Journal of Heat Island Institute International Vol. 9-2 (2014)

Academic Article

How much does urban green cool town?
Hirofumi Sugawara*1, Shogo Shimizu*2, Shinsuke Hagiwara*3, Ken-ichi Narita*4, Takehiko Mikami*5*1 National Defense Academy of Japan*2 Tokyo Metropolitan Univ. (present affrication: Hokkaido Government)*3 National Museum of Nature Study*4 Nippon Institute of Technology*5Teikyo Univ.
Corresponding author: Hirofumi Sugawara, hiros@nda.ac.jp
ABSTRACTThe cooling phenomenon brought by a large urban green park in Tokyo to its surrounding town area wasevaluated. Micrometeorological measurements conducted during summer nights revealed the heat balance for thepark’s forest canopy. Highly reliable measurement runs, in which independent evaluations agreed within 20%,
showed that the cooling amount was 60 Wm-2, which is equal to the nocturnal heating in the surrounding town area.
Key Words: Green cool island, Urban park, Advection

Normal thermometers cannot capture this transition because

1. Introduction

the time scale of turbulent mixing is close to the time response
Urban greening is an effective mitigation factor for urban heatof the thermometer, which is a few minutes for commonly usedislands. Many previous studies (1), (2) clearly showed that ansensors. Here, the time scale of mixing can be estimated asurban green park is cooler than the surrounding town in

L2/K, where L is the length scale (here, the building height insummer, i.e., a cool green island. However, none of thesesurrounding areas, 10 m), and K is the turbulent diffusion

studies, including ours, adequately evaluated the coolingcoefficient (on the order of 1 m2s-1). Therefore, what weamount generated by a green park. These studies were based

measured in previous studies was the already heated air aboveon the measured air temperature distribution, although theroads. The balance between heating by roads and advective

temperature distribution is not enough to evaluate the cooling

cooling (cooling amount) determines the air temperature

amount because it is a result of heat exchange between the

distribution. The cooling amount produced by green parks

green park and the surrounding town. For example, picking up

cannot be evaluated using the thermally balanced temperature

the cold air advection from the green park to the town, thedistribution.
The distribution of surface air temperature is a basic piece

advected air mass is heated by the warmer materials on roads.

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Academic Article

Journal of Heat Island Institute International Vol. 9-2 (2014)of information in urban climate study, showing the locations ofcovariance method, where the correction for air density andwarmer or cooler air masses. In contrast, the cooling amount,axis rotation for the terrain following flow were taken. The

which should be expressed as heat flux in Wm-2, is a moreradiometer (Kipp & Zonen CNR-1) measured upward andimportant quantity that needs to be determined in order todownward radiation in the shortwave and longwave ranges.

evaluate the worth of green parks in cities.

We captured outflow from the park by installing two sonicIn this study, we evaluated the cooling amount produced

anemometers (GILL windsonic) and two thermometers (T&D

by a large park in Tokyo, focusing on summer nights. We

RTR-52A) at the north and south boundaries of the park. We

estimated the heat balance of the forest canopy in an urban

also installed thermometers at 24 points inside the park topark, which is also a recent topic in forest meteorology (3). The

acquire the temperature distribution. The thermometers were

measurement accuracy of advective heat flux is a big problem

equipped with a radiation shield, and were ventilated naturally.

in rural forests because the horizontal temperature gradient in

The measurement height was 2.5 m, and the time interval wasforests is small. However, in our case of an urban forest, theone minute.

large temperature gradient between park and town produced a

In the summer of 2010, we took temperature profile

large advective heat flux. We also checked for closure of themeasurements at three points (the center mast, northern, andheat balance in the forest canopy in order to present results

southern edges) in the park. Thermometers of the same type aswith high accuracy.

those used in 2009 were hung at eight levels, from 2 m above
the ground to the top of the canopy (16 m).

2. Measurements

1km

The study site was the urban park of the National Museum of
Nature Study, located in Shirogane, Tokyo (Fig. 1). The
surrounding town is a highly built-up commercial area. The
park has an area of 0.2 km2, and is covered by a deciduous
forest whose mean height is 14 m.
Measurements were taken during the summer of 2009.
The sensor setup is shown in Fig. 2. We installed instruments
for turbulence and radiation measurements at the top of a mast
(20 m from the ground, 6 m from the forest crown). The mast
was located at the center of the park. A sonic anemometer(Kaijo SAT-540) and H2O/CO2 analyzer (Licor LI-7500)

Figure 1. Study area. Solid circle in inset map indicates themeasured the vertical turbulent heat flux with the eddy

position of the park. Gray hatched area is the Densely- 12 -

Academic Article

Journal of Heat Island Institute International Vol. 9-2 (2014)

Inhabited District. Triangle indicates mast location; circles

andindicate locations of anemometer and temperature profile

Qg = c g ρ g dmeasurements.

δT g
δt

.

(4)

Here, c is the specific heat, ρ is the density, and the subscripts
a, t, s, and g indicate air mass, trunk, stem (including leaves),
and ground soil, respectively. Rw is the area ratio of the tree
trunks (6% based on the every-tree inventory), and ht is themean height of the trees (14 m), which is equal to the height of
the analysis volume. Vs is the volumetric ratio of the stems andleaves (here assumed to be 15% based on the value for a

Figure 2. Schematic illustration of instrument setup.

typical Japanese forest). The variable d is the length scale ofunderground temperature change, which can be evaluated as

3. Analytical procedure12

 λg d ≈t cg ρ g We evaluated the heat balance of the forest canopy layer in the,(5)park. The analysis volume covers the forest canopy, with thewhere λg is the heat conduction coefficient of soils, and t is thesoil surface at the forest floor as the lower boundary, and thetime scale. The value of d was 3 cm because the time scale oftree crown as the upper boundary. The heat balance of theflux evaluation in this study was 30 min. For Ta, Tw, and Tg, wevolume is shown in Fig. 3 and written as

used air temperature measured 2.5 m above the forest floor

Q H + Q E + R net + Q g + Q adv + Q a + Q w = 0 ,(1)because the forest was so dense that radiative equilibrium, i.e.,

where each term is positive, as heat flows from the inside toisothermal conditions could be assumed inside the canopy. The

the outside of the analysis volume. QH and QE are the sensible

other constants are listed in Table 1.

and latent heat fluxes, respectively, at the top of the canopy,

QH

QE

Rnet

and are acquired from the eddy covariance measurements atthe tower. Rnet is the net radiative flux, also measured at the top

Qadv

of the tower. Qa, Qw, and Qg represent heat storage by the
canopy air mass, vegetation body (trunks, stems, and leaves),

Qw

Qadv
Qa
Qw
Qg

and ground soil, respectively. These are evaluated as

Qa = ca ρa (1 − Rw )ht

Qw = ct ρt Rt ht

δTa
,
δt

δTt
δT
+ c s ρ sV s s ,δtδtFigure 3. Definition of heat flux components in the forest(2)

canopy. The rectangle indicates the volume analyzed. Blackand white arrows indicate airflow in the divergent park breeze(3)

situation and heat flux, respectively.

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Journal of Heat Island Institute International Vol. 9-2 (2014)

points in this park in 2014 revealed a spatial variation of 0.1
Qadv is the advective heat flux at the park boundary, which

ms-1 for U when the divergent park breeze occurs, which might

indicates the cooling amount generated by the park. We

cause a 50–100% difference in Qadv_di evaluation.

evaluated Qadv using two methods. One method used the

Only the horizontal wind speed is included explicitly inresidual of eq. (1). We call this the indirect evaluation, andthis evaluation, but vertical advective flow at the top of thedenote it Qadv_in. The other method is named the direct

canopy is implicitly included in eq. (6). The reason is shown

evaluation, Qadv_di, expressed ashere by deriving eq. (6). When the divergent flow occurs at the()

Qadv _ di = ca ρaha L Tpark − Ttown U / S,(6)park lateral edges, the downward flow at the canopy top

where ha is the depth of cold air flow at the park boundary, L iscompensates for the mass budget in the volume analyzed. The

the circumference of the park, and S is the area of the park.

air mass budget can be written as

WS = ULha

This expression of advection was applied for the divergent.

(7)

park breeze case, whose flow system is shown in Fig. 3. Tpark

The left and right hand sides of eq. (7) indicates the heat mass

and Ttown are the representative air temperatures of the park andflux at the top of canopy and flux at the lateral boundary,surrounding town, respectively. We used the average of 24

respectively. W is the vertical wind speed at the top of canopy.

points measured in the park for Tpark, where the range of spatial

The heat flux at the top and lateral surface of the volume

variation is about 1 K.

analyzed are

Ttown was measured at the residential district 2 km away

QT = ca ρ a STurbanW

from the park, which was far enough from the reach of park

QL = ca ρa ha LTparkU

and

(8)

, respectively.

(9)

breeze. Using the temperature measured at only one point for

The advection flux in the volume analyzed is the residual of

Ttown suffers from the influence of the spatial heterogeneity ofout-flux at the lateral surface (QL) and influx at the top (QT),
Q adv _ di =urban air temperature, and obtaining the spatial average is quite

Q L − QT

= c a ρ a ha LT park U − c a ρ a STurban W .

difficult. Intensive measurements using a car revealed that the
spatial variation of air temperature in downtown Tokyo is 0.3(10)

Substituting eq. (7) into eq. (10) derives eq. (6).

K at night (4). The temperature difference (Ttown - Tpark ) in the

Analysis was conducted for each run of 30 min.

case shown in the section 5 (September 10-11, 2009) is 3 K, so

Independent evaluations of Qadv (indirect and direct) were

the spatial variation might cause 10% difference in the Qadv_di

compared, and runs with good agreement were selected forevaluation.

further analysis.

U is the wind speed component perpendicular to the park
boundary, and was an average of measurements at the south
and north edges of the park in the analysis. The spatial
variation of U is also problematic. Wind measurements at eight- 14 -

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Journal of Heat Island Institute International Vol. 9-2 (2014)

each level over a two hour period. Figure 4 indicatesaccumulated cold air at the bottom after 0 JST, and theTable 1. List of constants. Values for wet clay were used for

decreased correlation coefficient. This means that the air masscgρg and λg.

at the bottom has different thermal characteristics from that at
ctρt

7.2 × 103 Jm-3K-1

the top, and the depth of the cold air mass can be determinedcgρg

6.0 × 103 Jm-3K-1

from the decrease of the correlation coefficient. Figure 5 showscsρs

7.2 × 103 Jm-3K-1

profiles of the correlation coefficients. After 0 JST, theλg

2.0 Wm-1K-1

correlation coefficient decreased below 14 m height. We
determined the depth of cold air at 15 m, considering theuncertainty due to the 2 m interval measurements. The depth of

4. Depth of cold air outflow

cold outflow evaluated here is very close to that at other points
For the analysis, we need the depth of cold outflow ha in eq.

in the park and also close to the mean tree height.

(6), which was evaluated as follows. The vertical profile of air

Measurements in other parks in Tokyo also showed that thetemperature in the park was measured in 2010. Figure 4 shows

depth of park outflow is close to the tree height (5). Therefore

the time variations. The correlation coefficient shown was

the value of ha is assumed to be constant (15 m) in thecalculated between the air temperature at the top (16 m) and at

following analysis.

Figure 4. Time variation of air temperature and its correlation coefficient for August 29–30, 2010. The correlation coefficient is- 15 -

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Journal of Heat Island Institute International Vol. 9-2 (2014)

calculated using air temperatures at 16 m and at each height over a two hour period.
canopy, and divergent wind appeared at the surface. At thesame time, air temperature at the northern and southern park
boundaries dropped rapidly, indicating cold air outflow. The
divergent wind system was probably caused by the gravity
flow of cold air from the park to the surrounding town. This
divergent wind was also found at other parks in previous
studies (1), (6), and is often called the divergent park breeze. In
this situation, the independently evaluated Qadv values were in
Figure 5. Vertical profile of correlation coefficients for August

good agreement, probably because of the stable wind flow

29–30, 2010.

system. Rnet was close to Qadv, indicating that the green park
generated cold air by radiative cooling, and the outflow
brought most of the generated cold engergy to the town.

5. Heat balance analysis

The cooling amount in this situation is 60 Wm-2. ThisWe select one night time series, and discuss the wind flow

cooling amount is nearly equal to the heating amount in thesystem and heat flux in detail. Figure 6 shows the timeurban area at night (55 ± 22 Wm-2) measured above thevariation of temperature, wind, and heat flux on September 10–

building canopy in downtown Tokyo (7), which means that the11, 2009. The weather was cloudy, but no rain event was

green park quantitatively cancels out urban heating by featuresdetected. After 23 JST, the wind became calm above thesuch as road surfaces and cars.

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Journal of Heat Island Institute International Vol. 9-2 (2014)

Time [JST]Figure 6. Time variation of air temperature and wind speed at the northern and southern park boundaries (upper three panels), and heat
flux (lowest panel) for September 10–11, 2009. QH: sensible heat flux, Rnet: net radiation flux, Qadv_in and Qadv_di: advective heat flux
by indirect and direct evaluation, respectively.

Seden). Int. J. Climatol. 18 (1998), pp.681-700

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urban park, Shinjyuku Gyoen, Tokyo, Geographical Rev. of

C.， Canepa， E.， Lindroth， A.，Montagnani， L.，Rebmann，Japan 77 (2004), pp.403-420

C.，Sedlak，P. & Van Gorsel E. Direct advection measurements
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Evidence from three different forests. Agric. and Forest

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