Urban Temperatures and Urban Heat Islandsweb5.arch.cuhk.edu.hk/asi2015/en/Sources/lecture... · Urban Temperatures and Urban Heat Islands • James Voogt • Associate Professor &

ASI 1 ‐ Prof. James Voogt ‐ Part 2

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Urban Temperatures and Urban Heat Islands

• James Voogt

• Associate Professor &President International Association for Urban Climate

• Western University, London ON Canada

Part 2: Urban Heat Islands

• Beginnings – heat island types

• Conceptual basis – Lowry approach.

• Physical basis for UHIs: Relating UHIs to temperature changes and the energy balance

• UHIs in the future


2

3

Boundary Layer Heat Island (BLUHI)

Heat Island Types

Canopy Layer Heat Island (CLUHI)

Surface Heat Island (SUHI)Modified after Oke (1997)

4

SURFACE(SUHI)

‘True’ 3-DT°

- complete surface

Bird’s-eye 2-D T°- aircraft,

satellite

Zero-plane T°- model output

Ground T°- road

SUBSTRATE

Fixed- tower, sodar

Traverse- aircraft,

tetroon

Traverse- automobile

UBL(BLUHI)

UCL(CLUHI)

AIR

Standard- grass

Non-standard- roof, roadside

Fixed- screen

Types Sub-types

Depends on definition of ‘surface’

Depends on observation methods

After Oke (1995)

Urban Heat Islands


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Finding Urban Effects: Lowry Conceptual Model

Mi t x = Ci t x + Li t x + Hi t x

M - measured value of a weather element (e.g. temperature)C - background (“flat-plane”) climateL - departure from C due to topographyH - departure from C due to human effects (including urban

effects)

Subscripts:i - weather type t - time periodx - station location (see ‘Zones’ next slide)

Lowry (1977)

u – urban area

r – rural or natural area*

e – a rural or natural area surrounding theurban area that is influenced by urban effects

*rural areas are impacted by human activities such as agriculture; natural areas have none (i.e. Hitx = 0)

Consider temperature measurementsin these zones.

Defining Zones in the Lowry Model

Lowry (1977)

u

r

e

u e

r


4

Utility of the Lowry Framework

a) shows that measurements (e.g. temperature) in a city have contributions from multiple influences.

b) demonstrates that a reference location outside the urban area may not be fully free of human influences

c) provides a framework for experimental design and control related to identifying urban effects

A difficulty is that the framework has many assumptions that are difficult to fully satisfy

What is the Urban Effect?

• We want to isolate H for an urban site

Conceptually: Mi 0 x = Ci 0 x + Li 0 x + 0, soMi t x - Mi 0 x = Hi t x

But… pre-urban measurements are typically not available and C may change over time.

• An alternative: numerical model for the same site run without the urban area


5

Urban – “Rural” Differences

Mitu - Mite = (Citu - Cite)+ (Litu - Lite)+ (Hitu - Hite)

Is Mitu - Mite evidence of urban effects?

Issues:What is area e and can it be determined a priori ?Is area r a good representation of pre-urban conditions?Note that area e may be large from some climate variables

Lowry (1977)

?

The Lowry model and UHI assessment

• A UHI is an urban effect – the Lowry model provides a conceptual basis for identifying urban effects that is important to making UHI measurements (e.g. siting instruments)

• Lowry model is useful for many other urban effects and is applicable to modeling approaches as well as observations


6

Physical Basis for Heat Islands

• Urban heat islands are created by differences in urban and rural energy balances.

• Consider the physical basis of the main UHI types

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0 5 10 15 20 25 30

Traverse Distance (km)

5

10

15

20

25

30

35

Tr (癈

)

Agricultural Mixed Rural Warehouse

Fraser River

Residential

Comm./Res.

Lt. Indust. Downtown

Park

SN + 2 hrsSSSS + 4 hrsSS + 9 hrsNOAA-11

~6°

~8°

~9°

~7°

Land use areas with large amounts of visible impervious surfaces appear hot during the day (Warehouse, Lt industrial)

Large diurnal temperature range.

At night large and positive SUHI, maximum at end of night.

Surface Urban Heat Island

Traverse


7

Diurnal Evolution of Urban Surface Temperature

Basel Switzerland Sperrstrasse street canyon:

0700 Jul 11 2002 – 1100 Jul 12 2002

Surface Temperature Changes

• Temperature change can be related to the surface energy balance

• By day we have

• C is heat capacity of a very thin layer of depth z

∗


8

15

Surfaces - Troof >> Twalls > Tfloor > Tinterior SUHI - large positive: Ts urban > Ts rural

L, L

L K

K

L, L

L

L, L

QH

QH

QG

QH QGQHQG

QG

L

Troof

Twall 2Twall 1

Tfloor

Tinterior

RoofsSurfaces - unobstructed, dark, dry, insulated if winds weak, strong heating roofs very hot

CanyonsSurfaces - walls & floor part shaded, large area, dry, sheltered, good conduction good heat storage warm not hot walls & floor unless open & sunlit

SUHI: daytime surface heat exchanges

Oke et al. Urban Climates forthcoming, Cambridge University Press

L, LL, L

L

L, L

L L

Tinterior

Troof

Tfloor

Twall 1 Twall 2

QH

QG

QHQG QH QG

RoofsSurfaces - large sky view, dry, insulated if winds weak strong radiative cooling very cold

CanyonsSurfaces - dry, small sky view, near calm, good conduction weak radiative and convective drain, heat from storage and interior warm walls & floor

Surfaces - Tinterior > Twalls > Tfloor >> Troof SUHI - positive: Ts urban > Ts rural

SUHI: nighttime surface heat exchanges

Oke et al. Urban Climates forthcoming, Cambridge University Press


9

Urban & Rural Surface Temperature Changes

2 K0

SUHI(K)

SR SS

SN

0

T/t (K h

‐1)

Time of day

Roof

Rural

Canyon

Urban

H/W ≈ 1

Night time SUHI: Important controls

• SVF s: sky view factor

• : thermal admittance

• Largest SUHI occur with

small SVF, small rural , large urban

Oke et al. (1991)

Δμu‐r


10

SUHI Temporal Variability

Imhoff et al. (2010)

F – Forest, G – Grassland, D – Desert, M – Mediterranean Forest

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Canopy Layer Heat Islands

• Canopy Layer: UHI is a maximum at night, under calm and clear conditions (max 12°C, annual average 1-2°C). May be small or even negative during the day.

Data from Oke in Aguado and Burt (2004)

Oke (1982)


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21

• Unlike SUHI afternoon CLUHI is negative

• By sunset it is growing fast

• Maximum at end of night

• Rural has cooled 16° but city core by only 6°.

• Clearly CLUHI is due to thermal inertia - difficulty in warming and cooling

0 5 10 15 20 25 30


5

10

15

20

25

Ta

ir (癈

)

Agricultural

TunnelMixed rural

Warehouse

Residential

Comm./Res.

Lt. Industrial

Downtown

Park

SN + 2 hrsSSSS + 4 hrsSS + 9 hrs

~-1°

~5°

~9°

~10°

Canopy Layer UHI: Spatial Structure

Vancouver BC Canada Clear August (summer) day.

Voogt and Oke (unpublished)

Differences in urban & rural cooling underly the CLUHI

Sunset Sunrise

12 18 24 06 12TIME (h)

Hea

tin

g/C

oo

ling

R

ate

Air

Tem

per

atu

re

2°C

Rural

Urban(a)

TU‐R

Oke (1982, 2011)

Important differences in the early morning and near sunset


12

CLUHI temporal intensity

Sunset Sunrise

12 18 24 06 12TIME (h)

HE

AT

IS

LA

ND

IN

TE

NS

ITY

AIR

TE

MP

ER

AT

UR

E

2°C Rural

Urban

Tu-r = CLUHI

(a)

(b)

Oke (1982, 2011)

Temperature Change in a Layer1 ∗ 1

∗

Simplified situation: large, flat, homogeneous surfaceNo net horizontal transfers.No latent heat release.By day, when atmosphere well mixed, divQ*z ≈ 0

Divergence(div)

Convergence= ‐div

Oke (1987)


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Temperature Change in a Volume

1 ∗

e.g. Stull (1988; 2015)

for both radiation and sensible heat flux consider the volumetric divergence

Air - Ta roof > Ta canyon CLUHI - small: Ta urban ~ Ta rural

L, L

L K

K

L, L

L

L, L

QH

QH

QG

QH QGQHQG

QG

L

Troof

Twall 2Twall 1

Tfloor

Tinterior

RoofsAir - contact & convective cooling hot layer strong convection that feeds BLUHI

CanyonsAir - warm surfaces heat air but not more than rural (less evaporation more storage) thus small or negative CLUHI unless sunlit & exposed

CLUHI: daytime heat exchanges

Oke et al. Urban Climates; forthcoming, Cambridge University Press

Morning: divQHV in urban canyon is weaker than for rural


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L, LL, L

L

L, L

L L

Tinterior

Troof

Tfloor

Twall 1 Twall 2

QH

QG

QHQG QH QG

RoofsAir - contact & convective cooling cold layer, occasional katabatic slumps into canyons

CanyonsAir - warm surfaces convect & radiate to air & other surfaces hence slow cooling CLUHI, & weak plumes feed BLUHI

Air - Ta roof < Ta canyon CLUHI - large, positive: Ta urban > Ta rural

CLUHI: heat exchanges at night

Oke et al. Urban Climates; forthcoming, Cambridge University Press

Volume Processes: Night – Clear and Calm

Urban

• Weak ‐divQ*y, radiation from canyon walls; stronger divQ*z (but not as strong as in rural area due to sky view factor obstruction)

• Weak –divQHy: heat from canyon walls

• Net result: radiative cooling that is weaker than for the rural case

Rural

• Strong radiative divergence div Q*z under stable night time conditions

• Weak sensible heat flux convergence ‐divQHz from above

• Net result, strong radiative cooling


15

H/WSky view factor, s Canyon aspect ratio, H/W

Max

imu

m U

rban

Hea

t Is

lan

d (

K)

Oke (1981), and H/W Oke (1987)

Maximum UHI vs Surface Controls

Seasonal Variation of Rural Soil Thermal Admittance

Runnalls and Oke (2000)UHIUCL Pot = UHIUCL Obs / [u -½ (1 - kn2)]


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31

0.1 0.3 0.5 0.7 0.9TIME OF DAY

2

4

6

8

10

MO

NT

H

0.6

0.8

0.9

Temporal Evolution of Canopy Layer Heat Island

Note:• Daily pattern• Seasonal pattern with daylength• Seasonal effects of prevailing weather and rural surface conditions

Plotted by T.R. Oke with data from Klysik and Fortuniak

Łódź, Poland1997-1999

When is the canopy layer heat island best developed?

Normalized time of day

0 0.5 1 1.5 2

Sun Rise

Sun Set

Nor

mal

ized

hea

t isl

and

inte

nsity

1

0.8

0.6

0.4

0.2

0

Runnalls and Oke (2000)

Heat island intensities for Vancouver from 1991-1994


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Weather Controls on the CLUHI

Cloud cover relationn = fraction of cloud cover k = coefficient based on cloud typek larger for thick low clouds

Combined Effects: “Weather Factor w” w = u

‐0.5 (1 – kn2)

w how much will UHI be reduced from its maximum potential value; 0 w 1.

Wind speed: Tu-r = u-0.5

calm windyu

CL

UH

I

overcast clear

CL

UH

I

(Runnalls and Oke 2000)

Cloud CoverTu‐r = Tu‐r (1‐ k n2)

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Surface UHI and 3-D City structure

Voogt and Oke (2003)0 5 10 15 20 25 30


0

5

10

15

20

25

30

35

40

Tem

per

atu

re (癈

)

TairTsfc (3-D corr)

AgriculturalMixed RuralWarehouseResidential

Res./Comm.Lt. Indust.DowntownPark

water

Night: 0530 LDT; Sunset + 9 hrs

Day: 1530 LDT; Solar Noon + 2 hrs

Tsfc (nadir)


18

35

Modified after Oke (1997)

Boundary Layer Heat Island

UBL Heat Islands

• Increased absorption of K by air pollutants.

• Anthropogenic Heat (QF) ‐ heat production by space heating, particularly stacks.

• Increased QH from below (heat flux from roof tops and canopy layer).

• Increased QH from above (increased roughness leads to increased turbulent entrainment)

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Oke (1982)

Boundary Layer Urban Heat Islands

Day

Night


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Control of Urban Form on the Heat IslandUrban FormCharacteristic

Impact on Heat Island

Surface geometry Low SVF promotes high night time CLUHI. Shading reducesdaytime SUHI and CLUHI.

Surface reflectance High reflectance reduces all daytime HI, with some effectscontinuing to the nighttime HI

Surface thermalproperties

Materials that are good “storers” of heat will contribute to nighttime CLUHI, BLUHI and potentially to negative daytime coolislands. Some materials that are well insulated (e.g. roofs)may contribute to daytime SUHI and BLUHI due to their hightemperatures.

Greenspace Reduces all daytime heat island and possibly night time heatisland depending on the exact nature of the vegetation used(e.g. trees vs grass).

Irrigated areas Strong reduction of daytime SUHI, CLUHI with some transferto BLUHI

Based on Oke (1982)

Heatwaves and Cities

The urban heat island can magnify heat wave effects

Russian Heat Wave: 70,000 deathsEuropean Heat Wave: 55,000 deaths

Heat is the deadliest of weather hazards (US Data)


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Future Changes to UHI

• With large scale climate change, CLUHIs both increase and decrease

McCarthy et al. (2010)

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Factors Affecting Heat Islands

Oke (pers. comm.)

UHI

Synoptic Weather(Limits UHI)

• wind & cloud•stability

City Form • materials (fabric) • structure• cover

City Function • energy use• water use• pollution

City Size• fetch distance•density of use

Geographic Location • climate• topography• rural surrounds

Time• day

• season

Mitigation Measures

“manageable”


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Summary

• Lowry conceptual model is useful for thinking about how to measure and interpret urban heat islands and all urban measurements.

• Urban actions to address heat islands should recognize the different types and their specific processes.

• Physical basis of the urban heat island is rooted in energy balance differences that differentially affect urban and rural heating and cooling rates at the surface and in the UCL or UBL air volume.

• Cities of the future are likely to be hotter but the UHI may not necessarily be larger depending on how rural cooling rates are affected.

End, Thank you

James VoogtDepartment of Geography

Western University, London ON Canada N6A 5C2

Tel: 519-661-2111 x85018 Email: [email protected]:

http://www.geography.uwo.ca/people/faculty/voogt_james.html

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Urban Temperatures and Urban Heat Islandsweb5.arch.cuhk.edu.hk/asi2015/en/Sources/lecture... · Urban Temperatures and Urban Heat Islands • James Voogt • Associate Professor &