Urban microclimate
Sustainable Urban Systems
Dr Janet Barlow
Department of Meteorology
j.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 people
7,835 km-2 area
Source: “World Urban Areas: Population and Density”, 4th ed.(2008), Demographia
www.demographia.com
London, c. 1300
London,
1673
London,
2008!
Source: http://en.wikipedia.org/wiki/History_of_London
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!
San Francisco!
Manchester!
Nice!
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 air
Surface energy used up in evaporation (so surface cools!)
 Flux of energy into air = Latent heat flux
Surface energy budget
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
Net
radiation
=
Heat flux
into
ground
+
Q*
=
QG
+
Turbulent
heat flux
into air
QH
+
Vapour
flux into
air
+
+
QE
+
Storage of
heat in
surface
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)
Vancouver
Cleugh and Oke (1986)
“The father of meteorology”
Observations 1801 to 1841
T.J.Chandler
“The Climate of London”, 1965
Northing, miles
Graves H., Watkins R. et al. 2001
Building Research Establishment
Easting, miles
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.org
www.risc.org.uk/garden/
Urban heat island mitigation 2: other methods
High albedo
roofs
High albedo
pavement
Pervious
concrete
Uni. Of Arizona:
asusmart.com
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
z
zi~1km
windspeed
potential
free troposphere
temperature
boundary layer
mixed layer
~0.1zi
~2-5h
surface layer
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.
Łódź! Poland
07:30, summer
Boundary layer c. 1-200 m
Traps pollution! And heat…
• 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.
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 Acropolis: more
damage from 25 years
of pollution than the
previous 2500?
The Independent, Sunday 17th February 2002
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 (0.1m) 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
• Kerbside carbon monoxide concentrations c. 3 times urban background
1 Dec 04
1 Nov 04
1 Oct 04
1 Sep 04
1 Aug 04
Marylebone Road
1 Jul 04
1 Jun 04
1 May 04
1 Apr 04
1 Mar 04
1 Feb 04
Bloomsbury
1 Jan 04
1 Dec 03
1 Nov 03
1 Oct 03
1 Sep 03
1 Aug 03
1 Jul 03
1 Jun 03
1 May 03
1 Apr 03
1 Mar 03
1 Feb 03
1 Jan 03
-3
daily mean CO mg m
Marylebone Road – pollution
3
Exposure sampling periods
2
1
0
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