Urban microclimate

Sustainable Urban SystemsDr Janet Barlow

Department of Meteorologyj.f.barlow@reading.ac.uk

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?

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 world’s population is projected to be 60% urban, most of the growth in LDC.

• Tokyo-Yokohama – world’s largest urban area by population: 34,350,000 people7,835 km-2 area

Source: “World Urban Areas: Population and Density”, 4th ed.(2008), Demographia

www.demographia.com

Source: http://en.wikipedia.org/wiki/History_of_London

London, c. 1300

London, 1673

London, 2008!

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

London!

Nice!

Manchester!

San Francisco!

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 

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 ~ 0.05-0.1 grassland ~ 0.1-0.2 snow ~ 0.4-0.7

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

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 airSurface energy used up in evaporation (so surface cools!) Flux of energy into air = Latent heat flux

Surface energy budget

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

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

QS

• 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)

VancouverCleugh and Oke (1986)

“The father of meteorology”Observations 1801 to 1841

T.J.Chandler“The Climate of London”, 1965

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Easting, miles

Graves H., Watkins R. et al. 2001Building Research Establishment

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”

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

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?

livingroofs.orgwww.risc.org.uk/garden/

Urban heat island mitigation 2: other methods

High albedo roofs

High albedo pavement

Uni. Of Arizona: asusmart.com

Pervious concrete

Windflow around buildings

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

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 !!!

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

Atmospheric Boundary Layer

boundary layer

mixed layer

~2-5h

~0.1zi

z

surface layer

zi~1km

windspeed potentialtemperature

free troposphere

Diurnal cycle of boundary layer

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! Poland07:30, summerBoundary layer c. 1-200 mTraps pollution! And heat…

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

The Independent, Sunday 17th February 2002

The Acropolis: more damage from 25 years of pollution than the previous 2500?

Pollution in urban areas

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

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

Urban aerosols• Primary sources: dust, fuel combustion• Secondary sources:

oxidation of sulphur dioxide sulphate particles sulphuric acid (acid rain)nitrates

Particulate Matter (PM10, PM2.5)

• Physical processes are a function of size:

Small particles (0.1m) are more numerous but grow rapidly

Large particles (10-100 m) deposit easily to surfaces (hours)

Medium sized particles (1 m) reside longest in atmosphere (days)

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

Case study: London

Marylebone Road

Air quality monitoring site

Marylebone Road – wind patterns

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

Marylebone Road – pollution

• Kerbside carbon monoxide concentrations c. 3 times urban background

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Bloomsbury Marylebone Road Exposure sampling periods

Marylebone Road – people• Pollutant exposure depends on traffic mode, location in street, weather, health

Q: how representative are fixed monitoring sites?

More at www.dapple.org.uk

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

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

• http://www.urbanclimate.net- weather statistics, news, conferences about

urban areas• http://www.urban-climate.org

- website of the International Association for Urban Climate – free to join!• http://www.airquality.co.uk

- archived air quality data and information

Nighttime urban heat island(composite of thermal IR images taken at 03:27 on August 6-10 1998)

Paris by daytime(composite of thermal IR images taken at 13:28 August 6-10 1998)

Dousset and Gourmelon, 2003