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

Academic Article

Heat Island Mitigation Effects of Various Ground Cover Materials
in and around Yokohama Campus, Tokyo City University
Shoko Hashida*1, Hiroshi Omori2, AtsumasaYoshida3, Shinichi Kinoshita3

12

Graduate School of Frontier Sciences, The University of Tokyo
Department of Agricultural and Life Sciences, The University of Tokyo
3
Department of Mechanical Engineering, Osaka Prefecture University*Corresponding author: Shoko Hashida, shoko3.m@gmail.com
ABSTRACT
Yokohama Campus of Tokyo City University (TCU-YC) is located inland of Yokohama, yet it is endowedwith various kinds of greenery. People can enjoy nature even in this urban area. It has a water reservoir (a biotope,in this case, aquatic habitat) and we can find groups of trees and shrubs everywhere on the campus. The surface
temperature of various materials covering the ground was measured using infrared thermal images and the heatisland mitigation effect (HIME) was estimated in terms of the heat island potential (HIP) in summer. We found that,even though TCU-YC is located in an urban area, the HIME there is high thanks to the forest as well as the biotope.
Key Words: Urban surface, Ground cover material, Surface temperature, Heat island potential, Field measurement, Diurnalvariation, Heat island mitigation, Thermal design

should keep ‘Satoyama’(4).

1. Introduction

We find that TCU-YC is a good subject for this experiment andThe rate of temperature rise due to global warming is aboutthat it is desirable to improve it to the level of Satoyama.

0.7 ℃/100 years, while the rate due to heat island effect inEndangered species can be found on TCU-YC, and the campus

Japanese cities can be as high as ~ 2.2 ℃/100 years(1). Heathas both a sizable aquatic biotope and a forest. Our students are

island effect (HIE) has become serious in recent years due to thein charge of them; the faculty teach the their management andincreased thermal capacity of artificial materials on one hand,then students use them in educational activities for children.

and on the other hand, to decrease in land cover by greenery

In this research, we conducted a study of the thermal(vegetation) and wetlands. In the summer of 2016 in Japan,

environment, taking advantage of the aquatic biotope and forest

21,383 people were struck down by heat stroke and among them,at TCU-YC. We calculated thermal conditions quantitatively

59 people died(2). A nuclear accident associated with the greatusing Heat Island Potential (HIP)(5), which is a good index of theearthquake in Japan in 2011, made clear the importance ofheat island mitiagation effect (HIME). HIME is evaluated by theenergy saving. In order to suppress HIE and promote energy

decrease of sensible heat flux and is expressed by HIP. We usedsaving, cool roof materials were found effective and very

high-resolution thermography, and analyzed the data. By this(3)important in urban areas .

procedure, we unveiled differences in the HIME provided by

Symbiosis with nature is indispensable. In a well-planned

various kinds of plant cover (greenery) and artificial materials.

‘environmental city’, the quality of life must be guaranteed even

From the viewpoint of the HIME, we evaluated the present

after exhaustion of active cooling resources. The effect of solar

condition of thermal environment in TCU-YC by using HIP. On

shading by leaves and the transpiration of plants should be

the basis of the results, we estimated the future condition withconsidered. Moreover, forests and wetlands are important for thethe change of land coverage. The evaluation of HIME was(4)preservation of biodiversity . The ratio of green land cover (incarried out in the height of summer.

Tsuzuki-ku, Yokohama) decreased from 38.1% to 31.8% during
2. Measurement location and method

the 13 years from 2001 to 2014. For wild creatures to survive
there, it is essential to maintain at least the current balance. We- 54 -We measured the temperature of various surfaces on thethe relative humidity data. The results show Yokohama >university campus (such as a parking lot, RC, grass, shrubs, andMidori-ku Elementary School > TCU-YC, in Fig. 3. There wasthe biotope) and those in the surrounding residential area. Figureno rainfall during this period. Hot, clear days occurred before the1 shows the various land cover surfaces on TCU-YC. The letters

Temperature (deg.C)

star symbol ★ indicates a weather monitoring station. Theinfraredimages

werecapturedfromthefifthfloor(approximately 15 m high) of a building sandwiched between E
area and D area. The measurement was carried out every twohours until 10 a.m. on August 11 from noon of August 10, 2016.
It took approximately 15 min in each series of measurements.
We also performed continuous measurements of underground,ground,andwatertemperature(every10min).

TCU
Yokohama
Midori-kuSol. rad.

30

1500
1000
500

208/5

The

6789101112

Solar radiation (W/m2)

40

A–E indicate areas where we took infrared thermal images; the0Datemeasurement points were almost the same points as in thethermal images. The measurement sensors were water-proof

Fig. 2 Daily variation of air temperature and solar irradiation

thermisters. The depth of underground and water measurements

in Yokohama area

was less than 5 cm from the surface. When the ground

Absolute humidity (kg/kg)

temperature was measured, the sensor was fixed with aluminumtape. We also used meteorological data from a local
meteorological observatory close to the sea in Yokohama, anddata from an observatory in Midori-ku (in Yokohama City) ofthe residential area, excluding TCU-YC.

0.025
TCUYokohama
Midori-ku

0.020

0.015

0.010

8/5

6789101112DateFig. 3 Daily variation of absolute humidity in Yokohama area
40Temperarture (deg.C)

TCU-YC40m
Fig. 1 Measurement site and capture areas of infrared thermal
images (retrieved from Google Maps)

100
TCU August 10-11

Air temp / Relative humidity

Air temp.
RH

Relative humidity (%)

WeatherStation

90

3580
7030605025048121620048121640020JST

3. Measurement results and discussion(a) Air temperature and relative humidity

3.1 Meteorological data

Solar radiation (W/m )1500

university

campus,theYokohama

local

meteorological

TCU August 10-11

Solar radiation2

We compared the air temperature data from three sources: theSol. rad.

1000

observatory, and the elementary school in Midori-ku. Figure 2
shows these data for eight days. Data are included for several
days before the date of the thermal imaging. The result isremarkably clear: TCU-YC ＞ Midori-ku Elementary School＞ Yokohama. We note that the temperature difference is sizable

50000in the daytime but almost negligible in the evening. We also

4812162004812JST

calculated the absolute humidity using the air temperature and(b) Horizontal solar irradiation- 55 -16200

the average temperature of the chosen pixels became clear.

3
TCU August 10-11

Wind speed (m/s)

Wind speed

Figure 8 shows the diurnal variation of the surface temperatureWind speedof the various materials shown in Table 1. Except for the tiled
2roof, the surface temperatures of artificial materials were higher
all day than those of natural materials. The state that the artificial
surface temperatures were higher than the air temperature was1maintained even at night. The surface temperature of the tiled
roof became the highest in the daytime, but was about the same
0048121620048121620

as the natural surfaces by night. The water temperature was the0JST(c) Wind speedFig. 4 Meteorological data of measurement date in Yokohama
campus of Tokyo City University
measurement date. The meteorological data are shown in Fig. 4(a)–(c) for the capture measurement of infrared thermal image inTCU-YC. The hot clear days are typical of summer weather.
3.2 Thermography and ground temperature data comparing
artificial and natural ground cover
Figures 5–7 indicate thermal images captured at noon and justafter midnight in area A, B, and D. There is a biotope in thecentral part of area D, and the water is always drained. It was(a) Photographfound that the range of the surface temperature was remarkablein the daytime, but was not outstanding at night. The surface(℃)

temperatures of aritificial materials such as asphalt, concrete andbrick were much higher than those of the natural materials suchas plant and water surfaces in the daytime. The temperature of
the tiled roof of the single house was the highest, and exceeded
50 ℃ at noon. At night, the temperature difference became
small in comparison with the daytime, but the surface
temperatures of the artificial materials were clearly higher than
the natural surfaces. The cause appears to be the significant
thermal capacity of the artificial surface materials(6). Tiled roof
in area B and wood deck built over the biotope in area D became
the high temperature in the daytime, but, the temperature(b) Infrared thermal image at noon

difference with the nature surface was dissolved at night.
The surface materials which represented this campus werechosen among the thermal images as shown in Table 1. The(℃)

forest was composed of cherry and camphor tree. The species ofshrub was Boxwood (Buxus sempervirens). The species ofundergrowth was Ophiopogon japonicas. The latter was planted
around the roots of trees and had good exposure to sunlight. The
number of pixels used was decided according to the area of eachof the surface materials. The total pixels of the detector were 320×240. The spatial resolution was 1.78 mrad. The smallest
temperature indication was 0.06 ℃. The minimum number ofpixels was > 300, and the maximum number was < 5000
(according to the area). The standard deviation for each material(c) Infrared thermal image just after midnightat each measurement was small enough, and the significance ofFig. 5 Scenery and infrared thermal image in area A- 56 -(a) Photograph(a) Photograph

(℃)

(℃)

(b) Infrared thermal image at noon(b) Infrared thermal image at noon

(℃)

(℃)

(c) Infrared thermal image just after midnight(c) Infrared thermal image just after midnight

Fig. 6 Scenery and infrared thermal image in area B

Fig. 7 Scenery and infrared thermal image in area D

lowest (usually) all day long. Figure 9 shows the diurnal

analysis indicated a high HIME for the biotope. It contained

variation of the surface temperature of various plants. The

running water, so the temperature remained low. In the grassy

standard error of the average surface temperature from the image

area, the surface temperature in the daytime was notably high. Inwas nicely small thanks to the many pixels used for analysis ofcontrast we found that the temperataure was nicely suppressed in

the surface temperatures. This made it possible for us tothe other natural areas (shrubs, forest, and biotope).

distinguish the HIME between various plants using Welch’s

Figure 10 shows the diurnal variation of the underground,t-test. The level of significance was p < 0.1%. The difference inground, and water temperatures of various materials shown insurface temperature between trees and grass was significant. Our

Table 2, as measured by contact thermometers using thermister- 57 -Table 1 Surface materials and the areas from which infrared

Table 2 Surface materials and the areas in which temperaturethermal images were extracted

sensores were set up

Surface material

Area

Surface material

Area

Surface material

Area

Surface material

Area

Asphalt

C

ForestE

Asphalt

C

GrassA

BrickA

GrassA

BrickA

Shrub

B

RC roofB

Biotope

DWood deckDForestETile roof

B

Shrub

B

UndergrowthCRC roof

B

Biotope (sunny)D

Biotope (shady)

D

Surface temperature

August 10-11
Asphalt
BrickRC roof
Tile roof
ForestGrass
Biotope

605070Temperarture (deg.C)

Temperarture (deg.C)

704030201214161820220246810JST

Asphalt
Brick
Wooddeck
RC roof

Grass
Ground coverForestBiotope (sunny)
Biotope (shady)

504030200481216200481216200JST

Fig. 8 Diurnal variation of surface temperature of variousmaterials

Fig. 10 Diurnal variation of underground, ground, and watertemperature of various materials

50Temperarture (deg.C)

August 10-11Ground / water temperature
60Surface temperature

August 10-11GrassForest
Shrub (Baxus sempervirens)
Undergrowth (Ophiopogon japonicus)

40

4. Evaluation of heat island potential (HIP)HIP expresses the sensible heat budget added from all surfaces

30of the target area to the atmosphere and is indicated with atemperature unit. It is defined by the following equation.

2012141618

20

2202

46810JSTHIP All surfaces(Tsi  Ta )dsi(1)Fig. 9 Diurnal variation of surface temperature of various plants

Asensors. Here, Grass, Shrub, and Forest mean the soil

where Tsi, Ta, dsi and A indicate each constituent surface

temperature under them. Biotope (sunny) and (shady) mean thetemperature, air temperature, each constituent surface area andwater temperature in sunny and shady spots. The other data

total horizontally projected surface area, respectively. In otherindicates the surface temperature. The temperatures of shrubs,words. under the condition that the air temperature and the wind

forest, and biotope (shady) were the lowest throughout the day,speed are uniformly in the area, it means a surface integrated

and the daily changes were small. The temperature of the sunnydifference between the each constituent surface temperature andbiotope was slightly higher in the daytime owing to the influence

the air temperature as a horizontally projected plane inof the sunlight, but was the same as biotope (shady) at night. The

consideration of the uneven profile.

(6)

and grass became high in theThe current land use of the campus (TCU-YC) and thedaytime, but decreased rapidly at night. The time at which theyneighboring residential area is shown in the upper part of Fig. 11

showed their maximum values in the daytime was earlier thanand is based on aerial photographs. Three future development

for the other materials. The surface temperatures of asphalt,plans were assumed for the current TCU-YC.

brick, and the RC roof were high at night.

(1) Commercial area plan: RC building is enlarged, and all thetemperatures of the wood deck

remaining land is changed to an asphalt parking lot.

- 58 -Grass
16%

Biotope1%
RC roof Grass
13%
17%

20Asphalt
13%Tileroof
42%

Forest47%

UptownTCU current state

PresentPresent

1050Asphalt
53%

Asphalt
67%

Grass29%

Tile
roof
47%

RC roof33%

Forest53%

August 10-11TCU
Uptown15HIP (deg.C)

Brick9%Asphalt
42%1214161820

2202

46810JST
Fig. 12 Heat island potential (HIP) of uptown and campus

Biotope18%

Industrial area

New town development

Full planted

FutureFutureFuture(TCU-YC)
25

Fig. 11 Ratio of land coverage at present and simulated states

20(2) New town development plan: Housing land development is15HIP (deg.C)

conducted without greening.
(3) Full planted plan: Roof greening is introduced, and greening
or biotope is introduced on the grounds except for buildings
The land use of these plans is shown in the lower part of Fig. 11.
The heat island potential (HIP)(5) can be calculated from the airtemperature on the measurement date at TCU-YC, the surface

August 10-11
TCU -> Industrial area
TCU -> NewTownDevelopment
TCU -> Full planted1050

temperature obtained from the captured infrared thermal images,
and the land use ratio shown in Fig. 11.

-5Figure 12 shows the heat island potential (HIP) for the current
land use of the campus (TCU-YC) and the neighboring-10

121416

residential area. HIP values remained positive throughout the1820

220246810JST

day in both areas, because even on campus, artificial surfaces
make up about 40% of the total. It was found that thermalFig. 13 Simulated heat island potential (HIP) for different

emission from the surface toward the atmosphere occurs at night

development scenarios in the futureand that a fall in the air temperature at night is inhibited in thisdistrict. HIP values in the residential area were large in theIn urban areas where extreme development is proceeding,daytime in particular, because tiled roofs accounted for moreeducational facilities such as university campuses where creation

than 40% of the total surface area. This is expected to accelerateof large-scale green areas and water features is possible, arethe rise of the air temperature in the daytime, and to leads to theimportant for heat island effect mitigation. The heat island

substantial increase on extremely hot days (i.e., 35 ℃ or more).

mitigation effect was evaluated using the heat island potential

Figure 13 shows the HIP for the future development plans, andfor the current Yokohama Campus of Tokyo City University. The

these values mean the difference from the present conditions. It

current effect was verified, but was judged insufficient.

becomes clear that if the whole TCU-YC becomes residential

Promotion of roof top greening and addition of a large-scalearea, or if the RC buildings become commercial facilities

biotope are required. It was revealed that trees and shrubs weresurrounded by parking lots of asphalt, the HIP will be expected

more effective than other kinds of plants. A cool roof technology

to increase more than 10 ℃ in the daytime, and several degrees

is required by which to cool the city with water and greenery.

even at night, compared with the current TCU-YC. When theAcknowledgments

planting area and biotope were substantially increased for thecurrent TCU-YC, reduction of the HIP would occur at all times.

For the measurements on Yokohama campus of Tokyo City
5. Summary

University, the authors express their gratitude to Professor ShinjiYoshizaki, vice president of Tokyo City University, Professor- 59 -Kentaro Iijima, and Professor Akira Tanaka for their assistance.
This study was financially supported by the Ministry ofEducation, Culture, Sports, Science, and Technology under its
Grants-in-Aid for Scientific Research Planning (Grant Number
17H00802, Lead Researcher: A. Yoshida).
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Katsuo, T. Matsuura eds., Nipponno (2013), Nikkei-BP,
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