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Urban microclimate Sustainable Urban Systems Dr Janet Barlow Department of Meteorology

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1 Urban microclimate Sustainable Urban Systems Dr Janet Barlow Department of Meteorology

2 Outline: urban microclimate and pollution Why focus on urban climate? How does an urban area affect the atmosphere? How can we improve the urban climate? What are the sources of pollution and what is their impact?


4 Urbanization of world population 1800 – 3% urban 1900 – 14% 1950 – 30% (83 cities >1M) 2000 – 47% (76% in WDC, 40% in LDC) By 2030 the worlds population is projected to be 60% urban, most of the growth in LDC.

5 Tokyo-Yokohama – worlds largest urban area by population: 34,350,000 people 7,835 km -2 area Source: World Urban Areas: Population and Density, 4 th ed.(2008), Demographia

6 Source: London, c London, 1673 London, 2008!

7 Why study urban atmospheres? Higher percentage of population experiencing urban climate Urban microclimate has distinct characteristics Source area of many pollutants Better design can be used to mitigate climate

8 London! Nice! Manchester! San Francisco!

9 How is an urban surface different to a rural surface? random array of obstacles, in horizontal and vertical patchy – inhomogeneous surface type rough surface (causes turbulence) warmer surface (range of building materials) sources of heat and pollution released at ground level reduced surface moisture

10 Reflection of sunlight (shortwave radiation) Materials used quite dark (e.g. tarmac, slate tiles, stone) Reflect less sunlight Define albedo: the fraction of incident shortwave radiation which is reflected = 0 (no reflection) = 1 (total reflection) e.g. tarmac ~ grassland ~ snow ~

11 Emission or absorption of heat (longwave radiation) Some built materials have high heat capacity, low thermal conductivity they store heat, release it slowly (e.g. stone, brick, concrete) Some materials have low heat capacity, high thermal conductivity they heat up rapidly to high temperatures, and cool down rapidly (e.g. Cat on a Hot Tin Roof!) Typically, urban areas store heat, release it slowly

12 Turbulent transfer of heat from the surface Movie! Buoyant, hot air rising from concrete Hot air less dense Shimmering shows turbulence! Surface temperature more than air temperature (day) Flux of heat from ground to air If air temperature more than surface temperature (night) Flux of heat from air to ground = sensible heat flux Sunlight evaporates water Flux of moisture into air Surface energy used up in evaporation (so surface cools!) Flux of energy into air = Latent heat flux

13 Surface energy budget Net radiation Turbulent heat flux into air Vapour flux into air Storage of heat in surface Heat flux into ground =+++ Q * = Q G + Q H + Q E + Q S Surface reflects short-wave radiation (S) according to its albedo ( ), and absorbs and emits long-wave radiation (L) according to its temperature and emissivity. Q * = (1- ) S + L – L Q S

14 reduced latent heat flux increased flux into building fabric delayed peak in sensible heat flux delayed transition to downward heat flux during the night local solar time (hrs) Vancouver Cleugh and Oke (1986)

15 The father of meteorology Observations 1801 to 1841

16 T.J.Chandler The Climate of London, 1965

17 Northing, miles Easting, miles Graves H., Watkins R. et al Building Research Establishment

18 Urban Heat Island Urban areas can be several degrees warmer than surrounding areas. This effect is strongest at night with low wind and clear skies. Maximum temperature difference is observed not long after sunset, due to slow release of heat from storage in urban buildings vs. rapid cooling of rural area Oke, 1987, Boundary Layer Climates

19 Climate change in urban areas? Trends in minimum temperature in degrees per decade for period 1950 to 1990 for large urban areas in Mexico Compare with global warming background ~0.07

20 Urban heat island mitigation 1: green roofs Q: how does a green roof change the surface energy balance? Q: what impact does a green roof change have on energy use? water cycle?

21 Urban heat island mitigation 2: other methods High albedo roofs High albedo pavement Uni. Of Arizona: Pervious concrete

22 Windflow around buildings

23 Wind over urban areas…small scale Define street canyon: two parallel rows of uniform height buildings Flow in a street depends on aspect ratio, i.e. ratio of height (H) to width (W): a) isolated roughness H/W <0.3 b) wake interference 0.30.6 Flow pattern determines flux of heat or pollution out of street

24 Research: flow visualisation in a wind tunnel flat roof H/W=0.6flat roof H/W=1.0 high pitch H/W=0.6high pitch H/W=1.0 Model scale ~ 1:400 ~ 400 times faster than in nature !!!

25 Wind over urban areas…large scale The wind experiences friction with the ground, causing turbulence and wind strength increasing with height Turbulence causes exchange of momentum, heat, moisture and pollutants with the surface …also affects pedestrian comfort

26 Atmospheric Boundary Layer boundary layer mixed layer ~2-5h ~0.1z i z surface layer z i ~1km windspeed potential temperature free troposphere

27 Diurnal cycle of boundary layer

28 Boundary Layer Characteristics The boundary layer is the bottom layer of the atmosphere, characterised by its interaction with the ground. At the top of the daytime boundary layer is a temperature inversion which acts as a lid by inhibiting exchange of air with the free troposphere. Boundary layer depth varies diurnally between approximately 1000m by day to a few hundred metres at night. Łódź! Poland 07:30, summer Boundary layer c m Traps pollution! And heat…

29 Atmosphere adjusts to rural-urban transition Wind and temperature profiles adjust to the urban surface The influence of the surface is transmitted upwards by turbulence (creating an urban boundary layer) Wind and temperature profiles adjust back to rural surface

30 The Independent, Sunday 17 th February 2002 The Acropolis: more damage from 25 years of pollution than the previous 2500?

31 Pollution in urban areas

32 Ozone as a pollutant Ozone is produced in photochemical smog, i.e. requires precursor chemicals and sunlight to form Damages vegetation, buildings and materials, e.g. rubber Causes and exacerbates respiratory diseases

33 Ozone during summer th August 2003: Highest temperature in Kent (38.1ºC) Much of England and Wales experiencing >90 ppb of ozone (DEFRA high band) 1350 deaths attributed to ozone in first two weeks of August 2003

34 Urban aerosols Primary sources: dust, fuel combustion Secondary sources: oxidation of sulphur dioxide sulphate particles sulphuric acid (acid rain) nitrates

35 Particulate Matter (PM10, PM2.5) Physical processes are a function of size: Small particles (0.1 m) are more numerous but grow rapidly Large particles ( m) deposit easily to surfaces (hours) Medium sized particles (1 m) reside longest in atmosphere (days)

36 Effects and impacts Larger, absorbing, aerosols promote local greenhouse effect urban areas can be warmer! Smaller aerosols can be inhaled deep into lungs …440 deaths attributed to PM10 pollution during first two weeks of August 2003… Aerosols reduce visibility, soil buildings Aerosol absorbs radiation from ground and re-emits a smaller amount up and down

37 Case study: London

38 Marylebone Road Air quality monitoring site

39 Marylebone Road – wind patterns

40 Marylebone Road – traffic 3000 – 3500 vehicles per hour! Vehicles emit large amounts when they accelerate Multiple traffic lights, many at intersections hot-spots of high pollutant concentration

41 Marylebone Road – pollution Kerbside carbon monoxide concentrations c. 3 times urban background

42 Marylebone Road – people Pollutant exposure depends on traffic mode, location in street, weather, health Q: how representative are fixed monitoring sites? More at

43 Key points to learn characteristics of an urban area surface energy budget urban heat islands mitigation of heat in urban atmospheres flow patterns around buildings structure of atmospheric boundary layer main pollutants and sources meteorological and chemical conditions for formation Systems thinking: pollutant exposure is a function of Traffic emissions, weather, building layout, transport mode

44 Further reading Oke, T.R. (1987) Boundary Layer Climates, 2 nd ed, Methuen - chapter on urban climates Stull, R.B. (1997) An Introduction to Boundary Layer Meteorology, Kluwer Academic - good for boundary layer theory Turco, R.P. (2002) Earth under siege: from air pollution to global change, Oxford University Press - weather statistics, news, conferences about urban areas - website of the International Association for Urban Climate – free to join! - archived air quality data and information

45 Nighttime urban heat island (composite of thermal IR images taken at 03:27 on August ) Paris by daytime (composite of thermal IR images taken at 13:28 August ) Dousset and Gourmelon, 2003

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