult to generalise an urban effect. In general, the shortwave radiation received at the UCL may be reduced in intensity, be more diffuse and be depleted in some wavelengths. Reductions in KS will be offset by increases in L,l,. The altered nature of the urban surface also has a strong influence on the receipt and absorption of radiation because its convoluted, three-dimensional nature can result in shading and multiple reflection of the solar beam once it enters the urban canyon. Multiple reflections lead to greater shortwave trapping and absorption and so the albedo of the UCL is reduced. Urban albedos are also lower because of the dark nature of many materials used in urban construction (e.g. asphalt), however radiative trapping means that any threedimensional surface will absorb more radiation than a horizontal one constructed of the same material. AIDA (1982) found this difference to be of the order 20% based on an ex487

Urban climates

TABLE III ALBEDOS OF URBAN AND NON-URBAN SURFACES AND LANDSCAPES

Surface type/landscape

Mean albedo

Range

Asphalt Concrete Corrugated iron Suburban landscape Commercial/industrial land use Urban land use Pasture (short grass) Pine forest Wheat crop

0.12 0.27 0.13 0.15 0.27 0.14 0.25 0.10 0.22

0.05-0.20 0.10-0.16 0.10-0.18 0.11-0.16 0.05-0.15 0.18-0.25

From Ord~ (1987); ARNFIELD(1982); STEYNand OKE (1980). perimental arrangement of blocks; TAKAMURA (1992) also observed an overall decrease in the spectral reflectance (0.475-0.75/zm) over urban land use because of this altered urban canopy structure. Table III shows that urban albedos are typically 14-15%, ca. 10% smaller than pasture but little different from tall grass or forest. Longwave radiation emitted from surfaces within the UCL is absorbed and re-emitted by other surfaces and the air volume within the UCL. Thus we refer to longwave radiation "trapping" in the UCL. The sky view factor (SVF) for a point in an urban canyon is a measure of the relative amount of radiation emitted by each element occupying the point's hemispherical field of view that is intercepted by that point. More simply, the SVF is the percentage of a point's field of view that is occupied by sky, as opposed to buildings, trees or any other object in the landscape. Canyons with a large aspect ratio will have a small SVF; flat fields with an unobstructed horizon will have a large SVF. Because buildings emit greater amounts of longwave radiation than the cool sky, an urban canopy with a small SVF will yield a larger net longwave radiation (L*). Nocturnally, this larger L* will reduce radiative cooling and maintain a warmer UCL. Fig. 5, from OKE et al. (1991) illustrates this influence of SVF on surface temperatures. Despite these urban effects on the individual components of the radiation budget (equation (1), below), urban-rural differences in the net all-wave radiation (Q*) are often less than 5% (see AUER, 1981; OKE and MCCAUGHEY,1983; OKE and CLEUGH, 1986). Q* = K,,[,(1 - a ) + (L,[, - L'[') = K* + L*

(1)

This arises because of the negative feedbacks that exist between albedo/shortwave radiation absorption and surface temperature/emitted longwave radiation: surfaces with a low a and thus high shortwave absorption will also have very small longwave radiation gains because of their high surface temperature. Similarly, the reduction in KS is countered by an increase in L$. OKE (1979a) describes this conservative behaviour as a "fortuitous" offsetting of radiative effects. This may also explain observations of very limited spatial variability of Q* within the urban landscape (e.g. WHITE et al., 1978; SCHMID et al., 1991). Thus differences in net all-wave radiation are unlikely to cause changed urban climates. While changes in individual terms in equation (1) may lead to greater heating or cooling in components of the

488

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An understanding of the processes that contribute to an altered urban climate must include an analysis of the energy balance at an appropriate scale. The energy or heat balance simply shows the way that radiative energy (the energy source by day) is partitioned between various forms of heating: sensible (QH), which leads to a rise in air temperature; latent (QE) which leads to the phase change of liquid water at the surface into water vapour, and its subsequent transfer into the atmosphere; and conducted (Qc) sensible heating which leads to warming of the soil. The urban energy balance can be written as Q* + QF = QH + QE + AQs + AQA + AQp

(2)

Using this sign convention, all fluxes on the left-hand side are inputs and those on the righthand side are outputs. Readers should be aware that some authors use reversed signs; others define all output fluxes as negative and inputs as positive (i.e. all the terms in (2) sum to 0). The turbulent heat fluxes (QH and QE) refer to the fluxes emerging from the UCL and crossing the plane ABCD in Fig. 4a. The conductive sensible heat flux (Qo) has been replaced by a heat storage flux (AQs) which includes not only heat conducted into the UCL floor and walls, but the change in heat content of the air volume contained within the UCL. If the "box" illustrated in Fig. 4a (i.e. the UCL) extends for large distances up and downwind and the land use inside the "box" is homogeneous, then the net horizontal transfer of heat (AQA, the advective flux) will tend to zero. QF and AQp are the anthropogenicgenerated and photosynthetic heat fluxes, respectively. The terms on the right-hand side of equation (2) can be considered to be sinks or output fluxes. Obviously energy exchanges within the UCL will be exceedingly complex because of the many surfaces with differing moisture contents, geometry (especially their slope and aspect) and thermal properties. Albedo, shading, slope and aspect will exert the primary influence on the net radiative gain or loss for individual surface elements such as roads, walls and

489

Urban climates rooftops. The net all-wave radiation will be partitioned into either sensible or latent forms, depending upon the surface moisture status. Impervious surfaces will partition all their available energy into sensible heat. Microscale advection will also be dominant if the surfaces are moist, as shown by OKE's (1979b) measurements of evaporation and available energy (Q* - QG) in a well-watered suburban lawn. He observes a downward flux of sensible heat as a result of horizontal heat transfer from the warm paved surfaces to the moist lawn. Sensible heat transport towards the moist surface maintains an evaporation rate that exceeds the available energy. The array of surface elements within the UCL is so complex that we restrict our attention to either the scale of a complete urban canyon or the entire UCL to enable a more general understanding of urban energy partitioning. The urban canyon is a common feature in the urban landscape. Modelling and measurement studies of the urban canyon recognise that processes operating within this fundamental "unit" will be replicated across the UCL. NUNEZ and OKE (1977) instrumented a nonvegetated urban canyon in Vancouver (Canada) to measure all the energy balance components (Q*, Qrt, QE, AQs). Their results showed the influence of canyon geometry which perturbs the radiation regime for each of the canyon facets. When summed for the entire urban canyon, the diurnal course of the energy balance components showed features typical of fiat, "ideal" surfaces (Fig. 6). Similar features have been noted in other canyon energy balance studies (MILLS and ARNFIELD, 1993; YOSHIDA et al., 1991).

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490

Urban climate processes From a local and regional scale climate perspective, it is better to consider the UBL, where we write the "surface" (viz. the UCL) energy balance as presented in equation (2). This, combined with logistical constraints on measurements within urban canyons, has resulted in a greater number of studies focusing on the UBL. In the last 10-15 years, the first direct measurements of the terms on the right-hand side of equation (1) have appeared from cities in locations ranging from mid-latitude to tropical (including: Adelaide, Australia; Vancouver, Canada; Sacramento, St. Louis, Tucson, USA; Mexico City, Mexico). The following discussion reviews the salient points from these energy balance studies. The anthropogenic heat flux (QF) is a source of energy in cities that is absent from most rural energy balances and is often cited as one of the main reasons for elevated air temperatures in cities. The source for the anthropogenic heat flux is the energy released from domestic heating, combustion in the automotive and industrial sectors, and other forms of industrial energy. Estimates of the size of this flux are available in the studies by YAP (1973), GRIMMOND (1988) for Vancouver (Canada) and KALMA (1974) for Sydney (Australia). These studies indicate a peak value of--25 W m -2 at noon on an anticyclonic, summer day. A first order estimate of --15 W m -2 for daily QF appears appropriate, however the flux is extremely variable across cities and temporally (KALMA, 1974). Table IV presents annual average anthropogenic fluxes compiled by OKE (1988). These data plus results from other modelling and measurement studies reveal that QF depends on overall climate, population density and energy consumption. Although the magnitude appears relatively small in comparison to the other components of the energy balance, under some conditions (such as at night or in the winter) this source of energy can become very important especially in proportion to the size of Q* as illustrated by the results of GRIMMOND (1992). For daylight hours, QF exceeded 10% of Q* only in the winter months (January and February). However for the daily (i.e. 24 h) energy balance, QF was 46% of Q* in January and February, 12% and 9% for March and April and then dropped to ca. 5% through May and June. These data are from a mid-latitude city with a temperate climate, in more extreme climates QF will be an important energy source.

TABLE IV ANNUAL AVERAGE NET RADIATION AND ANTHROPOGENIC HEAT FLUX DENSITIES

City (latitude)

Studied years

Per capita energy use (GJ year-1)

Per capita energy use (W)

QF (w m-2)

Q* (w m-2)

QF/Q*

Fairbanks (64~ Moscow (56~ West Berlin (52~ Vancouver (49~ Budapest (47~ Montreal (45 ~ Manhattan (40~ Los Angeles (34~ Hong Kong (22~

1967-1975 1970 1967 1970 1970 1961 1967 1965-1970 1971

314 530 67 112 118 221 169 331 28

9,957 16,806 2,125 3,551 3,742 7,008 5,359 10,496 888

6 127 21 19 43 99 159 21 33

18 42 57 57 46 52 93 108 110

0.33 3.02 0.37 0.33 1 1.9 1.71 0.19 0.19

From OKE (1988).

491

Urban climates Given these sizes of QF, it is interesting to note the modelling results from SWAID and HOFFMAN (1990). Adding a daily mean anthropogenic heat flux of 54 W m -2 into an urban cluster with an aspect ratio of 0.5 created a diurnal air temperature increase of 1-2.5~ The maximum increase occurred at 2000 h, the minimum at 0600 h, and the daily average temperature excess was 1.8~ It is important to note, however, that these temperature increases are modelled values with no allowance for the site of energy release. An increased storage of sensible heat in the urban fabric and canopy air volume (AQs) is often cited as a means by which urban climates are modified. Such assertions are based on the belief that urban construction materials have a higher thermal conductivity and specific heat than natural surfaces. Oke suggests that the thermal admittance (Ps) is a more appropriate property for comparison as it combines both the specific heat and conductivity terms. The thermal admittance is defined as (ksCs)~ where ks is the thermal conductivity and Cs is the volumetric heat capacity of the material of interest. It is a measure of the temperature change resulting from some change in heat flux across the surface (OKE, 1987). In fact, as OKE (1987) points out, thermal admittance is a surface property; surfaces with a low /t s will show a large surface temperature change for a given heat flux across that surface. Typical/t s values fail to show a trend towards larger values among urban materials. However such comparisons neglect the role of the three-dimensional active surface and hollow buildings that feature in urban land use. DABBERDT and DAVIS (1978) and CARLSON et al. (1981) attempted to evaluate the thermal admittance at a landscape scale using satellite imagery. CARLSON et al. failed to identify any obvious urban-rural differences in thermal admittance while DABBERDT and DAVIS found higher values of thermal admittance in wooded sites, not urban areas. These landscape thermal admittance estimates may be biased because the satellite mainly "sees" rooftops which have characteristically low values of/t s (GOWARD, 1981). Landscape estimates of/~s compiled by OKE (1988) are: 800-1700 (suburban); 1200-2100 (urban); 1600 (farmland); 1600-3000 (mixed woods, swamp and farm) J m -2 S1/2 C -1. Urban heat storage fluxes can be derived as the residual in equation (2), if all other components of the energy balance are measured. Clearly there are limitations to this approach because all errors arising from measurement techniques, or from neglecting advective, photosynthetic and anthropogenic fluxes, are accumulated into the heat storage term. Table V illustrates typical magnitudes of AQs, in proportion to net radiation. All heat storage fluxes fall between 20 and 30% of net radiation (daytime fluxes only), with the exception of Mexico City (Mexico) and Tucson (USA). This ratio is larger than in rural areas (Table V), confirming the important role that this heat storage term must have in urban energy balances. Nocturnal values of AQs often balance the all-wave net radiation (NUNEZ and OKE, 1977). Nonetheless, most of the estimates for heat storage are based on models or are derived as residuals in equation (2). Accurate values of this term are still needed. Table V summarises the main features of urban energy balance partitioning and the Bowen ratio (fl = QH/QE) from published measurements. Although urban Bowen ratios are larger than in well watered, rural landscapes, they are not as extreme as many early studies assumed. In winter, when moisture is freely available, the Bowen ratio is less than unity but, in general, net radiation is preferentially channelled into sensible heat forms (AQs and QH). About 40% of the net radiation is partitioned into QH, with the following exceptions: Oke and McCaughey's anomalous results; urban (as opposed to suburban) land use; Tucson

492

Urban climate processes TABLE V TYPICAL ENERGY FLUX PARTITIONINGIN SUBURBANLANDUSE

Location/season

fl

AQs/Q* QE/Q* QH/Q*

H/W

Adelaideg: March Vancouver Summer Summer Spring (March, April, May) Summer (June only) Winter (January, February) St. Louis: Summerd Tucson: Summer Mexico City: end of dry season Sacramento: Summer Typical urban: Summer Typical suburb.: Summer Rural Wet Dry Ideal

2.5

N/A

N/A

N/A

10-15 m b N/A

COPPIN (1979)

0.16 1.28 c

0.23 f 0.22 f

0.67 0.34

0.11 0.44

0.34 0.34

64 a 64 a

OKE and MCCAUGHEY (1983) CLEUGH and OKE (I 986)

1.14

0.27 f

0.34

0.39

0.34

64 a

GRIMMOND (1992)

1.4

0.29 f

0.30

0.42

0.34

64 a

GRIMMOND (1992)

0.8 2.12 1.13 1.12

0.19 f 0.24 0.52 0.36

0.45 0.24 0.22 0.30

0.36 0.52 0.26 0.34

0.34 2stories b 5.8 mb 0.51

64 a 10 N/A 21 a

GRIMMOND (1992)

CHING et al., pers. commun. GRIMMOND and OrE (1990) OrE et al. (1992)

1.39 1.5 1

0.26 0.27 0.22

0.31 0.29 0.39

0.43 0.44 0.39

1 storeyb N/Ap N/Ap

N/A 15a 50 a

GRIMMONDet al. (1993) OrE (1988) OrE (1988)

22.23 0.46

0.07 e 0.12 e 0.04 e

0.63 0.04 0.66

0.11 0.80 0.30

0.05 b 0.5 b 0.15 b

N/Ap N/Ap N/Ap

GRIMMONDet al. (1993) GRIMMONDet al. (1993) CLEUGH and OrE (1986)

% gs

Author

Key: unless stated, all values are for suburban land use; the averaging period varies from several days to continuous monthly data; all values are daytime only. gs, greenspace; N/A: not available; N/Ap: not appropriate. aUsing plan area only; bbuilding height only, spacing not given; Clatent heat fluxes are estimated as a residual; ddata from St. Louis study are from 1000 to 1300 h only, obtained over 4 days; eat rural sites this is QG/Q* and is measured; fAQs is modelled, otherwise it is computed as a residual; gAdelaide study reported daily data only.

and Mexico City. The latter two cities are located at lower latitudes (subtropical and tropical) than any other city in Table V. Sensible heat fluxes are low and more energy is partitioned into latent heat and heat storage fluxes in these two cities. Fig. 7 illustrates the essential differences in partitioning between a suburban and an "ideal" site located respectively within and near to a mid-latitude city in summer. The sensible heat fluxes are the dominant diurnal energy sinks in the urban environment, Qn is a sink even after sunset and sometimes throughout the evening. The storage component is an energy source in the late afternoon and through the evening. This suggests that the sensible heat flux must be an important component in maintaining elevated air temperatures in the early evening in urban areas. QH has an asymmetric diurnal path, it peaks in the afternoon and remains positive until ca. 2000 h. Such behaviour was also observed by CHING et al. (1983) who noted that the late peak in QH is a feature which becomes magnified with increasing urbanisation. The results from Mexico City (ORE et al., 1992) are the first measured in a tropical city and illustrate some surprising features, such as the low Bowen ratio (the suburb had only 21% greenspace, Table V) which is smaller than for suburbs in mid-latitude cities where there is 64% greenspace. The latent heat flux appears to be maintained at the expense of Qn. The preferential channelling of energy into heat storage (AQs) possibly arises from the high

493

Urban climates

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Fig. 7. Measured differences between suburban and rural energy balance components (from CLEUGH and OKE, 1986). S, suburban; r, rural. building density and high occurrence of concrete rooftops which intercept large amounts of solar radiation at this tropical latitude (ca. 19~ In general urban latent heat fluxes are smaller than their rural counterparts, in proportion to the reduction in vegetation. However, even in drought conditions QE can still act as an important energy sink, by day, in suburban areas. These non-trivial latent heat fluxes are maintained by external water use (irrigation of gardens, lawns and parks). Considerable dayto-day variability in fl is also a common feature in suburban land use (e.g. KALANDA et al., 1980; OKE and MCCAUGHEY, 1983; CLEUGH and OKE, 1986; GRIMMOND et al., 1993). Some of this arises from variations in rainfall and irrigation and hence surface moisture availability (GRIMMOND and OKE, 1986). OKE and MCCAUGHEY (1983) suggested that variability in fl could be related to differences in incoming radiation, however neither of these factors provides a complete account. CLEUGHand OKE (1986) noted that such temporal variability was absent at their "control" site and argued that the turbulent flux partitioning is also linked to changes in the saturation deficit (i.e. dryness) of the overlying UBL. This is enabled by the roughness of the urban surface which enhances turbulent mixing and thus strong "coupling" between the UBL and surface fluxes. Aerodynamically rough surfaces, such as forest canopies and cities, are better coupled to the UBL because turbulence is enhanced. CLEUGH and OKE (1986) and GRIMMOND et al. (1993) suggest that this coupling enables synoptic control on turbulent flux partitioning which could explain their observed day-to-day variability of ft. Smoother surfaces, such as pasture in flat terrain, are not so well coupled to the PBL so surface turbulent fluxes are primarily determined by moisture supply and available energy (i.e. the net radiation minus the heat storage). Airflow

Modified surface heating and increased friction lead to perturbations in the mean wind velocity vectors and the turbulence within the UCL and UBL. This section describes the perturbations to mean flow in the UCL and UBL first, followed by a brief overview of changes in turbulence. The mean wind and its turbulent structure influence the sizes of the turbulent fluxes and the diffusion and dispersion of atmospheric pollutants released into the UBL. Within the UCL, the presence of buildings dominates the pattern of flow. The nature of airflow around individual buildings has been well documented in the literature (e.g. MERONEY, 1982). The introduction of sharp-edged and inflexible buildings into a moving airstream decreases the velocity of the approach flow. The streamlines are displaced and converge above

494

Urban climate processes

a

Isolated roughness flow

b Wake interference flow

r

Skimming flow

IOl lOF Fig. 8. Hypothesised effects of building spacing on airflow. The airflow is from left to right across the page and the urban canyons are assumed to be infinitely long in the cross wind direction (after Ord~, 1987). the building, yielding a jet of high velocity air above and a recirculating eddy located to the lee of the building. Airflow approaching from other wind directions will complicate this basic flow regime. The effect of an agglomeration of buildings will depend on their orientation and spacing. In the UCL, the important scaling variables for flow are thus the aspect ratio, the ratio of canyon length to height and building orientation to the airflow. The overall morphology of the city, in terms of the building height, length and spacing, determines the airflow patterns in the UCL plus its effectiveness as a momentum sink. These also affect the interaction between the UCL and the UBL and the microclimate within the UCL which is where people live and breathe. Fig. 8 depicts the types of flow and, by extrapolation, the microclimate which could be expected for a variety of building densities where the approach flow is oriented normal to the long axis of the urban canyon. The circulating vortex within the urban canyon exchanges scalars from within the canyon to the atmosphere above. Its strength depends on the speed and direction of the approach flow and the aspect ratio of the urban canyon. Note that airflow oriented along canyons is usually accelerated, with frictional slowing and uplift at the canyon walls. There are three mechanisms which alter mesoscale airflow in the UBL (AUER, 1981). Firstly, the changed density and characteristics of the roughness elements alter the friction of the urban surface and its effectiveness as a momentum sink. These surface modifications lead to, secondly, variations in the vertical transport of momentum via thermal and mechanical mixing. Lastly, the altered thermal state of the UBL leads to pressure and stability changes. Which of these effects dominates depends on the regional flow regime, atmospheric stability and the scale of interest. Calm conditions and/or increased distance from the aerodynamically-rough urban surface favour the influence of buoyancy, free convection and thus thermal effects. Closer proximity to the rough urban surface and/or stronger airflow allow frictional influences to dominate. When winds are light, the urban heat island produces a low pressure area at ground level and hence convergent flow (SHREFFLER, 1979). This is accompanied by uplift in the UBL 495

Urban climates

and hence subsiding and diverging air at the urban periphery. This uplift combines with enhanced vertical velocities (see below) from the increased turbulent activity and results in the UBL "doming-up", e.g. SHEA and AUER (1978) observe the UBL to be 300 m deeper over downtown St. Louis. This pattern of convergence and uplift tends to be stronger during the day, when the UBL is unstable and QH is large. The pattern of convergence also creates the potential for acceleration and turning of near-surface winds. In winds of moderate strength (>5 m s-l), thermal effects are overwhelmed by friction effects: large downward momentum fluxes, flow deceleration and uplift result (BRYANT et al., 1978). This is consistent with the barrier effect, following an abrupt change in surface roughness in the absence of a thermal influence. The uplifted airmass can form a "plume" of ascending air which will be advected downwind. In the lee of the city, another change in roughness is experienced by the airflow (rough to smooth) sometimes resulting in descending, diverging air and enhanced momentum transfer. Sometimes a local windspeed maximum at ground level is found, as a result of this greater downward transport of horizontal momentum. A decrease in horizontal wind velocities at any height in the UBL is accompanied by an increase in Zg, the altitude at which the geostrophic wind speed is reached. The urban-induced deceleration perturbs the balance between the pressure gradient forces and the Coriolis force leading to cyclonic turning of the flow (BORNSTEIN and JOHNSTON, 1977; BRYANT et al., 1978). This feature was observed by ANGELL et al. (1973) over Oklahoma City, where tetroon balloons turned cyclonically upon entering the built-up area of the city, then turned anticyclonically in the lee of the urban area. An extensive study of turbulence in the urban surface layer was conducted by CLARKE et al. (1982). Turbulent intensities exceeded rural values by 50% as a result of the frictiongenerated shear stress at the rough urban canopy/air interface and the effects of buoyancy. Increased shear stresses over the urban land use were observed both during the day and extending into the night. The instability of the nocturnal UBL maintains turbulent exchange of momentum between the UCL and UBL. Other studies report similar enhancement of turbulent intensities in the UBL plus increased momentum fluxes and friction velocities (e.g. BOWNE and BALL, 1977; HOGSTROM et al., 1982; STEYN 1980, 1982; HILDEBRAND and ACKERMAN, 1984; YERSEL and GOBLE, 1986; ROTH et al., 1989a; ROTH and OKE, 1993). A recent analysis of surface layer turbulence structure by ROTH and OKE (1993) and ROTH (1993) are among the most complete and include the first published urban moisture spectra. Their turbulent wind results reveal many of the features discussed above. The vertical wind speed (w) spectra peak was shifted to lower frequencies, in agreement with other turbulent studies (CLARKE et al., 1982; HOGSTROM et al., 1982; STEYN, 1982). The shape of the ucomponent spectrum showed the influence of building wakes with the peak energy being at higher frequencies (smaller spatial scales) with a relatively faster roll-off. The co-spectra of w'T' and u'w' (e.g. COPPIN, 1979) and the turbulent velocity statistics all conform to MOST

(Monin Obukhov Similarity Theory), in agreement with other studies. The turbulent moisture spectra showed influences from larger scale structures, in particular surface evaporation appeared to be driven by downdrafts of dry air. These findings suggest non-similarity between heat and moisture transport processes. Many of these basic effects on turbulence are propagated into the convectively dominated UBL. GODOWITCH (1986) found that vertical velocities were larger in the UBL compared to

496

Current and future urban climates a rural boundary layer and variances in urban vertical velocities were 50% greater than their rural counterpart. The UBL was characterised by a greater production of vertical turbulent energy across a wide frequency range with the dominant length scales of the updrafts and downdrafts being comparable to the depth of the UBL. All these observations also suggest that the UBL obeys convective similarity scaling. In summary, this overview of urban airflow and energy exchange processes reveals that the combination of an aerodynamically rough surface, the juxtaposition of wet and dry surface elements within the UCL, and an increased active surface area, yields an environment that has the potential for large heat or moisture fluxes. If the urban canopy is wet, then large evaporation rates can prevail, even larger than in well-watered rural environments. Under dry conditions, however, most net radiation is channelled into turbulent or stored sensible heat fluxes. The division into "wet" and "dry" surfaces is linked to anthropogenic control of surface watering, hence sizeable latent heat fluxes can be maintained long after rainfall has ceased. Large turbulent sensible heat and momentum fluxes lead to large vertical velocities and turbulent intensities throughout the UBL. By day, this can lead to enhanced entrainment from the stable atmosphere aloft, which can increase the thermal anomaly in the UBL (see below). Surface turbulent fluxes are therefore strongly influenced by mesoscale advective effects through coupling between the UCL and the UBL. The partitioning of net radiation into sensible heat depends also on canyon geometry, canyon construction materials and solar angle. Limited evidence from tropical energy balance studies suggests that high solar elevation angles may enhance AQ s.

Current and future urban climates In this section we address the way that these fundamental atmospheric processes lead to characteristic urban climates. The focus is deliberately at the local (viz. land use) and UBL (viz. meso-) spatial scale and the diurnal to annual temporal scale. The climate elements include temperature, humidity, rainfall and severe weather. Wind is also an important "climate" variable, but urban effects on airflow were covered in the previous section.

Temperature Temperature anomalies associated with urban areas have long been recognised. Luke Howard first observed elevated air temperatures in cities in 1833. The existence of this "urban heat island" has now been well documented in human settlements ranging from villages to megacities and in geographical locations from the high latitudes (Fairbanks, USA) to the tropics (including, among many others, cities in Nigeria, Mexico, India and Malaysia). Urban thermal anomalies have been observed both during short-term, intensive measurement campaigns (e.g. OKE and EAST, 1971) and in longer term climatological studies (e.g. CAYAN and DOUGLAS, 1984; ACKERMAN, 1985, 1987; BALLING and BRAZEL, 1987a; FENG and

PETZOLD, 1988; KARL et al., 1988; CHOW, 1992). Many of these longer term studies (e.g. CAYAN and DOUGLAS, 1984; KARL et al, 1988) report a decrease in daily maximum tem497

Urban climates peratures in all seasons except winter and increases in daily minimum and mean temperatures as a result of urbanisation. Most urban heat island studies conform to a common methodology: the temperatures are near-surface air temperatures measured in the UCL; the reported heat island strength (ATu_r) refers to urban-rural temperature differences either at a specific time, or in terms of the daily maximum, minimum and mean temperatures. The rural temperatures are measured at a site presumed to be uninfluenced by the nearby city. Intensive studies would involve a mobile temperature survey across the city and out to the rural periphery, and urban heat island strengths would refer to the almost simultaneous difference between urban and rural temperatures. Longer term, climatological studies (such as those mentioned above, e.g. by KARL et al., 1988) may simply use just two sites, one representing rural and one urban, and only use daily or monthly maximum, minimum and mean temperatures. Other studies use just an "urban" site, where such a site may well be located at the nearest airport, raising the question of whether it is urban or rural. For example, BALLING and BRAZEL (1987a) found little evidence of a temperature increase in Tucson (USA) because, they suggest, of the type of land-cover that urban growth is replacing. The so-called "urban" station for this study was in fact a nearby airport, removed from the city of Tucson. The suburban/rural energy balance comparison for Tucson (GRIMMONDand OKE, 1990) indicated that local climate differences should develop between suburban Tucson and its rural periphery. Most studies of the UCL (air temperature) heat island have been conducted in mid-latitude cities with temperate climates. From these, a fairly consistent picture of the UCL heat island has emerged; the reader is referred to Landsberg's text (LANDSBERG,1981) and the review papers by OKE (1982) and OKE (1979a) for details of these studies. The thermal anomaly that exists in the UCL is propagated upwards, leading to a warmer UBL. The UCL heat island is best expressed at night during stagnant synoptic conditions, such as exist with an anticyclone. Thus with light winds and clear skies where radiative cooling is optimal and turbulence is damped, and in flat terrain with minimal mesoscale flows, the spatial and temporal pattern of the UCL heat island would be similar to the hypothetical pattern shown in Fig. 9. Embedded in the general pattern of increasing temperatures from the rural periphery to the urban core are microscale variations associated with differences in land-cover: parks appear as cool "ponds" and commercial districts as warm "islands". Other variations in the temporal and spatial structure of the UCL heat island arise because of heating variations due to topography or the presence of water bodies. Measurements show that the heat island results from differential cooling rates between urban and rural land use, where rural really means well-watered, short vegetation (see below). Most studies agree that the UCL heat island reaches its maximum strength 3-5 h after sunset. Apart from wind speed and cloud cover, heat island strength could be expected to depend on specific features of individual cities, such as their size, density, etc. OKE (1973) used urban population as a surrogate for city size and found a log-linear relationship between population and heat island strength. Subsequent observations from outside North America revealed different log-linear relationships for European, Japanese and tropical cities (Fig. 10) that arise because of differences in building materials, building densities and building heights. Plotting heat island strength versus sky view factor reveals that heat island strength also strongly depends on the urban geometry.

498

Current and future urban climates

(D

E _

A

B -

City core

1

_

I Built-up

I

area

B 1/'"

A ...-"""

v

Built-up

area

Fig. 9. Temperature structure in an urban heat island (after Ord~, 1982). The upper figure shows nearsurface temperatures along transect A-B, while the lower illustrates isotherms of heat island strength. The modelling work by OKE (1981), VOOGT and OKE (1991) and OKE et al. (1991) provides insight into the processes involved in urban heat island development. These studies represent three approaches to simulating urban heat islands: a scale model (OKE, 1981); a model canyon in a real boundary layer (VOOGT and OKE, 1991) and a numerical simulation model (Surface Heat Island Model, SHIM) (JOHNSON et al., 1991; OKE et al., 1991). OKE (1981) showed the role of canyon geometry in determining the UCL heat island by using a scale model of an urban canopy. OKE et al. (1991) used a numerical model (validated by JOHNSON et al., 1991) with various combinations of sky view factor, urban and rural thermal admit-

o 16

o

I

I

~ 14

.~ if) E

~

I

I

NorthAmerican J / ~ o o"" e"/'E~r~ .../~/~ f _ . ~ o o'~

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-o 10 c-" ~ 8 ~

I

-

-

6

r-

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4 2 0

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Tropical

9

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-

9149

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10 100 1000 10000 Urban population ( thousands )

Fig. 10. Relationship between city size and UCL heat island strength (after JAUREGUI, 1986) for North American (closed circles); European (open circles) and tropical (triangles) cities.

499

Urban climates

tances, enhanced downward longwave radiation and anthropogenic heat fluxes to determine which factors are more important for urban heat island development. Their results highlight not only the influence of geometry on urban heat island development, but also the importance of urban thermal admittance, compared to rural, values. They suggest that some of the heat island strengths and temporal patterns observed in arctic and tropical cities can be explained by the contrast in thermal admittance between the surrounding landscape and the city. This is well illustrated by the results of JAUREGUI et al. (1992) where the strength of the urban heat island in Guadalajara (Mexico) had a seasonal dependence on the urban-rural contrast in thermal admittance. These contrasts will be largest in the dry season and smallest in the wet season. These results clearly show that urban thermal anomalies based on urban-rural intercomparisons depend on the nature of the rural surface. Recalling LOWRY (1977), the rural site is a surrogate for the pre-urban landscape. The different interpretations that result when using urban/rural as a surrogate for urban/pre-urban intercomparisons were illustrated by GRIMMOND et al. (1993). Their study compared suburban and rural air temperatures in and around Sacramento (USA), where the farmland surrounding the city is both irrigated and unirrigated. A maximum heat island strength of 4~

was observed in the comparison of the

suburban and irrigated rural site, but the suburban site appeared as a "cold" island by day (i.e. ATu_r was negative) when the unirrigated temperatures were used for "rural". The actual urban climate effect is probably better represented by the suburban-unirrigated comparison. TABLE VI HYPOTHESISED CAUSES OF THE URBANHEAT ISLAND(FROM OKE, 1987)

Feature of urban land use and urban activity

(a) Urban canopy layer heat island Canyon geometry - increased surface area and multiple reflection Air pollution and warmer UBL Canyon geometry - reduced sky view factor Domestic, industrial and traffic heat emissions Urban construction materials - increased thermal admittance Reduction in vegetation and increase in impervious surfaces Canyon geometry - increased aspect ratio and sheltering (b) Urban boundary-layer heat island Chimney and stack releases of heat UCL heat island- increased heat flux from roofs and canopy layer Increased sensible heat flux, effect of heat island and roughness Air pollution- increased aerosol absorption

500

Effect on atmospheric process, leading to a positive thermal anomaly

Increased absorption of shortwave radiation Increased radiative absorption and reemission of longwave radiation Increased longwave radiation from UCL building walls Increased anthropogenic heat source Increased sensible heat storage Reduced latent heat flux Reduced total turbulent heat transport

Anthropogenic heat source Increased sensible heat input from below Increased convective and mechanical turbulence hence entrainment of warm and dry air from above the UBL Increased absorption of shortwave radiation

Current and future urban climates The two "rural" sites yield quite different urban climate anomalies. The factors involved in the genesis of the urban heat island (both at the UCL and UBL scales) are summarised in Table VI, these were initially based on OKE's proposal (1982). As the preceding discussions have shown, many of these hypotheses have since been confirmed. Far fewer studies have been conducted outside of the mid-latitudes, viz. in the tropics or higher latitudes. We expect that urban climate processes will be different outside the midlatitudes because, firstly, the basic climate can be quite different. Seasons are determined primarily on the basis of rainfall, wind direction and cloudiness rather than temperature, hence the seasonal variation in the urban climate will differ from mid-latitude cities. Secondly, some subtropical cities are located in arid landscapes where irrigation of the surrounding lands is frequent (e.g. Phoenix, Sacramento, USA) or the landscape is desert or semi-desert (e.g. Kuwait City, Kuwait; Karachi, Pakistan; Cairo, Egypt). The nature of the urban effect will vary drastically between such locations, especially because of variations in the thermal admittance. In desert areas dust can raise the atmospheric turbidity, and lower K,I, in both rural and urban locations. Thirdly, the solar zenith angle is smaller at tropical latitudes, hence radiation absorption by rooftops becomes important and solar penetration into urban canyons is increased. Finally, artificial heating is not required, however in the wealthier cities or suburbs, artificial cooling demand will be high and anthropogenic heat fluxes potentially large (recall Table IV). Urban climate studies in the tropical latitudes are important because this is the region of future urban expansion. Thermal comfort may already be compromised in many tropical cities where the combination of an already warm and humid climate is exacerbated by excess urban heating. Increased illness and mortality plus excess energy consumption are just two of the potential impacts from the heat island effect in cities whose regional climates already predispose them to be close to the limits of thermal comfort. Unfortunately, the issue of urban climate does not rank particularly highly in many developing, tropical nations where the struggle to provide food, shelter and clean water is of more immediate importance. Nonetheless studies are emerging which describe the urban climate of tropical, urban cities. Some of these are reviewed next. ADEBAYO (1987) compared rural, suburban and urban temperatures in Ibadan (Nigeria), a low latitude, humid city. He found that urbanisation led to increased minimum and mean temperatures, and slightly reduced daytime maximum temperatures. Heat island strengths were larger during the dry season. These results are similar to other tropical locations, including Guadalajara, Delhi, Mexico City, Nairobi and cities in Malaysia. JAUREGUI (1987) compared urban and rural cooling rates for four cities in Mexico (three in inland valleys and one on the coast) using mean monthly temperature data. Although some results were broadly similar to mid-latitude cities, there were some important differences in the urban heat island timing: peak heat island strengths were observed at sunrise or shortly thereafter and cool islands developed in the afternoon. As reported in many studies, maximum heat islands were observed in the dry season. NASRALLAHet al. (1990) reviewed 23 years of annual mean, maximum and minimum temperatures in and around Kuwait City. They found only a "modest" urban heat island; increased minimum temperatures were not confined to sites within Kuwait City and so no significant differences between urban and desert minimum temperatures could be found. Differences in maximum temperatures between subur-

501

Urban climates

ban and desert sites were significant; the authors suggest that this could be the result of a growing urban heat island. In comparison to other arid cities, Kuwait City's heat island is very small. The authors argue that this is a result of the lack of trees; the similarity of thermal properties of construction materials between the city buildings and the desert; lower building heights; and the moderating influence of the Arabian Gulf. These studies show that urban heat islands exist in tropical cities, but they are less pronounced and the timing of the peak heat island strength is altered compared to mid-latitude cities. Heat island strengths and occurrence are greater in the dry season. Changes to surface energy partitioning in urban land use should be reflected in surface temperatures. For this reason, plus the wealth of spatial information provided, several studies (e.g. PRICE, 1979; VUKOVXCH, 1983; ROTH et al., 1989b) have investigated the spatial expression of the urban heat island using surface temperatures measured from satellites. GALLO et al. (1993), for example, found a linear relationship between the normalised difference vegetation index (NDVI) and observed urban-rural minimum air temperature differences for 37 cities in the USA. Satellite-derived temperatures, however, do not always reveal the same spatial and temporal patterns as air temperature surveys. Several common features found in satellite studies were summarised, and confirmed, by ROTH et al. (1989b). They noted, firstly, that heat island intensities were largest by day and smallest by night, almost the reverse of the diurnal cycle of the UCL air temperature heat island. Secondly, the spatial temperature structure was closely linked to the land use patterns (i.e. features such as parks, shopping centres) during the day, but not at night. ROTH et al. found that the warmest urban surfaces (by day) were not in the city core but in areas with large fiat-topped buildings or extensive pavement. Some of these discrepancies may simply be linked to the difference between a radiative surface temperature and near-surface air temperatures where the latter depend on the surface energy balance plus heating and cooling in an air volume. Satellites may also preferentially view horizontal surfaces such as pavements, rooftops and treetops. The amount of the three dimensional active urban surface "viewed" or sampled by a satellite sensor then depends on its view angle and the ratio of active to plan area of the urban landscape being sampled. GOWARD (1981) showed that rooftops have much lower thermal admittance values and thus will show a greater surface temperature amplitude which may explain the satellite thermal patterns. The discrepancy between the UCL air temperature heat island and that viewed from a satellite simply illustrates our limited knowledge of urban processes. But it also means that satellitebased observations of urban climate processes must be interpreted with care.

Humidity Much less is known about urban effects on humidity. Humidity is expected to differ between urban and rural locations because of the following factors: 9

smaller input of vapour as a result of a reduction in vegetation in the UCL and hence smaller transpiration rates (but note that the reduced atmospheric stability at night may allow transpiration and evaporation to continue into the night);

9 9

emissions of water vapour by industry and transport; intercepted water on impervious surfaces may lead to enhanced evaporation following rainfall;

502

Current and future urban climates * 9

anthropogenic control of water application; juxtaposition of wet and dry surfaces means microscale advection can enhance evapo-

ration. Like the thermal anomaly, urban humidity effects need to be separated into the UCL and UBL effects. Cities are often asserted to have lower relative humidities because of the misconception that urban areas are devoid of water. Most measurements of relative humidity in the UCL support these assertions but only because the relative humidity index depends both on atmospheric water vapour content and temperature. In fact, relative humidity is more sensitive to temperature changes than to humidity changes, so lower relative humidities often simply reflect the extra warmth of cities. Studies comparing actual humidity show that cities are often drier than their surroundings but there is a diurnal and seasonal dependence. HAGE'S (1975) study found, in summer, that urban areas were drier than rural by day and more moist by night. Seasonally, HAGE found that the vapour density was always greater in winter. Similar results (see OKE, 1979a) have been reported from other cities, e.g. in Chicago, ACKERMAN (1987) found lower vapour pressures and dew point temperatures in the forenoon and on spring afternoons. Urban-rural humidity differences, like temperatures, were found to be very sensitive to wind speed and cloud cover. For tropical Ibadan, ADEBAYO (1991) observed slightly reduced vapour pressures, especially in the afternoon, based on 2 years of data.

The UBL climate These thermal and moisture anomalies are not restricted to the UCL. The UCL warm, dry "island" extends upwards into the UBL and can be advected downwind. The temperature and humidity in the UBL also result from entrainment, advective influences and the effects of radiative heating and cooling. Measurements conducted in St. Louis in summer daytime conditions also showed a thermal excess and a specific humidity deficit in the UBL temperature profiles (ACKERMAN and MANSELL, 1978; TAPPER, 1990) which were displaced downwind. These thermodynamic anomalies varied inversely with wind speed

(SHEAand

AUER, 1978) and extended to heights of 500-1000 m (AUER, 1981); they also resulted in higher convective condensation levels. The deeper UBL slightly dilutes and reduces the heating impact. These observations were in summertime, anticyclonic conditions. In wintertime, or nocturnally, the UBL can be more moist. The drier UBL is due to both a decreased surface evaporation flux and enhanced entrainment of dry air from the stable inversion layer above the UBL, as demonstrated by HILDEBRAND and ACKERMAN (1984). They found that QH decreased with height in both urban and rural boundary-layers, but urban QH fluxes were always larger. Note that these large fluxes, accompanied by large vertical velocities in the UBL, enhance the entrainment process. Their urban/rural comparison found large rural QE fluxes at the surface which decreased with height in contrast to the large QE flux at the top of the UBL. Evaporation in the UBL thus increases with height and leads to a drying of the UBL.

Precipitation Two of the basic ingredients for raindrop formation; condensation nuclei and strong up-

503

Urban climates

drafts, are prevalent above and downwind of cities. It is therefore no surprise that cities have long been suspected of creating positive rainfall anomalies. CHANGNON (1968) identified the La Porte rainfall anomaly; HARNACK and LANDSBERG (1975) also observed enhanced summer rainfall over Washington, USA; HUFF and CHANGNON (1973) showed anomalies in precipitation and storm conditions for cities with populations greater than 1 million. Furthermore, the results of AYERS et al. (1982) show the potential for urban modification of rainfall. They found that the fluxes of cloud condensation nuclei (CCN) from Australian cities were an order of magnitude greater than the CCN fluxes generated by the continental landmass. The METROMEX study in St. Louis aimed to identify and explain links between summer rainfall anomalies and urbanisation. Summer rainfall was observed to increase from west to east across St. Louis, reaching a maximum northeast of the Mississippi River and the locus of the St. Louis urban heat island. Climate studies showed that although there was a peak in rainfall totals at this location, there was no observed shift in the diurnal pattern of rainfall. The rainfall maximum occurred at the site with the largest incidence of severe weather phenomena such as thunder and lightening. The major finding of the METROMEX project was that a precipitation anomaly arose from altered UBL dynamics that resulted from a range of urban influences, the most important including: 9 modified inputs of latent and sensible heat from the urban surface into the UBL; 9 input of cloud condensation nuclei from urban activities; 9 increased convergence and updrafts throughout the UBL due to perturbations in momentum and heat exchange across the city; 9 enhanced moisture input from industrial sources (BRAHAM, 1981, cited in CHANGNON, 1991). These combined influences enable cumulus clouds to grow taller and shift the droplet size spectrum to smaller median droplet diameters. The input of giant nuclei from industry, plus stronger updrafts, enhance the collisioncoalescence process with a resulting increase in rainfall. Nocturnal rainfall increases in St. Louis (CHANGNONand HUFF, 1986) were linked to urban influences on already heavy rainfall events. There are other studies that report the existence of "rain islands" in association with urban areas, including several from tropical cities. BALLING and BRAZEL (1987b) observed both increased rainfall amounts from, and frequency of, late afternoon and early evening storms in Phoenix (USA). They suggest rainfall anomalies may be linked to the urban heat island whose existence in Phoenix is well documented. However the failure to find any statistical significance in these observed differences highlights the difficulties in ascribing anomalies in rainfall occurrence and amount to an urban influence. Nonetheless, there is agreement that in some cities, positive rainfall anomalies exist. ELSOM and MEADEN (1982) examined the distribution of tornadoes in the Greater London district in the period 1830-1980 and found that the inner city shows a much smaller occurrence of tornadoes. They argue that the increased roughness of cities and excess heating combine to either dissipate weak tornadoes or suppress their formation altogether. They believe that the coincidence of an area of reduced tornado activity with the most densely populated area of London is significant. Similar tornado-free zones have been observed in Chicago and Tokyo (FUJITA, 1973, cited by ELSOM and MEADEN). Increased thunder activity was found over London (ATKINSON, 1969, 1971), St. Louis and other cities (CHANGNON

504

Future urban climates et al., 1977). ELSOM and MEADEN argue that the same factors that lead to enhanced convective precipitation and thunderstorm activity could also lead to a reduction in tornado activity and cite FUJITA's (1973) studies that show urban heating and frictional effects prevent tornadoes from forming. The thermodynamic anomalies in the UBL which influence convective and severe storms also have the potential to modify the passage of fronts. A classic study in New York City (LOOSE and BORNSTEIN, 1977) found that frontal movement was accelerated with a well developed urban heat island and retarded by the city's frictional influence if an urban heat island was absent. A series of observational and modelling studies in Tokyo have revealed that the movement of the seabreeze front through the Central Business District and suburbs is modified. The elevated air temperatures lead to local low pressures over Tokyo's Central Business District. The increase in pressure gradient found between the city core and the outer suburbs prevented the seabreeze from advancing through the city (YOSHIKADO, 1992).

Future urban climates

The previous sections have provided a general picture of urban effects on those atmospheric processes (radiation, heat, mass, momentum exchanges) that determine weather and climate. The urban influence on the atmosphere can extend vertically to the top of the PBL and, by day, can enhance the entrainment of air into the UBL. The regional wind advects this "plume" of warm, dry and polluted air downwind, enabling cities to "export" their atmospheric impacts to neighbouring areas. There is thus an urban "footprint" extending ca. 100 km downwind of a city. This does not include long range transport of atmospheric pollutants (which may extend to many thousands of kilometres). At the local scale, the urban temperature regime is characterised by higher daily minimum temperatures with no consistent change in maximum temperatures, thus daily mean temperatures are also elevated. The local scale climate tends to be warmer and drier, especially at night. There is a slight to medium reduction in the quantity and quality of solar radiation received at the top of the UCL, however these reductions are offset by enhanced longwave radiation. Within the UCL shading and specular radiation will dominate the microscale radiation regime. Wind speeds and directions are modified both within the urban canopy, where perturbed flow around buildings dominates, and in the UBL depending on the regional wind speed. Turbulence, and hence turbulent exchanges, are modified because of the altered thermal and mechanical properties of the urban surface. Cities tend to store more heat within their fabric, and in dry conditions heat is preferentially channelled into sensible rather than latent forms. These climate effects are primarily local to mesoscale in extent. The extra friction and heating generated by the UCL also modifies the occurrence and amount of rainfall, thunder and lightening, tornadoes and frontal movements. These effects may extend to regional scales. What will be the nature of future urban climates? Some have queried whether ongoing urban expansion will begin to modify continental and global scale climates. The total area of land in "urban use" was estimated as 1 million km 2 in 1980, growing at a rate of 2 • 104 km 2 year -1 (OKE, 1980). Thus in 1993 the total land area occupied by cities will be 505

Urban climates (very approximately) only 0.25% of the total surface area of Earth. It seems very unlikely that global-scale climate change could result from the modifications to albedo, roughness and moisture availability as a result of such a small fraction of urban land use. It is instructive here to review our current knowledge of the effect of cities on climates at this larger scale. The search for a greenhouse-induced signal in the near-surface air temperature record for the globe has focused considerable attention on the issue of urban warming and its large scale impacts. The US has the longest and most spatially comprehensive measurement network and so most understanding relates to urbanisation in the US. In 1986 KUKLA et al. analysed the US temperature record from 1941 to 1980 using paired urban/ rural meteorological sites and found that urban stations showed an average temperature increase of order 0.11 ~

Eliminating those station pairs suspected of introducing er-

rors made little difference to this urban warming figure, except for the small subset of stations that had no change in instrument or station location over the data record. These showed a warming trend of 0.34~

This range (i.e. 0.11-0.34~

encom-

passed that found in their survey of other studies of long-term urban warming. KARL and JONES (1989) suggested that the urban warming bias present in the entire US climate record was between 0.1 and 0.4~ in the 84 years from 1901 to 1984; JONES et al. (1989) estimated a similar-sized urban bias for the US which they extrapolated to 0.01-0.1 ~ in 84 years for the global temperature record. These studies show that in the period of rapid urban expansion in the US (1901-1984), urban areas may have warmed by between 1 and 3 ~ been estimated to have yielded a 0.1-0.4~ entire US.

This has

rise in the near surface air temperature for the

JONES et al. (1989) extrapolated their estimates of urban warming for the US to the globe, yielding a 0.01-0.1 ~ warming in 84 years for the global temperature record. This is an order of magnitude less than the observed warming, suggesting that urban impacts on the global temperature are not important. In summary, there is clearly some suggestion that a portion of the increased temperatures observed in the US is attributable to urban warming, however there is no evidence of a similar affect at the global scale. At the local scale, however, urban dwellers in the US have undergone "climate change" of a similar order to that being predicted by current GCMs to result from enhanced greenhouse warming. People living in cities in developed nations probably are exposed to a climate that has elevated minimum and mean temperatures; is potentially less humid; the sunlight they receive may be more diffuse, contain less UV radiation and be less intense; nocturnal temperatures are warmer; and rainfall and storm patterns may be altered. CHANGNON(1992) notes, interestingly, that this climate change has occurred by and large without comment. A social attitude assessment conducted in St. Louis (FARHAR, 1979, cited in CHANGNON, 1992), found that residents in that part of the city shown to have an altered climate were either unaware of the changed climate, or were unconcerned. While urban-induced climate change does not appear to be directly important to urban dwellers, it does affect their lives and environment in many indirect ways. Cities located in cold regions are slightly warmer during winter which saves heating costs at both the personal and environmental level; conversely more energy will be consumed in those cities already located in warm climate regimes. Microclimate effects within the UCL influence the level of pollutants, radiation and turbulent gusts to which pedestrians are exposed. In warm climates the thermal stress resulting from urban heating superimposed on pre-existing heat-wave conditions (which may be

506

Future urban climates

heightened even further due to global warming) can contribute to illness and premature mortality (MATZARAKIS and MAYER, 1991). It is unlikely that there will be any further, sizeable change in the urban climates of cities in developed nations because the period of rapid urbanisation is over. These cities will continue to grow and encroach on the surrounding landscape but they are unlikely to continue increasing their population and, if anything, urban consolidation may slow down this encroachment. There will therefore be some expansion of the urban footprint, but this is unlikely to greatly increase urban climate effects. SEAMAN et al. (1989) simulated the effects on mesoscale airflows of doubling the size of St. Louis. Surprisingly, they found an amelioration of urban effects, namely a reduction in the urban generated convergence and vertical velocities. Future changes in local and meso scale urban climates may arise from efforts to reduce urban sprawl and energy consumption in developed nations. Calls by governing bodies to increase building densities in urban areas through urban consolidation will lead to larger aspect ratios, greater sheltering and reduced SVF in the UCL. Using the relationship developed by OKE (1987), a doubling in H/W ratio from 1 (common for European, Australasian and North American cities, OKE, 1987) to 2 (towards the upper end for some North American cities) leads to a decrease in SVF from ca. 0.5 to 0.3. This would be accompanied by a 3~ increase in the maximum heat island strength. As Fig. 8 illustrates, an increase in aspect ratio may move an urban block or land use zone from a well-ventilated "wake interference flow" landscape to a "skimming flow" landscape. Urban temperatures will be further increased if urban consolidation is accompanied by reductions in greenspace; external water use and pervious surfaces. In cold climates, the resulting warmer temperatures will further reduce heating demand, but urban consolidation will have a real cost in warmer climates, unless other steps are taken to reduce the excess thermal stress. Improved architecture and construction of buildings in cities will enhance passive energy sources and further reduce energy consumption and anthropogenic heating. Reductions in QF potentially mean a reduction in the energy inputs and hence a potential reduction in warming, however these simple considerations ignore the importance of the site and height of the energy release (recall equation (2) and discussion). Future reductions in photochemical smog may arise from improved technology which will increase the receipt of shortwave radiation in those cities where photochemical smog is prevalent. Such increases are more likely, however, in cities such as Shanghai where coal burning is currently the major fuel source. Switching to alternative fuels, likely within the next 20 years, will significantly increase shortwave radiation receipts in such cities. In short, cities in developed nations without a heavy industrial base are unlikely to show any further significant warming at the continental-global scale. As explained in the Introduction, most urban growth will occur in cities in poorer nations, often located in the tropical latitudes. Already, most of the world's megacities are located within either the poor, or rapidly industrialising (e.g. Korea, Thailand), nations. Unlike most developed or industrialised nations, urbanisation is still increasing in many of the poorer nations. A United Nations report in 1989 found that in less developed nations, the urban population has been increasing at a rate of 3.6% per year and this was projected to continue until 2000. By way of comparison, urban populations in the developed world were increasing at or below 1% per year. Urban populations in Asia and Africa were projected to in507

Urban climates

crease by 2.3 billion between 1990 and 2025. To put this in perspective, this is equivalent to the total urban population for the globe in 1990, and represents 82% of the global increase in urban dwellers for the period 1990-2025. Recalling the findings of Karl et al. on the size of the urban warming signal for the US, it is of interest to note that this increase in urban population for Asia alone (1990-2025) is about ten times the increase in urban population in the US from pre-1900 to 1985. Increasing urbanisation in the African and Asian regions is not only concentrated into megacities. The same 1989 United Nations report found that the fastest growing cities (1985-2000) in the less developed nations (and for the globe) were projected to be those with populations less than 7.5 million (in 1985). Small towns will grow into medium-sized towns; middle-sized cities will grow to become megacities. If the urban contribution to the total warming observed in the US from 1901 to 1984 could be generalised (viz. 0.1--0.4~ then these large increases in urban populations and cities have the potential to make a significant contribution to the global temperature. Similarly, if the population-urban heat island relationships developed by KARL et al. (1988) are valid globally, then a large amount of the urban heating will result from this increase in population and size of medium-sized towns. JAUREGUI's (1986) graph of (log) population versus urban heat island strength shows a smaller slope for tropical, compared to mid-latitude, cities (recall Fig. 10). Furthermore KARL and JONES (1989) note that the urban warming apparent in the US record was not evident in the global temperature record. Clearly the observational evidence of global warming due to the urban growth during the 20th century is equivocal. It is thus very difficult to quantify the impact of the enormous increases in urban dwellers and city sizes projected into the 21st century. Regional and local scale climate changes will occur in those regions undergoing large-scale urbanisation. The IPCC (1991) predicted that with doubled CO2, tropical regions will show only small temperature increases but possibly increased rainfall. Urban effects will be superimposed on this backdrop of greenhouse-induced climate change. The projected increases in urban populations discussed above are almost beyond our comprehension, if they are correct then many areas in the Asian and African regions which will have a much different climate than exists today. Many of these changes will be similar to those already experienced by urban dwellers in industrialised and developed nations, and described above. Minimum and mean temperatures could be at least 5~ warmer, using the US data as an example. There will also be some differences: the amount of heating due to the urban heat island may be roughly similar in magnitude, but its strength will depend on the season. Changes in the timing and length of the wet and dry seasons in the tropics will therefore impact on the duration of urban-induced warming. Many cities in the poor and rapidly industrialising nations will have a markedly different radiation climate. Unless alternative fuel technologies are developed rapidly, these cities will experience reductions in the incoming solar radiation; UBL warming may result from radiation absorption or alternatively there will be a large increase in diffuse radiation. Improvements in fuel technology should mean that these cities pass through this "stage" of solar dimming (STANHILLand KALMA,1993) more rapidly than industrialised nations. If current patterns of urban growth in less developed nations prevail, much of the urban land-use will be informal squatter settlements. The effect of this altered land use depends, of course, on the pre-urban land-use. There is insufficient knowledge to quantify the change in energy balance partitioning that would arise from a transition from a

508

References grassland or a forested landscape into a squatter settlement. The results from OKE et al. (1992) would suggest that the convective sensible heat flux is the smallest heat flux in a tropical city energy balance. If so, possibly energy balances will not be altered greatly. The model simulations of SEAMAN et al. (1989) show that rainfall modifications are more likely to occur in those small to medium sized towns that will increase in population and extent. Although these results cannot be generalised, such modifications to rainfall patterns, storms and possible flooding will be of far greater significance. These local scale climate impacts are important, especially in terms of energy consumption, human health and their potential for causing larger scale climate change. Their importance, however, pales in comparison to the likelihood of increased disease and mortality arising from limited air and water quality plus hazards of increased flooding and landslides that will occur in the very poor cities. It can only be concluded that the combined effects of weather and climate changes, possible sea level rise (more than 20% of the world's cities are sited on coasts) and increased climate variability predicted to result from the enhanced greenhouse effect plus urban-induced local climate change and worsening air and water quality will severely limit the quality and sustainability of the urban environment in less developed and poor nations.

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