Climate regulating ecosystem services:
Introduction to urban microclimates
Dr Gina Cavan
April 2012
[email protected]
Outline
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Climate regulating ecosystem services
Fundamentals of urban climatology
Urban heat island effect
Relevant studies from the literature
Modelling impacts of urban microclimate
Monitoring the urban microclimate
Climate regulating services
House et al (2005: Ch 13)
House et al (2005: Ch 13)
Focus on climate regulating services
In particular:
• Today’s presentation will focus on climate regulating services
relating to temperature regulation; the water balance and
related impacts including run-off
• Precipitation and evapotranspiration are not discussed in any
detail
– Covered in detail by CLUVA hazards, risk and multi-risk assessment
course, Naples, 2011; available on CLUVA website at www.cluva.eu
• Atmospheric motion (wind parameters) not included
Urban climatology
• Branch of climatology concerned with interactions between
urban areas & atmosphere, impacts and processes
• Concept of scale is fundamental in understanding how urban
environment interacts with atmosphere
• Much focus on urban canopy energy budget modelling
Surface radiation balance
(Oke, 1987)
• Amount of reflected SW radiation depends on incident
radiation ( ) and albedo (α)
• Amount of reflected LW radiation depends on atmospheric
temperature & emissivity (in absence of clouds)
• Proportion of ( ) not reflected is absorbed
•
and
determine differences in radiation budget between
local surfaces (in particular α & T0)
Surface energy balance
(Oke, 1987)
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= turbulent transport of heat between surface & atmosphere
(sensible heat)
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= heat released or absorbed when water changing state (latent
heat; evaporation flux)
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= heat transferred from or to soil below the surface
(Flux = rate of flow of some quantity)
Surface energy balance in
urban areas
(Oke, 1987)
• QF energy flow directly controlled by human decisions;
depends on average energy use per person and population
density
• In some cities QF is greater than Q*
• Reduced evapotranspiration, resulting in energy into sensible
forms – QH and Qs
• Greater urban storage due to greater surface area for
absorption due to geometry, reduced latent heat, etc.
Radiative properties of natural
materials
(Oke, 1987)
Urban heat island effect
• Urbanisation alters thermal properties through:
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Changing albedo (reflectance of solar radiation)
Vegetation cover
Surface geometry (important for wind moderation)
Building mass, and
Anthropogenic heat emissions
Albedo
• Most influential property in the formation of UHI
• Albedo of vegetation much greater than urban structures
(US EPA, 1999)
Causes of urban heat islands
Characteristics of UHI #1
• Related to the size of a city – strong relationship
between population and maximum heat island
intensity (Oke, 1987)
• Strong correlation between geometry of street
canyons in city centres & maximum UHI intensity
• Surface heat islands show similar spatial and
temporal pattern to air temperature within urban
canopy later.
• Surface temperatures more strongly related to site
characteristics such as sky view factors
Characteristics of UHI #2
Average percentage differences between urban and rural areas
Annual
Cold season
Warm season
Solar radiation -22 %
-34%
-20%
Temperature +2
+3
+1
Humidity
-6
-2
-8
Rainfall
+14
+13
+15
Thunderstorms +15
+5
+30
(from Briggs and Smithson, 1997)
Urban heat
island effect &
population
(Roth, 2007)
Water balance
On land:
In urban areas:
• Water input into urban system greater due to F and I (ignoring
irrigation), there is no counterpart in rural areas
• Urban E and sub-surface storage,
, are less than rural areas
due to replacement of vegetation by impervious materials
with poor infiltration
• Therefore urban runoff, , is greater in urban areas.
(Oke, 1987)
Monitoring and modelling
evaporation
Measuring:
• Hydrological
• Water balance pans
• Atmometers
• Lysimeters
• Vegetation (porometer)
• Meteorological
• Eddy Correlation
• Bowen Ratio
Modelling:
CLUVA course on Hazard,
Risk & Multi-risk
assessment, Naples, 2011.
Module 3.2 Drought
• One-step model
• Penman-Monteith
• Two-step model
• Energy (Thornthwaite, Blaney-criddle)
(Source: Prof. Clive Agnew, UoM)
Studies on urban microclimate
relevant to African cities
• CLUVA D2.6: A database of international evidence on
ecosystem services of urban green infrastructure
• Evidence from relevant climate zones to African case studies
• >30 entries for climate regulating services
Dropbox: CLUVA WP2/T2.2/Database…
Urban modification of the surface
energy balance in Ouagadougou
UHI characteristics in tropical semiarid cities:
• Strong seasonal differences between wet and dry seasons
• Maximum intensity reached during dry season, though wet & dry
heat islands may exist
• Seasonal changes caused by seasonal variation in thermal
characteristics of rural surfaces
Results:
• Low thermal mass of sheet metal roof used in residential areas
reduces heat storage relative to concrete roofs in urban centre
• Unplanned peri-urban development may have lower impacts on local
climate than planned developments in the city
• If urbanisation leads to more paving, increasing height and density of
buildings will lead to higher night-time urban temperatures
(Offerle et al. 2005)
(Offerle et al. 2005)
(Offerle et al. 2005)
Urban greening
• Urban greenspace offers potential to help adapt cities for
climate change (Gill et al., 2007; Bowler et al. 2010).
• Urban greenspace can help to mitigate flooding:
– Reduces rate & volume of surface water runoff as rainfall is
intercepted by vegetation
• Urban greenspace can help to moderate microclimate:
– Cooling through evapotranspiration
– Storing & re-radiating less heat than built surfaces
– Direct shading
0%
1.2 Field crops
1.3 Mixed farming
2.2 Mixed forest
2.3 Riverine,…
2.4 Mangrove
2.5 Bushland
3.1 Mineral…
4.1 Parks
4.2 Stadium
4.3 Beach
4.4 Other open…
4.5 Sports ground
5.1 Major road…
5.3 Rail
5.4 Port
5.5 Bus stations
6.1 Energy…
6.2 Water tanks…
6.4 Cemeteries
7.1 Condominium
7.2 Villa & single…
7.3…
7.4 Mixed
7.5 Scattered…
8.1 Education &…
8.2 Medical
8.3 Religion
8.4 Institutional
9.1 Malls
9.2 Formal…
9.3 Open markets
9.4 Mixed formal…
10.1 Manufacturing
10.2 Offices
10.3 Storage &…
10.4 Garages
13.1 Marsh/swamp
14.1 Hotels
14.2 Entertainment
15.1 Military
Land cover types (%)
Modelling: potential application
of land cover analysis
100%
90%
Cultivated
crops
80%
Grasses
70%
Palm trees
60%
Small trees
50%
Large trees
40%
30%
Water
20%
Sand
10%
Bare ground/
soil
Modelling impacts:
The STAR Tools
http://www.ppgis.manchester.ac.uk/grabs/
Surface temperature tool #1
Surface temperature tool #2
Surface temperature tool #3
Presenting outputs:
Surface temperature
Runoff tool #2
Runoff tool #3
Presenting outputs:
Runoff
Monitoring urban microclimates
• Separate workshop/small group session to discuss
monitoring in case study locations (potentially on
Thursday afternoon)
• Including discussion of:
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Equipment
Site selection
Sampling techniques
Important considerations (e.g. calibration, correcting
physical environment effects)
References / further reading
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Bowler et al. (2010). Urban greening to cool towns and cities. A systematic review of the
evidence. Landscape and Urban Planning 97(3): 147-155.
Gill (2006). Climate change and urban greenspace. PhD thesis, University of Manchester.
House et al (2005). Chapter 13: Climate and air quality. In: Hassan et al. Ecosystems and
Human Well-being: Current State and Trends. Volume 1 Millennium Ecosystem Assessment.
Oke (1978). Boundary layer climates. Routledge.
Roth M. (2007). Review of urban climate research in (sub)tropical regions, Int. J. Climatol., 27,
1859-1873 .
Smith et al (2011). Climate regulating services. In: The UK National Ecosystem Assessment
Technical Report. UK National Ecosystem Assessment, UNEP-WCMC, Cambridge
US EPA (1999). Heat island, http://www.ghcc.msfc.nasa.gov/urban/urban_heat_island.html
Other resources:
• Case studies in international evidence database available on dropbox
• STAR tools www.ppgis.manchester.ac.uk/grabs