CLIMATE OF CITIES

Introduction

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

Global, regional, and local effects

Cities, towns, and settlements cover only a small fraction of the Earth (<2 percent of the land surface). Thus, in terms of direct surface changes individual cities do not impact global weather or climate patterns. However, given the large and ever increasing fraction of the world’s population living in cities, and the disproportionate share of resources used by these urban residents, cities and their inhabitants are key drivers of global climate change. Cities affect greenhouse gas sources and sinks, both directly and indirectly. Urban areas are the major sources of anthropogenic carbon dioxide emissions from such sources as the burning of fossil fuel for heating and cooling; from industrial processes; and from transportation of people and goods. Moreover, the demands for goods and resources by city dwellers, both historically and today, are the major drivers of regional land-use change such as deforestation. While the exact values are subject to debate, it is widely suggested that more than 70 percent of anthropogenic carbon emissions can be attributed to cities. Thus despite their small surface area globally, the effects of cities are significant regionally and globally, as well as locally.

Table 10.1 Controls on urban climate are dependent on location as well as urban specific characteristics

Variable

General controls

Urban controls/effects

Incoming solar radiation (K|)

Latitude; synoptic

conditions/cloud cover

Air quality/industrial sources influence scattering

Outgoing solar radiation (KJ)

Surface materials; surface morphology'/geometry

Incoming longwave radiation (L|)

Synoptic conditions/cloud cover

Air quality/industrial sources affect absorption

Outgoing longwave radiation (LJ)

Thermal properties of materials; radiative properties; surface morphology'/geometry

Net all wave radiation (Q*)

Latitude; synoptic

conditions/cloud cover

Materials & morphology air quality

Sensible heat flux (QH)

Temperature gradient;

atmospheric stability;

synoptic conditions

Building volume; built fraction

Latent heat flux (QE)

Moisture gradient;

atmospheric stability;

synoptic conditions

Fraction greenspace; irrigated surfaces; enhanced runoff; detention ponds

Storage heat flux (AQs)

Materials & morphology' urban surface; orientations walls; mass/ volume urban surface

Anthropogenic heat flux (Qf)

Latitude; continentality;

regional setting

Heating/cooling requirements;

industrial activity'; socio-economic conditions; population/building density; transportation routes &’ methods

Air temperature

Latitude; continentality; regional setting

Materials & morphology' of urban surface; release of anthropogenic heat; air quality'

Humidity

Latitude; continentality;

regional setting — proximity to water bodies

Reduced vegetation; fewer moist surfaces

Localized releases (industrial sources) as by-product combustion; urban air temperature

Wind field

Synoptic conditions

Building & tree density'; morphology’ buildings & roofs affect roughness & displacement lengths, channelling through urban canyons

Precipitation

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

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

Note: For further details see Oke et al. (2017).

Schematic figure to show the link between the energy and water exchanges at the urban surface

Figure 10.1 Schematic figure to show the link between the energy and water exchanges at the urban surface

Source: Gnmmond and Oke (1991)

A city’s geographical setting influences both its climate and its effects on climate. Latitude has an influence through basic solar forcing; continentality influences seasonal extremes; and the sequence of expected fronts, low pressures systems, and other synoptic scale influences, affect the ranges of meteorological conditions a city experiences. These all influence the design of a city (e.g. building styles and materials) and the behaviors and activities of inhabitants (their demands for heating, cooling, etc).

At the mesoscale, the geographic setting of a city will influence regional wind systems forced by topography (e.g. mountain—valley) or the presence of water bodies (e.g. sea or lake-land breezes). These in turn affect such things as the redistribution of air pollutants. The presence of the city itself can, under some synoptic conditions (e.g. anticyclones), create regional winds, the so-called rural-to-city breezes (e.g. in Paris documented by Lemonsu and Masson 2002). Cities also influence areas downwind. They are a source of warm, polluted air and can modify precipitation patterns (Lowry 1998; Shepherd et al. 2002; McLeod et al. 2017).

Within a city, neighborhoods with similar land-use and land-cover, generate distinct local scale climates (Stewart and Oke 2012). These are a function of the shape and spacing of buildings and their materials, amounts of vegetation, and human activity. Repeated patterns, based on features such as the height of the buildings, width of the canyons between them, the shape of roofs, and the areal fraction of vegetation can be clearly identified (Figure 10.2). Urban climatologists commonly characterize cities at this scale in terms of the height, width, density of buildings, the fraction of greenspace, and amounts of heat released by human activities (Table 10.2). Many numerical models simulate urban climates at this scale (e.g. the Weather Research and Forecasting Model WRF, Meso-NH) (see further description in Grimmond et al. 2010; Chenet al. 2011).

Schematic representation, with photographs, of the three horizontal-vertical scales commonly used m studies of urban atmospheric processes

Figure 10.2 Schematic representation, with photographs, of the three horizontal-vertical scales commonly used m studies of urban atmospheric processes: (a) the planetary boundary layer (PBL) and urban boundary layer (UBL) (shown here for daytime convective conditions); (b) the urban canopy layer (UCL) with vertical dimensions related to the mean height of the roughness elements; (c) the micro- or individual property scale.

Within neighborhoods, arrays of microscale climates exist. At these smaller spatial scales (10"—101 m), a person walking down the street can experience a range of conditions: the sunlit or shaded sides of the street; the channeling or blockage of wind by a building; the influence of a park or shade trees.

Thus, key to an understanding of urban climates, is a clear understanding of spatial scale, both horizontal and vertical (Figure 10.2). Fundamental in any study of urban climate is the question as to ‘what is actually of interest’ — the overall effect of the city, conditions in a neighborhood, or the climate of an individual house or garden? Confusion results if this is not clear and if there is a spatial mismatch between the entity of interest, observations collected or used, or the spatial resolution of models run to simulate or predict effects.

 
Source
< Prev   CONTENTS   Source   Next >