Exchange Processes at the Land Surface for a Range of Space and Time Scales
(Proceedings of (he Yokohama Symposium, July 1993). IAHS Publ. no. 212, 1993.
181
Heating of paved ground and its effects on the near
surface atmosphere
TAKASHI ASAEDA, VU THANH CA
Department
Japan
of Civil and Environmental
Engineering, Saitama University,
Saitama,
AKIO WAKE
Institute of Technology, Shimizu Corporation,
Tokyo, Japan,
Abstract The below-surface heat storage of pavement and bare soil
during summer days was measured and analyzed. The results showed
that the below-surface heat storage during the day was largest for the
asphalt pavement and smallest for the bare soil surface. Accordingly,
the temperature of the asphalt surface rose to more than 60 °C at its
maximum at noon and was about 5°C higher than the atmospheric
temperature even in the early morning. Consequently, the asphalt
pavement released an additional 150 W/m 2 in infrared radiation and 200
W/m 2 in turbulent sensible heat at its maximum to the atmosphere
compared with the bare soil surface. Even in the early morning, it still
released about 70 W/m 2 of heat to the atmosphere. Most of the infrared
radiation from the ground was absorbed within 200 m of the lower
atmosphere, affecting the air temperature near the ground.
NOTATION
Cj
D^v
Djv
DTa
q
g
K
Kx
/
L
t
T
TQ
z
X
Pi
px
pv
\p
heat capacity of the pavement material
matric head diffusivity of vapour in \p-T system
temperature diffusivity of vapour in \p-T system
transport coefficient for absorbed liquid flow
specific heat of liquid water
gravity acceleration
hydraulic conductivity
heat conductivity of the pavement material
thickness of the cover slab
latent heat of vaporization of water
time
temperature
reference temperature
downward vertical coordinate of the point
heat conductivity of soil
density of the pavement material
liquid water density
density of water vapour.
matric head
Takashi Asaeda et al.
182
6
8a
volumetric liquid water content
volumetric air content
INTRODUCTION
It is well established that during summer, in Greater Tokyo, the average air
temperature is about 3 to 5°C higher than that in the surrounding rural area. There is
no doubt that among other factors, the paved surfaces, which can absorb a large
amount of sun radiation during the day and release it to the atmosphere at other times,
play an important role in the formation of the heat island problem. In spite of its
obvious importance, the role of the paved ground on the heat island problem has not
been thoroughly studied.
When the temperature of the ground surface is much greater than the air
temperature, as in the case of the pavement, not only the turbulent sensible heat, but
also the net long-wave radiation contribute significantly to the heat exchange between
the surface and the atmosphere. Thus, the study of the absorption of the long-wave
radiation in the lower atmospheric layers is necessary for understanding the influence
of the paved surfaces on the urban atmospheric thermal environment.
To understand the role of the paved ground in the development of the heat island,
the heat fluxes at the ground surface must be well established. Also, the heating of the
atmosphere by turbulent transfer of the sensible heat between ground and air and the
absorption of infrared radiation from the ground by atmospheric substances must be
studied. This study addresses the partition of the net radiation at the ground surface
and the absorption of the infrared radiation from the surface by the lower atmosphere.
EXPERIMENT DATA
An experiment was carried out by the Environment and Hydraulic Laboratory, Saitama
University in 1990 and 1991, with extensive measurements performed on 23-25
August 1990, 8-9 and 25-27 August 1991. The experiment site is inside the Saitama
University's Campus, 35°52'N, 139°36'E.
The distribution of temperature under surfaces covered by asphalt, concrete,
rubble, sand and soil were measured together with meteorological data such as solar,
atmospheric infrared radiation, albedo of the surface, air temperature, wind velocity
and atmospheric humidity at every hour during the day and every two hours at night.
The water content of the soil at the surface and at 10 cm below the surface was
measured four times per day. More information about the experiment is given in
Asaeda et al (1991) and Asaeda & Wake (1992).
The measured temperature distributions under different surfaces at selected times
on 25 August 1990 are depicted in Fig. 1. The data show that at 5 a.m., before
sunrise, temperatures of all materials were low and decreased upwards. This suggests
that the heat stored during the previous day was being released. The heating period of
the pavement was from sunrise to about 1 p.m. during which the surface temperature
of the asphalt rose to more than 60°C, which was 15°C higher than that of the blacktop concrete and much higher than the surface temperature of other surfaces. In spite
of the small value of albedo of the soil surface throughout of the day, the temperature
Heating of paved ground and the near surface
atmosphere
183
temperature C
30
P.
50
70
10
20
a) 5 am
temperature C
30
g
50
70
10
20
c) 5 pm
Fig. 1 Temperature distribution in pavements (1990 observation), (a) 5 a.m., (b) 1
p.m., and (c) 5 p.m.
- , black-top c o n c r e t e ; ; normal
; asphalt;
colour concrete;
macadam layer;
, sand layer; and —
bare soil.
under the bare soil surface was always smaller than that under the other surfaces. This
indicates that a large portion of the net radiation to the soil surface was used for the
evaporation of water inside soil.
THE SURFACE HEAT BALANCE
From the experiment data, a numerical model was developed to simulate the nearsurface transfer of heat and moisture. The governing equations were the heat and mass
conservation equations. For the pavement, the thermal characteristics of concrete or
asphalt are assumed constant. Thus inside the pavement slab, 0 < z > /, the heat
Takashi Asaeda et al,
184
conservation equation is
dT
n
fl,C,
„ d2T
(1)
= A.,
1 l
dt
V
The equations of mass and heat conservation inside below the pavement, z > I, follow
Milly (1982) as
Pi
H
= mK + D.W
Pi d\p
+
dt
+
dt"
dT
Pv
Pi
(Dn+DTJVT\-
dpv
dB
C+Lea—-(p,W+pL)^= V[\VT+pl(LDiv
1
dT
+
p, dT
It
3K
~dz
Ua—-(p,W+PJL)-
gTDra)VJ/]-tY,,VT
(2)
#(4)
dt
(3)
In the case of an unpaved surface, the upper boundary conditions are the mass and
heat fluxes at the surface. For the paved surface, only the heat flux boundary condition
at the upper surface is needed. The mass flux is evaporation rate minus rainfall rate.
The total heat flux at the surface is the summation of gains by net solar radiation, net
infrared radiation, turbulent diffusion sensible heat and , in case of bare soil, the loss
due to the latent and sensible heat carried away by water vapor. In case of the paved
surfaces, the condition at the contacted surface is the continuation of temperature and
heat flux. The lower boundary condition is a constant temperature and matric head at
large depth. The initial conditions are the matric head and temperature, given as a
function of depth at the time of the beginning of computation.
A fully implicit, finite difference type numerical model was developed to compute
the mass and heat transfer inside the soil in case of bare soil (equations (2), (3)), heat
conduction in the covering slab (equation (1)), and heat and mass transfer inside the
underlying soil (equations (2), (3)) (Asaeda & Vu, 1992).
The results of calibration of the numerical model for fine weather days showed
that there is a good fit between computed and observed data, suggesting that the model
works well during fine weather.
Study of the change of vapor head and moisture in the soil underneath a paved
surface indicated that during the day, when soil temperature increased, water in the
soil was evaporated and a part of vapor was transported downwards. The evaporation
reduces the liquid water phase content in the soil and increases the vapor phase
content, thus reducing the heat conductivity of the soil. At night, when surface
temperature decreases, water vapor inside soil condensed and the upward transfer of
water vapor carried heat upwards, increasing release of stored heat by the surface. In
order to better understand the role of water vapor movement in subsurface temperature
distribution, a numerical experiment was performed by running the model with
different thicknesses of pavement slabs under the same atmospheric boundary
conditions. The computational results of below-surface temperature distribution
Healing of paved ground and the near surface
atmosphere
185
indicated that with an increase in the thickness of pavement slab, the heat island
problem becomes more significant.
Model and experimental data are used to evaluate the heat fluxes at the pavement
surface. The net radiation, turbulent diffusion sensible and latent heat and heat
conduction to deeper layers are depicted in Fig. 2. It is clear that for paved surfaces,
during the day, the heat conduction to deeper layers is much larger than for unpaved
surfaces.
It was estimated that the below-surface heat storage in one day is about 7.7xl06 J
for the asphalt surface, 6.5xl06 J for the concrete surface and 4.1xl06 J for the soil
surface. In the afternoon, the porous surfaces and soil surface cooled down rapidly.
On the other hand, due to the large heat storage, the temperature of the surface of the
pavement was always higher than the air temperature. Even at 5 a.m., the temperature
of the asphalt surface is about 5°C higher than the air temperature, and the surface
released about 70 W/m2 of heat to the atmosphere in the form of sensible heat and
long-wave radiation.
_
S
.
12
16
—
20
24
Time (hour)
(a) Net Radiation to the Surface
4
8
12
16
OJ
U '
1
1
4
3
•
1
12
16
.
20
Time
(b) Turbulent Sensible Plus Latent Heat
20
24
Time (hour)
(c) Heat Conduction into the Ground at the Surface
Fig. 2 Net radiation and heat fluxes from different surfaces.
concrete; — —•—• — • bare soil.
asphalt;
-T-
24
(
)
hour
Takashi Asaeda et al.
186
THE ABSORPTION OF THE LONG-WAVE RADIATION FROM SURFACE BY
THE ATMOSPHERE
From sunrise to about 4 p.m., the infrared radiation from the asphalt surface to the
atmosphere is about 30 to 90 W/m2 larger than that from the concrete black top, which
in turn is about 40 to 50 W/m2 larger than that from the normal concrete surface. Part
of this infrared radiation is absorbed by absorbers such as water vapor, carbon
dioxide, aerosol and ozone in the atmosphere affecting the air temperature.
A numerical model was developed to study the absorption of the long-wave
radiation by the absorbers (Asaeda et ai, 1992). Water vapor, carbon dioxide and
aerosol are considered as radiation absorbers while the effect of ozone is approximated
by attenuating the incident influx. The solar radiation was divided into the direct and
i
(
i
i
r
I
i
i
3000
\\
\
1000
-50
1
1
1
\\
2000
—concrete
w
b a r e soil
\
1000
\\
0
1
r-
—asphalt
0
1
1
w
a)
w—V
vA
\
w
w
w
2000
i
i
3000
w
1
0
50
0
50
integrated net absorption of infrared radiation
integrated net absorption of infrared radiation
W/m2
W/fn2
3000
2000
1000
0
-50
0
50
integrated net absorption of infrared radiation
W/fn 2
Fig. 3 Net absorption rates of infrared radiation in the lower atmosphere, (a) 8 a.m.,
(b) 2 p.m., and (c) 6 p.m.
.asphalt;
, concrete; and
bare soil.
Heating of paved ground and the near surface atmosphere
187
diffusive components. They were once again divided into absorption and non
absorption bands. The transmission function of the atmosphere was determined using
the pressure correlated optical path length. The infrared radiation was also divided into
wavelength intervals. In order to simplify the problem, a linear atmospheric
temperature distribution with height from surface was adopted (Asaeda et al., 1992,
Welch & Zdunkowski, 1976, Manabe & Môller, 1961) Fig. 3 depicts the integrated
values of the net absorption of the infrared radiation computed by the model at
selected times in a day. The upward infrared radiation from the surfaces is absorbed
quickly within less than 500 m above the surface. The absorption rate of the infrared
radiation from asphalt surface is about 65 W/m2 larger than that of the infrared
radiation from soil surface. The modelling results indicated that this is much larger
than the absorption of short-wave radiation above respective surfaces. This difference
shows the effect of the paved surface on the atmospheric temperature. The modelling
results also show that above 300 m from the surface, the atmospheric emission exceeds
the absorption of the radiation from the ground.
REFERENCES
Asaeda, T. & T.D. Vu (1993) 'Hie subsurface transfer of heat and moisture and its effects on the environment: a
numerical model. Boundary-Layer Mcteorol. (in press)
Asaeda, T., T.C. Vu, & M. Kitahara (1991) Pavement effect on the atmospheric thermal environment. Env. Sys.
Res. 19, 89-93.
Asaeda, T., A. Wake, & T.C. Vu (1992) Solar healing of pavement and its effect on lower atmosphere, (submitted
to Atmos. Env.)
Manabe, S. & F. Môller (1961) On the radiative equilibrium and heat balance of the atmosphere. Mon. Weath.
Rev., 89, 503-532.
Milly, P.CD. (1982) Moisture and heat transport in hyslerelic,' inhomogeneous porous media: a matric head-based
formulation and a numerical model. Water Rcsour. Res. 3. 489-498.
Welch, R. & W.G. Zdunkowski (1976) A Radiation model of the polluted atmospheric boundary layer. J. Atmos.
Sci., 33, 2170-2183.