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Urban microclimate Sustainable Urban Systems Dr Janet Barlow

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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?

3 World’s population is becoming increasingly urbanised

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

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

6 London, c. 1300 London, 1673 London, 2008! Source:

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! San Francisco! Nice! Manchester!

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 Surface reflects short-wave radiation (S) according to its albedo (), and absorbs and emits long-wave radiation (L) according to its temperature and emissivity. QS Q* = (1-) S + L – L Net radiation Turbulent heat flux into air Vapour flux into air Storage of heat in surface Heat flux into ground = + Q* = QG QH QE QS

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 Vancouver Cleugh and Oke (1986) local solar time (hrs)

15 “The father of meteorology”
Observations 1801 to 1841

16 “The Climate of London”, 1965
Chandler 1962: line this up with the next map to show how london has grown T.J.Chandler “The Climate of London”, 1965

17 Building Research Establishment
Northing, miles Watkins et al from GLA report: during summer 2000, 1st July to 30th Sep, average of 6 Uhi events Graves H., Watkins R. et al. 2001 Building Research Establishment Easting, miles

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 Pervious concrete Uni. Of Arizona:

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 0.3<H/W<0.6 c) skimming flow H/W>0.6 Flow pattern determines flux of heat or pollution out of street

24 Research: flow visualisation in a wind tunnel
flat roof H/W=0.6 flat roof H/W=1.0 high pitch H/W=0.6 high 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
mixed layer ~2-5h ~0.1zi z surface layer zi~1km windspeed potential temperature free troposphere boundary layer

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

31 Pollution in urban areas
follow structure in lectures species, impact building shape impact on dispersion regional? link back to some of climate change things

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 2003 10th 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 Acropolis?

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, 2nd 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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