Urban Heat Islands

11 Urban Heat Islands 11.1 Introduction .............................................................................................................................143 11.2 Principles ...................................................................................................................................144 Urban Scales • Conceptual Framework • Heat Island Types, Characteristics, and Underlying Processes • Heat Island Controls 11.3 Methods of Analysis ................................................................................................................151 Observations • Models 11.4 Impacts and Applications ...................................................................................................... 154 Solutions to Mitigate Urban Heat Islands • Human Comfort and Health • Forecasting Urban Weather and Climate and Developing UHI Mitigation Strategies • The Urban Heat Island and Global Climate Warming 11.5 Major Challenges .....................................................................................................................157 M. Roth Science • Observations National University of Singapore References .............................................................................................................................................158 11.1 Introduction heat-related illness and mortality, air pollution, energy demand for air conditioning, and indirectly greenhouse gas emissions. For millions of people living in cities, increased temperatures Heat waves, for example, which heat islands can exacerbate, are a growing fact and concern. The urban heat island (UHI) is are the leading weather-related killer in the United States. a phenomenon whereby urban regions experience warmer tem- Urbanization and UHIs have impacts that range from local to peratures than their rural, undeveloped surroundings. The UHI is global scales, and cities are an important component of global the most obvious atmospheric modification attributable to urban- environmental change research. ization, the most studied of climate effects of cities and an iconic Urban climatology has a rich and long history beginning with phenomenon of urban climate. It can be found in settlements of Luke Howard’s pioneering work. Howard was a chemist and all sizes in all climatic regions and arises from the introduction amateur meteorologist and is most famous for his classification of artificial surfaces characteristic of those of a city that radically of clouds. In 1815, he conducted the first ever systematic urban alters the aerodynamic, radiative, thermal, and moisture proper- climate study measuring what is now called the UHI effect based ties in the urban region compared to the natural surroundings. on thermometers in the city of London and in the countryside The heat island is defined on the basis of temperature differences nearby (Howard 1818). It is also remarkable that Howard identi- between urban and rural stations, and the isotherm patterns of fied virtually all causes that are responsible for the development near-surface air temperatures resemble the contours of an island. of the UHI during this early study. Subsequent urban climate The evaluation of the influence of settlements on the local research replicated Howard’s findings from urban-rural pairs climate has been an important task for a long time, and the of thermometers at about 2 m height in many cities and a large thermal environment in particular has received widespread number of UHI maps generated from mobile traverses appeared attention because it has practical implications for energy use, in the literature during the 1930s. Much of the research from this human comfort and productivity, air pollution, and urban period, with a focus on German work, was summarized by the ecology. During the winter, heat islands can be beneficial to Benedictine Father Albert Kratzer in a monograph entitled “Das cities in colder climates by helping to reduce heating costs and Stadtklima” (The Urban Climate) (Kratzer 1937), which also pro- cold-related deaths. The negative impacts of summertime heat vides the first systematic review of the influence of settlements islands, however, outweigh the benefits of wintertime warm- on air temperature. A comprehensive summary of UHI stud- ing in most cities because the largest population centers are ies (maps and statistics) carried out in primarily European and located in (sub)tropical climates. Summertime UHIs increase North American cities was subsequently published by Helmut Handbook of Environmental Fluid Dynamics, Volume Two, edited by Harindra Joseph Shermal Fernando. © 2013 CRC Press/Taylor & Francis Group, LLC. ISBN: 978-1-4665-5601-0. 143 144 Handbook of Environmental Fluid Dynamics, Volume Two Landsberg in his book entitled “The Urban Climate” (Landsberg conceptual framework required to model each type. In applied 1981). During the 1970s and 1980s, focus shifted from the largely studies, the target may be human comfort, building energy use, or descriptive early work toward exploring the processes responsible city-wide mitigation which all occur at different scales (Oke 2009). for the urban effect with many fundamental contributions from Three basic urban climate scales are usually recognized Tim Oke, laying the foundation for a modern treatment of this (Figure 11.1). Individual buildings, trees, and the intervening spaces topic, much of which will be referred to in the following. The pres- create an urban “canopy” and define themicroscale , found inside ent review also benefits from a recent, comprehensive summary the roughness sublayer (RSL), which itself is a manifestation of the of current knowledge of urban climate to allow cities to become small-scale variability found close to the urban surface. Typical more sustainable (Grimmond et al. 2010). microscales extend from one to hundreds of meters. Similar houses in a district combine to produce the local scale, which is an integra- tion of a mix of microclimatic effects. Typical local scales extend 11.2 Principles from one to several kilometers. Plumes from individual local scale 11.2.1 Urban Scales systems extend vertically and merge to produce the total urban boundary layer (UBL) over the entire city, which is a mesoscale phe- Urban climates are characterized by a combination of processes nomenon. During daytime, this layer is usually well mixed due to occurring at a variety of scales imposed by the biophysical the turbulence created by the rough and warm city surface, extend- nature of cities and the layered structure of urban atmospheres. ing to a height of 1 km or more by day, shrinking to hundreds of Recognition of the impact of scale is key to understand the work- meters or less at night. The UBL has typical scales of tens of kilo- ings and phenomena of urban climates in general and heat islands meters and can be advected as an urban “plume” downstream from in particular. Scale is important when studying the form and the city by the prevailing synoptic winds. genesis and for the measurement and modeling of heat islands. Scale sets the fact that there are not only one but several types of 11.2.2 Conceptual Framework heat islands. The size of the source area from where the thermal influence originates and how it changes over time is determined The formation of the UHI is related to the energy balance of by scale. Scale also determines the controls and processes affect- the urban area. Urban structures and materials, land cover ing and creating each heat island and therefore determines the change, and human activity alter the individual components of Urban “plume” Mixing layer PBL UBL Rural BL b) Surface layer Rural Urban Rural (a) Inertial Surface sublayer layer Roughness sublayer Roughness c) sublayer UCL UCL (b) (c) FIGURE 11.1 Idealized vertical structure of the urban atmosphere over (a) an urban region at the scale of the whole city (mesoscale), (b) a land- use zone (local scale), and (c) a street canyon (microscale). Gray-shaded areas (thick line following surface in (c)) show “locations” of the three UHI types corresponding to each scale (see Section 11.2.3 and Table 11.1). (Modified after Oke, T.R., Initial guidance to obtain representative meteorological observations at urban sites, Instruments and Observing Methods Report No. 81, WMO/TD-No. 1250, World Meteorological Organization, Geneva, Switzerland, 2006.) Urban Heat Islands 145 the energy balance in urban areas and subsequently the atmo- short- and longwave radiation and respective net fluxes. The urban spheric state. The UHI must therefore be the result of urban/ atmosphere is both polluted and warm, and the surface is warmer rural energy balance differences. The urban energy balance is compared to the countryside, which also affects the net longwave defined as (Oke 1988) radiation balance. Overall the urban effects tend to offset, and Q* in cities is close to that in nearby rural settings. Sometimes −2 the lower albedo of the city can result in positive urban-rural Q* QQ* +=FHQQ++ESQQ+∆ A ()Wm (11.1) daytime differences, and negative nocturnal differences are often observed as a consequence of extra longwave emissions from the where warmer urban surface temperatures (Figure 11.2). Q* is the net all-wave radiation flux The warm and rough nature of the urban surface promotes Q and Q are the turbulent sensible and latent heat fluxes, H E turbulent mixing, and unstable conditions prevail during respectively daytime and mildly unstable or neutral conditions at night, Q is the net uptake or release of

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