Urban Heat Stress and Mitigation Solutions

An Engineering Perspective

Vincenzo Costanzo, Gianpiero Evola, Luigi Marletta, Vincenzo Costanzo, Gianpiero Evola, Luigi Marletta

• Publisher: Routledge
• Year: 2021
• ISBN:
  9781000431520
• Edition: 1
• Topic: Arquitectura
• Subtopic: Planificación urbana y paisajismo

This book provides the reader with an understanding of the impact that different morphologies, construction materials and green coverage solutions have on the urban microclimate, thus affecting the comfort conditions of urban inhabitants and the energy needs of buildings in urban areas. The book covers the latest approaches to energy and outdoor comfort measurement and modelling on an urban scale, and describes possible measures and strategies to mitigate the effects of the mutual interaction between urban settlements and local microclimate.

Despite its relevance, only limited literature is currently devoted to appraising—from an engineering perspective—the intertwining relationships between urban geometry and fabrics, energy fluxes between buildings and their surroundings, outdoor microclimate conditions and building energy demands in urban areas. This book fills this gap by first discussing the physical processes that govern heat and mass transfer at an urban scale, while emphasizing the role played by different spatial arrangements, manmade materials and green infrastructures on the outdoor microclimate. The first chapters also address the implications of these factors on the outdoor comfort conditions experienced by pedestrians, and on the buildings' energy demand for space heating and cooling.

Then, based upon cutting-edge experimental activities and simulation work, this book demonstrates current and forthcoming adaptation and mitigation strategies to improve the urban microclimate and its impact on the built environment, such as cool materials, thermochromic and retroreflective finishing materials, and green infrastructures applied either at a building scale or at the urban scale. The effect of these solutions is demonstrated for different cities worldwide under a range of climate conditions. Finally, the book opens a wider perspective by introducing the basic elements that allow fuel poverty, raw materials consumption, and the principles of circular economy in the definition of a resilient urban settlement.

Part I
Physical processes and outdoor comfort in urban areas

1 Understanding heat and mass transfer at the urban scale

Vincenzo Costanzo, Gianpiero Evola, and Luigi Marletta

University of Catania (Italy)

Introduction

The evaluation of urban settlements and their local climate has received increasing attention starting from the 19th century, when a strong urbanisation process took place because of the new industrialised society. The study of the urban thermal environment has raised particular interest because of its implications on the energy use in buildings, human comfort, air pollution, and urban ecology. In this sense, the pioneering work of Howard, who conducted the first-ever systematic urban climate study in London [1], laid the basis for what is now recognised as the Urban Heat Island (UHI) effect, i.e. the warmer air temperatures experienced in urban areas if compared with those of the undeveloped (rural) surroundings.

The increase in the air temperature contributes to worsening the liveability of urban areas, thus determining the so-called urban heat stress. However, an outstanding contribution to urban heat stress also comes from the radiant heat emitted by built-up outdoor surfaces and received by the human body. This contribution is again higher than in rural areas, where the absence of dark and impermeable manmade surfaces, along with the presence of vegetation, would significantly reduce the ground surface temperature and the radiant energy emitted (or reflected) to the human body.

The scientific community has agreed on the correlation between extreme heat stress and mortality: indeed, the human body is not able to manage the excessive exposure to heat, and its reduced ability to cool down can result in dehydration, circulatory collapse, and eventually death.

With the aim of understanding urban heat stress, this chapter describes the physical phenomena that affect the urban energy balance and the human energy balance, by taking into account all the relevant climatic, morphometric and thermal properties of urban areas. This paves the way to a discussion of the possible mitigation strategies and their effectiveness.

The different scales of the urban heat island

The understanding of the UHI effect involves different scales of analysis, both spatially and temporally. In particular, three urban climate scales are usually involved in the establishment of the UHI effect (see Figure 1.1):

Figure 1.1 Schematics of the various scales for thermal stress: (a) mesoscale, (b) local scale, (c) microscale.

• The microscale, where individual buildings, trees and other manmade constructions create an urban canopy that extends in height from the street level to the tallest building (so-called urban canopy layer, UCL) and horizontally for about hundreds of metres
• The local scale, made up of similar houses and urban contexts, extending from one to several kilometres horizontally and up to the roughness sublayer (RSL) in height. The RSL, which extends up to a few building heights, is the air volume where the turbulence effects, originated by manmade surfaces, mainly take place. Within a local climate zone, many different microscales coexist, which implies a great variability in the urban climate and the corresponding perceived heat stress
• The mesoscale, which includes various local scales horizontally and extends up to the urban boundary layer (UBL) vertically. The UBL extends from the top of the canopy layer up to the mixing layer and is characterised by the fact that its height depends on diurnal cycles. During daytime the UBL is typically well mixed because of the turbulence originated by rough and warm urban surfaces (with a typical extension of more than one kilometre), while during the night it shrinks to hundreds of metres

According to this classification, heat islands can be regarded as a phenomenon occurring at the urban surfaces scale (surface UHI), at the canopy layer scale (canopy-layer UHI) and at the boundary layer scale (boundary-layer UHI), respectively [2].

Surface UHI is usually measured using land surface temperatures (T_S) derived from remote thermal sensing, which provide an opportunity to characterise this phenomenon at various temporal (diurnal, seasonal and annual) scales [3,4]. However, the currently available resources used for deriving T_S do not have a high spatiotemporal resolution because of satellite technical constraints and due to disturbance from cloud cover [5]. Satellite derived T_S data have been used also for calculating air temperature (T_A) in the UCL of cities with the support of limited meteorological observations through a statistical regression between T_A and T_S [6]. However, the published relationships between T_A and T_S remain empirical, and a general relationship has yet to be found [3,4]. For this reason, but also because the high air temperature experienced by pedestrians within the canopy layer can lead to reduced mental and physical performance and to physiological and behavioural changes [7], the canopy-layer UHI remains the most widely studied effect.

The main causes of the canopy-layer UHI are:

• Decreased long-wave radiation loss to the sky at night (atmospheric window), due to the presence of buildings and other obstructions
• Increased sensible heat storage due to the use of materials with high thermal admittance (e.g. concrete, asphalt, brick)
• Increased absorption of short-wave radiation due to the increased reflections from the surrounding surfaces, especially if they are covered with low-albedo materials
• Decreased evapotranspiration from pervious surfaces, water bodies, and vegetation, due to their reduced presence in urban areas
• Increased anthropogenic heat production
• Decreased convective heat transport due to reduction in wind speed

The urban energy balance

The diurnal variation of the UHI effect at the canopy layer is marked and confirmed by numerous experimental studies [8–10]: during daytime, the urban-rural temperature difference is usually small (below two degree Celsius) or even negative (i.e. a cool island effect may arise) in dense settlements or in presence of tall buildings, mainly because of the beneficial shading effect on urban surfaces. On the contrary, after sunset and until sunrise, the reduced cooling rates in citie...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Contributors
• Preface
• Part I: Physical processes and outdoor comfort in urban areas
• Part II: Urban energy modelling
• Part III: Adaptation and mitigation measures
• Part IV: Towards a resilient urbanscape
• Index