Numerical Simulation Research of Urban Heat Island
in Macao
Wang Zhishi, Jing Chengtao Faculty of Science and Technology of University of
Macau
1. Introduction of Urban Heat Island
Urban heat island is a phenomenon of that temperature of the urban is higher than
the suburb. As the global warming and the developing of the urban, urban heat island is
getting harder and harder. Urban heat island is bad to the people who live in urban. High
temperature makes people feel uncomfortable. The pollutants in the air are hard to flow
out of the urban. There are lots of cars in the urban, so the pollutants are getting more
and bad for people’s health.
The urban heal island (UHI) effect is related to the problem of global warming. The
majority of weather stations are in cities or nearby; thus, the temperature trend, which is
the signature of global warming, can be influenced by urbanization. (Urban Heat Island
Modeling in Conjunction with Satellite-Derived Surface Soil Parameters) In the past,
people measured the temperature of the urban and the suburb to realize urban heat
island. They can find that the temperature of urban is higher than suburb evidently year
by year. Now the primary method of consider urban heat island is remote sensing. It is
easy to get data from the satellite. After analyzing the data people can find the
distribution of the ground temperature in and around the urban. We may also use some
relationship which is got from experiment and measurement change the ground
temperature to air temperature. It is a good and convenient method to study the urban
heat island. The disadvantage of this method is: it is difficult to observe the change of
heat island with time. So it cannot predict what will happen.
It is a good method to use computer simulation to study urban heat island. The
computer is developing very fast and incredibly during these years. People also design
and compile many program and software to study the weather and temperature. And the
theory is mature, we have the corresponding equations and relationships to calculate
and predict the change with time. So people can know what will happen. It is easy to
study the change and cause of formation of urban heat island using computer.
ARPS (Advanced Regional Prediction System) is a completely nonhydrostatic
model system which is developed by the Center for Analysis and Prediction of Storms
(CAPS) at the University of Oklahoma. It is a multi-scale nonhydrostatic atmospheric
simulation and prediction model. The ARPS includes its own data ingest, quality
control and objective analysis packages, a data assimilation system which includes
single-Doppler velocity and thermodynamic retrieval algorithms, the forward
prediction component, and a self-contained postprocessing, diagnostic and verification
package.
As the developing of Macao, it is evident urban heat island in Macao. So using
ARPS to study urban heat island in Macao is very important to people. After finding out
the factors of the heat island, we can make scheme and method to control it.
2. Numerical Simulation Research of UHI
2.1 Theoretical analysis and calculation model
2.1.1 Land-Surface energy budget
This model is based on the soil-vegetation model developed by Noilhan and
Planton (1989) and Pleim and Xiu (1995). It is designed to simulate the essential
processes involved in surface-atmosphere interactions with the minimal amount of
computation time and the fewest parameters and complexities (Wong et al., 1994). At
present, only snow-free and non-frozen soils are considered. It requires the horizontal
distribution of soil texture at the land-surface.
The equations of Land-Surface energy budget are
Ts
2
 CT ( Rn  H  LE ) 
(T  T )
t
 s 2
T2 1
 (Ts  T2 )
t 
where Ts : Ground surface temperature
T2 : Deep ground temperature
CT : Land surface heat capacity
Rn : Net radiation heat flux
H : Sensible heat flux
LE : Latent heat flux
 : Characteristic time
2
(Ts  T2 ) represents the heat flux between ground surface and
The term

underground.
2.1.2 Radiation flux
In ARPS, we have two models of the net radiative flux for surface heat balance. The
first net radiative flux is given by(Wong et al., 1983)
Rn1  Rsw1 (1   )   g Ts4   a Ta4
in which  g is the emissivity of the earth’s surface,  a  0.725 is the emissivity of the
air,   5.67 108Wm2 K 4 is the Stefan-Boltzman constant, and Ta is the air
temperature at an atmospheric level. The short-wave radiation is determined from
Rsw1   rg wv S0 (
a2
) cos Z
r2
in which the solar constant is S0  1353.0Wm 2 , and
a2
is Earth-Sun distance factor.
r2
The second net radiative flux is
Rn 2  Rsw2 [(1  irbm ) fdirir  (1  uvbm ) fdirpar  (1  irdf ) fdifir  (1  uvdf ) fdifpar ]   g Ts4   a Ta4
a
Rsw 2  ( f dirir  f dirpar  f difir  f difpar ) S0 ( ) 2 cos Z
r
2.1.3 Sensible heat flux
Sensible heat (H) is heat energy transfer between the surface and air when there is a
difference in temperature between them. Sensible heat flux is one of the parameters
controlling the height of the mixed layer. Net increase in sensible heat flux given by
surface heat balance is high in developed areas and low in rural areas. The spatial
distribution of H implied that H is due primarily to anthropogenic heat discharge and
heat storage in concrete buildings during the daytime. The industrial areas, in which
huge amounts of energy are consumed, have extremely high net increase in sensible
heat flux. They have concluded that the increase in sensible heat flux due to
anthropogenic heat discharge.
Sensible heat flux from the total surface of urban blocks ranged from 130Wm-2 to
300Wm-2 at noon on a clear sky summer day. Spatial form of urban blocks, as well as
materials used, influenced the sensible heat flux from the total surface of urban blocks.
When the surface is warmer than the air above, heat will be transferred upwards
into the air as a positive sensible heat transfer. The transfer of heat raises the air's
temperature but cools the surface. If the air is warmer than the surface, heat is
transferred from the air to the surface creating a negative sensible heat transfer. If heat
is transferred out of the air, the air cools and the surface warms. This situation may take
place at night when the sun goes down and there is no input of solar radiation. At this
time, the ground cools due to longwave emission and the air directly above the surface
is warmer.
Figure 2.1 Direction of positive and negative sensible heat flux
In ARPS, the sensible heat flux is
H   a c p CdhVa (Ts  Ta )
where  a is the air density, c p is the specific heat at constant pressure, Cdh is the
exchange coefficient depending upon the thermal stability and roughness, Va is wind
speed at an atmospheric level.
2.1.4 Latent heat flux
The heat used in the phase change from a liquid to a gas is called the latent heat of
vaporization. We say it is "latent" because it is being stored in the water molecules to
later be released during the condensation process. We can't sense or feel latent heat as it
does not raise the temperature of the water molecules.
Evapotranspiration (evaporation and transpiration) is an effective moderator of
near-surface climates. Given the right conditions, evapotranspiration can create ‘oasis’
that are 2-8℃ cooler than their surroundings. Urban areas, with extensive impervious
surfaces, have generally more runoff than their rural counterparts. The runoff water
drains quickly and, in the long run, less surface water remains available for
evapotranspiration, thus affecting the urban surface energy balance. The lower
evapotranspiration rate in urban areas is a major factor in increasing daytime
temperatures. Latent heat flux is one of the sources of humidity in the atmosphere,
which affects both cloud and rain formation.
When evaporation is taking place we say there is a positive latent heat flux. A
positive latent heat flux is illustrated with an arrow pointing up away from the surface
of the earth. This indicates that the surface is losing energy to the air above.
Evaporation is a cooling process for a surface because energy is removed from the
water as molecules escape the surface. This causes the surface temperature to decrease.
Figure 2.2 Direction of positive and negative latent heat flux
In ARPS, the latent heat flux is the sum of the evaporation from the soil surface Eg ,
transpiration Etr , and evaporation from wet parts of the canopy Er :
LE  L( Eg  Etr  Er )
in which L is the latent heat of vaporization.
Eg  (1  veg )  a CdqVa [hu qvsat (Ts )  qva ]
Etr  veg  a
1  Fw
[qvsat (Ts )  qva ]
Ra  Rs
Er  veg  a
Fw
[qvsat (Ts )  qva ]
Ra
2.1.5 Relationship between heat fluxes
There are four types of heat fluxes to consider for calculating the ground surface
energy: net radiation flux, sensible heat flux, latent heat flux and ground heat transfer.
During the day net radiation is a positive value as incoming radiation exceeds outgoing
radiation allowing the surface to gain energy. The energy is distributed over the three
major categories of energy use, LE, H, and G. During the day, the available radiant
energy is used to evaporate water into the air, raising the air's humidity. Sensible heat is
transferred upwards to warm the air above the surface. Heat is also conducted down
into the subsurface. At night the processes reverse. At night with no incoming solar
radiation there is more outgoing radiation than incoming creating a negative value for
net radiation. Under these circumstances the surface cools due to a loss of energy and
heat is transferred from the air toward the surface. As air cools through the evening the
loss of energy allow condensation to occur, so long as the air's humidity is at or near
saturation.
For net radiation flux, sensible heat flux, latent heat flux and ground heat transfer,
they are different situations on moist surface or dry surface. Water is available at the
surface for evaporation and latent heat transfer into the air at moist locations. Without
available water, no transfer of latent energy occurs, hence the absence of an LE flux for
the dry surface. Most of the available energy, net radiation flux, is allocated to sensible
heat transfer creating warm air temperatures.
Figure 2.3 Different heat fluxes between moist and dry surface
From the analysis above, we can see that vegetation is important for UHI. If
there is lots of vegetation in the city, the LE will be large and H will be small. So there
is no large energy to heat the air above the ground. But if there is no vegetation in the
city, H will be large. So there is lots of energy to heat the air and the air will be hot
and it will be form urban heat island.
2.2 ARPS numerical simulation model
ARPS is a program to simulate the weather, temperature, wind, humidity and so
on. It is very useful for numerical prediction. We use this program to analyze urban
heat island in Macao.
In ARPS we make the domain is 10km×15km and the grid size is 200m×200m.
The grid number is 50×75. Figure 2.5 shows the divided grid.
Figure 2.4
Macao map
Figure 2.6 Macao ground utilization
Figure 2.5 Grid and altitude
Figure 2.7 Different ground types
Figure 2.4 and 2.6 are the profile of Macao. In terms of different types of the
ground surface, we divide the ground into 6 parts approximately in ARPS in figure 2.7.
They are water (blue), construction (light blue), greenbelt (light green), bare ground
(green), hengqin island (red) and breeding plant (yellow).
Figure 2.8 Greenbelt in Macao
Figure 2.9 Vegetation distribution
Figure 2.8 shows the distribution of greenbelt in Macao. In terms of this figure,
we make figure 2.9 in ARPS. Figure 2.9 shows vegetation distribution in ARPS. We
can see that most of vegetation is located in Coloane and Taipa.
2.3 Case study
2.3.1 Comparison with observed data
From the meteorological bureau of Macao, we get the weather data such as
temperature, wind speed, wind direction, humidity and so on. These data are collected
hourly for three places: meteorological bureau, emplacement and Sun Zhongshan
Park. Through comparing numerical simulation data with observed data, we can
improve the program better to work in Macao.
Figure 2.10 Three places in Macao marked by +
There are two cases for comparison: case g2 and case k3c. They both begin
calculation at 5:00 July 3th, 2005. The initial condition is 3m/s wind speed, 140º
wind direction (southeast), 26.5℃ air temperature, 28℃ ground surface temperature,
28.9℃ water surface temperature and 92% air humidity. Case g2 is set zero gradient
condition for top layer. It means that at top layer, there is no change in wind speed and
direction between neighboring vertical grids. Case k3c is set constant wind speed and
direction for top layer. There are 3m/s and 140º. And this case is set lower latent heat
flux for improving lower temperature.
Figure 2.11 to 2.13 show the air temperature comparison between simulation and
observation at three places. At first and second day, the air temperature of case g2 is
much lower than observation. And at third and fourth day, the simulation is better but
still a little lower. The result of case k3c is better than case g2. It is close to the
observation.
Figure 2.11 Air temperature comparisons at meteorological bureau
Figure 2.12 Air temperature comparisons at emplacement
Figure 2.13 Air temperature comparisons at Sun Zhongshan Park
Figure 2.14 to 2.16 show the air humidity comparison between simulation and
observation at three places. From the figures we can see that in terms of observation,
air humidity is high at night and is low at daytime. But the simulation is not good for
this parameter. As the time pass by, air humidity goes down evidently. So ARPS
program need to improve for air humidity.
Figure 2.14 Air humidity comparisons at meteorological bureau
Figure 2.15 Air humidity comparisons at emplacement
Figure 2.16 Air humidity comparisons at Sun Zhongshan Park
2.3.2 Surface temperature and air temperature
Figure 2.17 to 2.19 show surface temperature and air temperature comparison at
three places in case g2. At first and second day, the result is not good enough. At third
and fourth day, surface temperature is a little higher than air temperature at daytime
and a little lower at night at emplacement and Sun Zhongshan Park. That is according
to the fact. But at meteorological bureau, surface temperature is always lower than air
temperature. It is related to the greenbelt surface condition and changed heat flux.
Figure 2.17 Case g2 surface and air temperature comparison at meteorological bureau
Figure 2.18 Case g2 surface and air temperature comparison at emplacement
Figure 2.19 Case g2 surface and air temperature comparison at Sun Zhongshan Park
Figure 2.20 to 2.22 show surface temperature and air temperature comparison at
three places in case k3c. It is also not good at first and second days. At third and
fourth day the result is good because surface temperature is a little higher than air
temperature at daytime and a little lower at night at three places.
Figure 2.20 Case k3c surface and air temperature comparison at meteorological bureau
Figure 2.21 Case k3c surface and air temperature comparison at emplacement
Figure 2.22 Case k3c surface and air temperature comparison at Sun Zhongshan Park
2.3.3 Heat flux analysis
Figure 2.23 to 2.25 show heat fluxes at three places in case g2. The result at first
day is not good. At other days, solar radiation is very large (red line). Sensible heat
flux and latent heat flux are interactional fluxes at daytime. In the three figures, if H is
small, LE will be large, vice versa. But this is not reasonable. At emplacement and
Sun Zhongshan Park at third day and fourth day, latent heat flux becomes small and
sensible heat flux becomes large. Actually, it should not change fiercely during these
days. Ground heat transfer is always small. It is a little positive at daytime and a little
negative at night.
Figure 2.23 Case g2 heat fluxes at meteorological bureau
Figure 2.24 Case g2 heat fluxes at emplacement
Figure 2.25 Case g2 heat fluxes at Sun Zhongshan Park
2.3.4 Heat island analysis
Now we consider urban heat island in Macao. In the simulation, the data at first
day and second day are not good. So we take the third and fourth day to analysis.
Figure 2.26 is the distribution of air temperature in case g2. The time of the left one is
July 5th 14:00, namely the third day, and the time of the right one is July 6th 14:00,
namely the fourth day. From the figure, the temperatures of most parts of Macao and
Taipa are very high for these parts’ ground surfaces are construction. The
temperatures of east part of Taipa and coloane are low because their ground surfaces
are greenbelt. At daytime the green plants can store the energy from the sun and adjust
the temperature. Therefore the greenbelt part’s temperature is low. The difference
between high temperature part and low temperature part is about 4℃.
Figure 2.26 Distribution of air temperature in case g2 (left: July 5th 14:00, right: July 6th 14:00)
Figure 2.27 is surface temperature with the same case and the same time of
figure 2.26. The highest temperature part is the bare ground between Taipa and
Coloane. The surface temperature is almost 40 ℃ . In Macao and airport the
temperature is also very high. The low surface temperature parts are east and west part
of Taipa and Coloane, which almost same with air temperature. The difference
between high temperature part and low temperature part is about 17℃.
Figure 2.27 Distribution of surface temperature in case g2 (left: July 5th 14:00, right: July 6th 14:00)
At night we get a reverse result. There are distribution of air and surface
temperature in case g2 at 2:00 of July 5th and 6th in figure 2.28 and 2.29. At daytime
Macao and bare part between Taipa and Coloane give high temperature and at night
they give a little low temperature. Because at night the bare and construction ground
release the energy, which get from the sun at daytime, is faster than the greenbelt
ground. It makes the temperature decline faster.
Figure 2.28 Distribution of air temperature in case g2 (left: July 5th 2:00, right: July 6th 2:00)
Figure 2.29 Distribution of surface temperature in case g2 (left: July 5th 2:00, right: July 6th 2:00)
Case k3c gives the similar results with air and surface temperature.
3. Plan for next year
ARPS is an useful program to simulate weather. It provides valuable result for
urban heat island effect in Macao. But the numerical simulation still not perfect. For
example, it cannot provide good result in air humidity, and heat balance still has
problems.
Next step, we should adjust heat balance in terms of theories. Heat balance is
important for urban heat island. Also we should find out the reason the air humidity
turns small. Now we think there is no source for water vapor. So we consider the sea
and the air-sea interaction to adjust air humidity. We also consider other parameter
impact the urban heat island. If we make the program better, it will be very useful to
research the cause of urban heat island in Macao.
Simulation is not enough. Next year we will set up about seven weather stations.
They use wireless system. The weather stations will provide practical data about
temperature, wind, humidity and so on. Combining metrical data with simulation, we
can deeply analyze urban heat island in Macao.