Journal of Heat Island Institute International Vol. 12-2 (2017)

Academic Article

Analysis of wind and radiant environment in street canyons for
production of urban climate maps at district scale
1

Hideki Takebayashi*

1

Yuki Kiyama*

1

Naoki Yamamoto**1 Department of Architecture, Graduate School of Engineering, Kobe University
Corresponding author: Hideki TAKEBAYASHI, thideki@kobe-u.ac.jp
ABSTRACT
Relationships between the characteristics of buildings and urban blocks and wind and radiant environment instreet canyons are analyzed in order to produce urban climate maps at district scale. From the analysis of theradiation environment, the priority of adopting mitigation strategies for heat islands is low in areas placed at H/3
from the southern building wall and in areas placed at 2H/3 from the eastern and western building walls that arecharacterized by a gradient of about H/4 from the southern tip of the building. From the analysis of the windenvironment, a high weak wind risk area is defined in the three following cases: (1) the road width is 0 to 5 m; (2) aroad parallel to the main wind direction has width between 5 m and 15 m, or a road perpendicular to the main winddirection has width between 5 m and 10 m, and the building height is less than 30 m; and (3) a road perpendicular
to the main wind direction has width between 10 m and 15 m and building height is less than 40 m.
Key Words : Radiant environment, Wind environment, Street canyon, District scale

1. Introduction
In order to mitigate the negative impact of urban heat islands,
different strategies have been developed(1), such as solarNorthradiation shade, urban ventilation and mist spray, among others.
The appropriate strategy should be applied depending on thecharacteristics of each location. It follows that the urban climate
map is an effective tool for the identification of places that needintervention and, at the same time, for the evaluation of which

Fig.1 Simple building model with height H, east–west width X

adaptation technique should be applied at each location. Many ofand north–south width Y

the existing studies analyze urban climate maps at urban scale
and are focused on air temperature and wind distribution in the

Boundary at
20 MJ/m2

entire urban area(2), (3). Differently, in this study, the analyzed
area is at a district scale, and the focus is on the radiation andwind effects on pedestrians. Spatial distribution of air

Northtemperature and humidity are little for human thermal
environment in a street canyon. Effects on wind and radiantSimplebuilding
model
H: Height

environment due to building and urban block characteristics areanalyzed in order to produce reliable urban climate maps at(MJ/m2)

district scale, by using GIS building data instead of detailed
calculation.
Fig.2 Distribution of daily integrated solar radiation around the
2. Analysis of the radiation environment

simple building model (The white, dashed line is marking theboundary at 20 MJ/m2, which correspond to the 80 % of theDistribution of the daily integrated solar radiation around a

maximum daily integrated solar radiation)- 78 -

simple building model is calculated based on the orbit of the Sun.

3H/8

(W/m2)
1200

y (m)Calculation method of the daily integrated solar radiation is

H/3

1000

shown in the literature by the authors(4). A simple building model

H/4

800

with height H, east to west width X, north to south width Y, is

H/8

shown in figure 1. Weather values are taken for a sunny, summer

0

About 6 MJ/m2= 20 % of daily
integrated
solar radiation

600
400

0

4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00

16:00

15:00

14:00

13:00

11:00

day (August 5, 2007). The site in analysis is the city of Osaka.

12:00

9:00

10:00

8:00

200

Fig.3 On the left panel, the distance of the shadow from theAs visible in figure 2, the boundary (white dashed line) marking2

the 20 MJ/(m day) (which corresponds to the 80 % of thesouthern building wall on an east–west road, on the right panel,maximum daily integrated solar radiation) is located at about

the solar radiation in a typical summer day

H/3 from the southern building wall and at about 2H/3 from theeastern and western walls with a gradient of about H/4 from theSolar radiation is incident

southern tip of the building.
About
H/3
45°
H/3

2.1 Analysis of the east–west road caseLowerthan H/3

H/3
H/3

13:00

11:00

The distance of the shadow from the southern building wall on

X < 2H/3

X = 2H/3

X > 2H/3

an east–west road and the solar radiation in a typical summer

Fig.4 The distance of the shadow from the southern building

day are shown in figure 3. The distance y [m] of the shadow

wall on an east–west road at 11:00 and 13:00 (From left to right,

from the southern building wall is calculated using equation (1).

the value of X, corresponding to the east–west width of they = H * sinA * cotβ

building, is >2H/3, 2H/3 and <2H/3 respectively)(1)owhere H [m] is the building height, A [ ] is the angle between the(W/m2)
1200

x (m)
H

A is calculated using equation (2), where α [ ] corresponds to the
o

3H/4

約2H/3

1000

2H/3

2H/3

solar azimuth.

800

H/2

A = α − 90 (90 < α < 180),

400

H/6
0

(2)

200

9:00
15
9:30
45
10:00
15
10:30
45
11:00
15
11:30
45
12:00
15
12:30
45
13:00
15
13:30
45
14:00
15
14:30
45
15:00

270 − α (180 < α < 270)

From 11:00 to 13:00, the maximum distance of the shadow is

About 6.5 MJ/m2= more than 20 %of daily integrated
solar radiation

600

H/3

0

4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00

Sun and the southern building wall, and β [o] is the solar altitude.

approximately H/3 (left panel of figure 3) and the corresponding

Fig.5 On the left panel, the distance of the shadow from thesolar radiation is about 20% of the daily integral value (rightwestern building wall on a north–south road, on the right panel,

panel of figure 3). It follows that the need for the adoption of

the solar radiation in typical summer day

mitigation measures for the heat island is low in areas placed at a
distance of H/3 from the southern building wall that do noty’ (m)
H/2

receive solar radiation from 11:00 to 13:00.
3H/8

It is observed that the solar radiation hits the road surface
H/4

the solar azimuth at 11:00 and 13:00 is about 45o. The

H/8

corresponding distance of the shadow is H/3 from the southern0building wall and it occurs when the east–west width of the

H/4

12:00
12:30
13:00
13:30
14:00
14:30
15:00
16:00

passing between the southern buildings. As shown in figure 4,

building, X, is wider than 2H/3. Conversely, when the width X isFig.6 Distance of the shadow from the southern tip of thenarrower than 2H/3, the shadow does not reach the distance ofbuilding

H/3 from the southern building wall.
About 20° Solar azimuth at 19:00

2.2 Analysis of the north–south road case

H/4
Y

The distance of the shadow from the eastern and western

Y

H/4
H/4

H/4

building walls on a north–south road and the solar radiation in a
2H/3

typical summer day are shown in figure 5. The distance x [m] of

2H/3

Y

Lower than H/4
Lower than 2H/3

the shadow from the eastern and western building walls is

Fig.7 The distance of the shadow from the western building wall

calculated using equation (3).

on a north–south road at 19:00 (From left to right, the value of Y,

x = H * cosB * cotβ

corresponding to the north–south width of the building, is >H/2,(3)

where B is calculated using equation (4).

H/2 and <H/2 respectively)- 79 -

B = α (α < 90), α − 90 (90 < α < 180),
270 − α (180 < α < 270), 360 − α (270 < α)

between the two methods are shown in figure 15. Even for thisoblique case, estimations of low/high priority areas obtained(4)

The distance of the shadow is approximately 2H/3 at 14:30 andwith the two methods compare well. Errors at the corners of thethe corresponding solar radiation is about 20% of daily integral

buildings, however, appear to be more frequent than in thevalue from then to sunset. As shown in figure 6, at this time, thenorth–south and east–west road cases.

distance of the shadow from the southern tip of the building isabout H/4. This is calculated using equation (1); however, by
replacing A with B. It follows that the need for the adoption of
mitigation strategies for the heat island is low in areas placed at
a distance of 2H/3 from the eastern and western building wallsthat are characterized by a gradient of about H/4 from southernNorth

Low priority
High priority

tip of the building and do not receive solar radiation from 14:30
to sunset.
As shown in figure 7, the solar azimuth at 19:00 is about 20o.

Fig.8 Priority of adopting a mitigation strategy around a simple

Then, the distance of the shadow reaches 2H/3 from the westernbuilding model with an orientation of 45o with respect to thebuilding wall when the north–south width of the building, Y, iswest direction

wider than H/2. Conversely, when the width Y is narrower than
H/2, the shadow does not reach a distance of 2H/3 from the

0.20H

0.13H

0.10H

western building wall.
2.3 Analysis of the oblique road case

θ=10°

θ=20°

0.06H

θ=30°
North

The priority of adopting mitigation strategies around a simple
building model oriented at 45o with respect to the west direction

H/4
2H/3

θ=40°

is shown in figure 8. The priority of adopting mitigation

Low priority
High priority

θ=45°

strategies for the heat island in analysis is low in areas placed at

Fig.9 Priority of adopting a mitigation strategy around a simple

a distance of 2H/3 from any of the eastern and western walls ofbuilding model (From left to right, top to bottom, the building

the building, with a gradient of about H/4 toward the northernorientation presents an increasing angle, θ, with respect to theside. As shown in figure 9, the low priority area in the northernwest direction)side of the building decreases in size as the angle θ between thebuilding and the west direction approaches the 45o. The angle of

5H/12

orientation of the building with respect to the west and the

H/3

distance of the low priority area from the southern building wall

H/9

is shown in figure 10.

H/6
H/12
0

2.4 Validation of results obtained using buildingheights only

0

10

20
θ ( o)

30

R2=0.9440

Based on the previous analysis, the priority of adopting

Fig.10 Distance of the low priority area from the southern

mitigation strategies for the studied heat island is estimatedbuilding wall according to the orientation of the building

using the distance of the shadow from the building and building(expressed using the angle θ)height. Results obtained with this method are verified comparing
them to results obtained using calculations. The two methods areNorthapplied to the north–south road case (figure 11), and to theeast–west road case (figure 12). The differences between the two

Lowpriority

methods for each case are shown in figure 13 (north–south roadLowpriorityHigh priority
High priority(b) Results using(a) Results using
calculations
building heights only

case in the left panel, east–west road case in the right panel). As
shown in figure 13, low/high priority areas are estimated with

Building
storeyssufficient accuracy both in the north–south road case and in theFig.11 Low/high priority areas for the north–south road case as

east–west road case. Errors occur only at the corners of theobtained with the two different methods: (a) results obtainedbuildings. The comparison between the two methods for theusing building heights only, (b) results obtained using

oblique road is presented in figure 14. Again, the differences

calculations- 80 -

Table 1 Model parameters for estimating wind distribution

North

High priority

High priority

Low priority

Low priority

Turbulence model
Advection term
Inflow boundary

Standard k-e model
Up-wind difference schemePower-law, 3.2m/s at 54m
high, power: 0.25
Zero gradient condition
Outflow boundaryUp, side boundary
Free-slip condition
Wall, ground surface Generalized log-law
Grid resolution
10 m (x), 10 m (y), 1 m (z)in the target area

Building
storeys(b) Results using(a) Results using
calculations
building heights only

Fig.12 Low/high priority areas for the east–west road case as
obtained with the two different methods: (a) results obtainedusing building heights only, (b) results obtained using
calculations
cannot findHigh priority
by buildingheights only

match
match

North

match

false estimate
High priority
by buildingheights only

Building
storeysmatch

Fig.13 Left panel: difference in the estimation of the priority
3.2m/sareas between the two methods for the north–south road case,right panel: same as left panel for the east–west road caseLowpriority

Low priority
North

High priority(a) Results using building heights only

High priority

0.0

Building
storeys(b) Results using calculations

Wind(m/s)
velocityWind velocity
at(m/s)2 m height

5.0

Fig.16 Distribution of wind velocity at 2 m height in Osaka cityfrom Takebayashi et al.(5)Fig.14 Low/high priority areas for the oblique road case as
obtained with the two different methods: (a) results obtained
1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

cannot findHigh priority
by buildingheights only

0.2

Road width (m)

0.0

15-20
20-25
25-30
30-35
35-40
40-45
45-50
50-55
55-60
60-65
65-

15-20
20-25
25-30
30-35
35-40
40-45
45-50
50-55
55-60
60-65
65-

0.0

5-10
10-15

false estimate
High priority
by buildingheights only

5-10
10-15

calculations

Wind velocity ratio to upper windWind velocity ratio to upper windusing building heights only, (b) results obtained using(Error range means standard deviation)

North

Road width (m)Fig.17 Relationship between road width and ratio of wind
Building
storeysvelocity at 2 m to wind velocity at 42 m (left panel: for a roadparallel to the main wind direction, right panel: for a roadFig.15 Difference in the estimation of the priority areas between

perpendicular to the main wind direction)

the two methods for the oblique road case
Table 2 Weak wind risk estimations for different road widths
3. Analysis of the wind environment
Road
width

Wind velocity distribution is estimated using a numerical modelfor the whole Osaka City (223 km2), which is divided into 50

High weak wind risktarget area. The parameters of the model are shown in table 1.
Middle weak wind risk

Individual building shape data has been provided by the

Low weak wind risk

municipal office of the city of Osaka. Trees, signs, cars, humanbodies, etc. are not reproduced. The resulting distribution ofwind velocity at 2 m height in Osaka city is shown in figure16(5).
The relationship between road width and the ratio of wind

- 81 -

velocity at 2 m to wind velocity at 42 m is shown in figure 17.

(m)

3.2m/s

According to the wind environment evaluation scale proposed(6)

by Murakami et al. , the wind is considered discomforting due
to small wind velocity when the latter is less than 0.175 [m/s].
Intersection

0

3.1 Weak wind risk classification based on road width
The probability of occurrence of discomforting wind due toFig.18 Example of road width (left) and wind velocity at 2 msmall wind velocity is shown in table 2 for different road widths.

height (right)

When the risk of occurrence of discomforting wind is more than
70%, the corresponding area is defined as a high weak wind risk
area.
The classification of weak wind risk based on road width is thenWeak
Not weakIntersection

compared to results obtained using CFD. In figure 18, an
example of map of road width and wind velocity at 2 m height isdisplayed. The comparison between the corresponding weakwind maps estimated using the road width and CFD results isshown in figure 19. It is evident from figure 19 that the largest

Fig.19 Example of weak wind map estimated using road widthdifferences between the two evaluation methods occur in roads(left) and CFD results (right)

characterized by width from medium to large.
100%
80%
60%
40%
20%
0%

calculations to the weak wind risk estimations based on road

0-5m
5-10m
10-15m15-20m20-25m25-30m30-35m35-40m40-45m45-50m50-55m55-60mwidth is shown in figure 20. It is remarkable that in the case with

100%
80%
60%
40%
20%
0%

road width 5 to 15 m, weak wind risk ratio based on CFD islarger than the one based on road width. It is speculated that theRoad width
Higher than the results
based on road widthbuilding height may be responsible for this.

0-5m
5-10m
10-15m15-20m20-25m25-30m30-35m35-40m40-45m45-50m50-55m55-60mRatio of the weak wind risk estimations based on CFD

Match to the results
based on road width

Road width
Lower than the results
based on road widthFig.20 Ratio of the weak wind risk estimations based on CFD
3.2 Weak wind risk classification based on road width

calculations to the weak wind risk estimations based on roadand building height

width (left: road parallel to the main wind direction, right: road

Relationship between building height and wind velocity ratio isperpendicular to the main wind direction)

shown in figure 21. It is observed that in the low building heightboth in case of a road parallel to the main wind direction as wellas in the case of a road perpendicular to the main wind direction.
The probability ratio of occurrence of discomforting wind due tosmall wind velocity is shown in table 3 for a combined effect of

1.0

1.0

0.8

0.8

0.6

0.6

0.4

R2=0.2519

0.4

0.2
0.0

Wind velocity ratio

Wind velocity ratio

case, wind results are discomforting due to small wind velocity

R2=0.2343

0.2

both different road width and building height. If building height0 10 20 30 40 50 60 70 0.00 10 20 30 40 50 60 70 80 90 100 110
Building height (m)Building height (m)is less than 30 m, high weak wind risk areas are defined whenFig.21 Relationship between building height and wind velocity

the road width between 5 m and 15 m in roads parallel to theratio (left: road parallel to the main wind direction, right: roadmain wind direction, and between 5 m to 10 m in theperpendicular to the main wind direction)

perpendicular case. If building height is less than 40 m, highweak wind risk areas are defined when the road width isTable 3 Weak wind risk estimations based on road width andbetween 10 m to 15 m in roads perpendicular to the main windbuilding height

direction. The ratio of the weak wind risk estimations based on
Road
width

CFD calculations to the weak wind risk estimations based onroad width and building height is shown in figure 22. Whenbuilding height is less than 30 mbuilding height is less than 40 m
High weak wind risk

comparing figure 20 and figure 22, it emerges that considering

Middle weak wind risk

the two effects of road width and building height instead ofLow weak wind risk

considering road width has only improved the matching ratefrom 48% in figure 20 to 64% in figure 22.

- 82 -

Road width
Higher than the results
based on road width

References

100%
80%
60%
40%
20%
0%

(1) Ministry of Environment of Japan, Guidelines for measuresagainst heat in the town (2016)
0-5m
5-10m
10-15m15-20m20-25m25-30m30-35m35-40m40-45m45-50m50-55m55-60m0-5m
5-10m
10-15m15-20m20-25m25-30m30-35m35-40m40-45m45-50m50-55m55-60m

100%
80%
60%
40%
20%
0%

Match to the results
based on road width(2) J. Baumueller, U. Hoffmann, U. Reuter, Climate booklet for
urban development, Ministry of Economy Baden-Wuerttemberg,

Road width
Lower than the results
based on road width

Environmental Protection Development (1992)

Fig.22 Ratio of the weak wind risk estimations based on CFD(3) C. Ren, E. Y. Ng, L. Katzschner, Urban climatic map studies:calculations to the weak wind risk estimations based on roada review, International Journal of Climatology 31, 2213-2233

width and building height (left: road parallel to the main wind(2011)

direction, right: road perpendicular to the main wind direction)(4) H. Takebayashi, E. Ishii, M. Moriyama, A. Sakaki, S.
Nakajima, H. Ueda, Study to examine the potential for solar

4. Conclusionenergy utilization based on the relationship between urban
morphology and solar radiation gain on building rooftops andRelationships between the characteristics of buildings and urban

wall surfaces, Solar Energy 119, 362-369 (2015)

blocks and wind and radiant environment in street canyons are(5) H. Takebayashi, K. Oku, Study on the evaluation method ofanalyzed in order to produce urban climate maps at district scale.

wind environment in the street canyon for the preparation of

From the analysis of the radiation environment, the priority ofurban climate map, Journal of Heat Island Institute International

adopting mitigation strategies for heat islands is low in areas

9(2), 55-60 (2014)

placed at H/3 from the southern building wall that do not receive(6) S. Murakami, Y. Morikawa, Criteria for assessing

solar radiation from 11:00 to 13:00 and in areas placed at 2H/3

wind-induced discomfort considering temperature effect, Journal

from the eastern and western building walls that areof architecture, planning and environmental engineering, AIJ

characterized by a gradient of about H/4 from the southern tip of358, 9-17 (1985)

the building and do not receive solar radiation from 14:30 to(Received Apr. 27, 2017, Accepted Jul. 7, 2017)

sunset.
From the analysis of the wind environment, a high weak windrisk area is defined in the three following cases: (1) the roadwidth is 0 to 5 m; (2) a road parallel to the main wind direction
has width between 5 m and 15 m, or a road perpendicular to themain wind direction has width between 5 m and 10 m, and thebuilding height is less than 30 m; and (3) a road perpendicular to
the main wind direction has width between 10 m and 15 m andbuilding height is less than 40 m. It is observed that whencomparing results obtained using building characteristics against
results based on CFD computations, the matching rate between
the two methodologies is improved from 48 % to 64 % byconsidering not only the road width but also the building height.
It has to be stressed that while the results of the radiation
environment analysis are valid also for other Japanese cities
located at similar latitude, this is not true for the results relative
to the wind environment analysis. In fact, when interested inother cities, it is necessary to re-examine the wind environment
taking into consideration the specific shapes of the buildings thatare present in the city for the analysis.
Acknowledgments
This work has been supported by JSPS KAKENHI Grant
Number 16H04464.

- 83 -