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

Academic Article

Influence of Heat Transport by Sea Breezes on Inland Temperaturein the Osaka Area
Atsumasa Yoshida*1

Junichi Yashiro*1

Xinbo Xiao*1

Ryusuke Yasuda*1*1 Department of Mechanical Engineering, Osaka Prefecture University
Corresponding author: Atsumasa YOSHIDA, ayoshida@me.osakafu-u.ac.jp
ABSTRACTHeat transport from Osaka city to inland areas by sea breezes in mid-summer was evaluated using amesoscale meteorological model. The magnitude of the advection, diffusion, and radiation terms in the conservation
equation of potential temperature was individually evaluated.
The advection term was found to always show a cooling effect, and its magnitude and time variation weresimilar to the diffusion term, but with opposite sign. The cooling effect of the advection term decreases from thecoastal area to the inland area, but hardly turns to heating. Another simulation of increasing the anthropogenic heatin Osaka city area was executed and the results show that heat transport from the central area by sea breezes does
not lead to temperature rise in the inland area.
Key Words : Urban heat island, Sea breeze, Heat advection, Numerical modelincrease in temperature in the inland areas.

1. Introduction

In this study, the influence of heat transport from the central area
In Osaka, the most populous city in western Japan, and in otherof Osaka city on the inland areas in mid-summer using observa-

Japanese megacities, such as Tokyo and Nagoya, countermeas-tion data and a meteorological model was investigated. Fur-ures to the urban heat island (UHI) problems in summer are anthermore, a sensitivity test was performed by increasing theimportant issue. Based on the “wind passage” concept, the local

anthropogenic heat released within Osaka city and its impact on

government of Osaka city has created a future urban plan for thethe air temperature in the inland areas was evaluated.

area

(1)

to utilize the cooling potential of sea breezes. In the plan,

widening of streets and the creation of open spaces along the

2. Observational data analysis

paths of sea breezes are proposed to promote ventilation in thecity; however, it has been pointed out by some researchers thatFig. 1 shows Osaka area, which is surrounded by mountains to

the UHI of megacities causes temperature increases in down-

the north, east, and south. The western side faces the Osaka Bay.

stream areas. Zhang et al.(2) investigated the relationship be-

To investigate the relationship between wind direction and hightween heat wave in 2007 and upstream urbanization over thetemperatures in mid-summer, we categorized the local wind

Baltimore-Washington corridor in the US using a numericalsystem of this area into several patterns using hourly data ob-model. They found that the UHI effects over Baltimore would beserved at Automated Meteorological Data Acquisition Systems

1.25 °C weaker, or reduced by 25%, without the upstream ur-(AMeDAS) stations. The period investigated was July and Au-(3)investigated the Shanghai megacity ingust from 2006 to 2009. Under clear day conditions (solar radia-

China and concluded that the UHI leads to temperature rise intion > 18 MJ/m2/day and precipitation < 0.5 mm/day) two con-the downstream area. Masumoto et al.(4) examined the tempera-

trasting patterns were focused on: large-scale sea breeze (EW-L)

ture distribution in Osaka city using observational data and con-days and easterly wind (EE) days. EW-L is a typical land-sea

cluded that strong westerly to south-westerly winds (sea breezes

breeze circulation in this area. In this type, a small-scale sea

in the area) in the daytime transport sensible heat from the cen-breeze from Osaka Bay develops early in the morning and atral area of the city to its eastern area. Therefore, enhancement oflarge scale sea breeze from the Pacific Ocean overrides it in theventilation in Osaka city as a UHI mitigation plan may lead to anlate afternoon. The number of EW-L days was 45% of all clear

banization. Kang et al.

-6-Diurnal temperature range(℃)

days. In the EE days, easterly winds (land breeze in this area)
persist all day long in a synoptic pressure pattern. On clear days
this pattern appears less frequently (7%).
Fig. 2 shows the mean diurnal temperature variations at theOsaka and Hirakata AMeDAS stations (P1 and P2, respectively;
see Fig.1) for each wind pattern. P1 is located in the central areaof the city. P2 is in the inland area, about 20 km to the northeastof P1; therefore, P2 is in the downstream of the sea breeze. Since
the daily maximum temperature (Tmax) and the daily temperature range (Tmax–Tmin) in EW-L days are smaller than in EE

121086Osaka
Hirakata

40246
Wind speed at Osaka (m/s)

Fig. 3 Relationship between mean wind speed in the daytime

days at both points, it can be inferred that the sea breezes are not(1200–1800 LST) and daily temperature range in EW-L daysnecessarily associated with high inland temperatures. Fig. 3in August 2006–2009; solid: Osaka, dashed: Hirakata.

shows the relationship between the mean wind speed in the daytime (1200–1800 LST) at P1 and the daily temperature range at
P1 and P2 in EW-L days. The correlation coefficient is -0.57 and3. Numerical method-0.38 at P1 and P2, respectively. Because the mean wind speed,which represents sea breeze strength, negatively correlates withWe used the Weather Research and Forecasting (WRF) model-the daytime temperature rise, it is unlikely that the heat trans-ing system ver.3.2.1(5) to simulate land-sea breeze circulations in

ported from the central area causes a temperature rise in thethe study area. The calculation parameters are shown in Table 1.

inland areas.

Initial and boundary conditions for the meteorological variables
were from the Final Operational Global Analysis database of theNational Centers for Environmental Protection, with 1° resolution and 6-hour interval. Grid data of the altitude (50 m resolution) and land use (100 m resolution) were from the National
Land Numerical Information database of the National and Regional Planning Bureau. A single-layer urban canopy model(6)was activated on the urban grids specified by the land use data.
Fig. 4 shows the calculation domain. Two domains were set witha two-way nesting configuration. The depth of the lowest layer
was about 60 m. The calculations started at 0900 LST two days
before the target day. After the 39 hours spin-up, hourly data of
0100–2400 LST on Grid 2 were used for analysis.
Table 1 Calculation parameters of the numerical model.

Fig. 1 Map of the study area. P1 and P2 are Osaka and Hirakata
AMeDAS stations, respectively. The red line denotes the location

Items

Grid1

Grid2

of the vertical cross-section in Fig. 9. P0 is the point from which theGrid interval (horizontal)

3.0 km

1.0 kmNumber of grids (horizontal)

120×120103×103

distance from the coast is measured.

Temperature (℃)

3528

Center of grids (Lat., Lon.)( 34.672°, 135.480°)

Each process of heat transport; i.e. advection, diffusion and radi-30

25

Number of grids (vertical)

ation, was evaluated from the conservation equation of potential

Osaka EW-L
Osaka EE
Hirakata EW-LHirakata EE

612LST (hour)

18temperature:∂Θ∂t24Fig. 2 Diurnal variation of mean air temperature at Osaka andHirakata in each wind pattern in August 2006–2009.

-7-TotalΘ
AdvectionΘDiffusion

Θ 1Radiation,1

where ρ, Cp, Θ, and T are the density, specific heat, potential

values in the parentheses shown above are benchmarks recom-temperature, and air temperature, respectively. Uj is a wind ve-mended by Emery et al.(7). The agreement is excellent because

locity component in the j direction. Dθ is the horizontal/verticalall of the benchmark indices are within the recommended values,diffusion coefficient and F is the vertical radiation flux. Sensible

except the MBE of wind speed, but its exceedance is very small.

heat flux from the ground surface was included in the diffusion

Fig. 5 shows the horizontal distribution of air temperature andterm. A heating/cooling effect by condensation/evaporation ofwind vectors at 1200 and 1800 LST. At 1200 LST, a local sea

hydrometeor elements was ignored in this analysis because thesebreeze arising in Osaka Bay penetrates the land. In the coastal

processes do not contribute substantially in lower layers on aarea, the wind directions are nearly normal to the coastline. Atclear day.

1800 LST, a sea breeze front can be seen deep inside the Osaka

These terms were reconstructed from the hourly outputs of thearea. Since the origin of the stream is not in Osaka Bay but in

WRF model. For the reconstruction, Equation 1 was transformed

the south of the analysis area (the Pacific Ocean), this stream is a

into a η-coordinate system as formulated in the model, and eachlarge-scale sea breeze. At both times, the air temperature in-term was discretized according to the dynamical/physical op-

creases from the coast to the inland areas along the sea breeze

tions used in our simulation, such as an upwind scheme ofpath, i.e. air temperature is higher in the leeward side; however,

5th-/3rd-order for the horizontal/vertical advection, and theit is difficult to ascertain whether this temperature gradient is

Mellor-Yamada-Nakanishi -Niino level 2.5-closure scheme forcaused merely by the declination of the sea breeze cooling in thethe vertical mixing. Note that instantaneous fluctuation is in-inland areas or by the downstream transport of sensible heat

cluded in the estimated values because the evaluation is madefrom the central area of the city. Whether sensible heat advectedbased on the instantaneous value of hourly outputs.

by sea breezes increases air temperature in inland areas is discussed in the next section.

Grid 11200 LST

Grid 21800 LST

Fig. 4 Calculation domains of the numerical model.
4. Modeling results and discussion
From 2006 to 2010, several days were selected and simulated astypical days for each local circulation pattern. In the following
section, the result of Aug 5, 2006 is presented as a typical EW-Lcondition.
4.1 Meteorological field reproduced by numerical model
The model outputs were compared with the observation data ofnine AMeDAS points in Osaka prefecture. For the air temperature at 2 m, the Mean Bias Error (MBE, ≤ ±0.5 °C), Mean Ab-Fig. 5 Horizontal distribution of air temperature at 2 m and windsolute Error (≤ 2 °C), and Index of Agreement (IA, ≥ 0.8) arevectors at 10 m on Aug 5, 2006 (left: 1200 LST, right: 1800

0.32 °C, 1.10 °C and 0.96, respectively. For the wind speed at 10LST). The thick line shows a sea breeze front and the whitem, the MBE (≤ ±0.5 m/s), Root Mean Square Error (≤ 2 m/s)

block arrow indicates the direction of the sea breeze.

and IA (≥ 0.6) are 0.51 m/s, 1.07 m/s, and 0.66, respectively. The-8-

1; 90 W/m2 at maximum).

asFig. 8 shows the difference of the daily maximum air tempera-Fig. 6 shows the thermal effect of each term in the conservationture along the red line shown in Fig. 1. A large temperature dif-equation of potential temperature in the lowest layer at 1200 andference exists only in the Osaka city area where the heat released

1800 LST. The horizontal axis is distance from the coast alongis increased, and the maximum air temperature in the inland

the red line shown in Fig. 1. As shown in Fig. 6, the diffusionareas does not change much. In Fig. 8, Osaka city is 2–22 km

term (surface heat flux) and advection term show large positivefrom the coast, and a temperature rise is found within 10 km

and negative values, respectively, and they almost cancel eachfrom the downwind edge of the city, but the increment is lessother. At 1200 LST the change rate of potential temperature overthan 0.2 °C, which corresponds to only one quarter of the value

land is determined substantially by the heating of the radiationin the city area. Fig. 9 shows the distribution of the potentialterm. The cooling effect of the advection term is more prominent

temperature difference (Case 2–Case1) and wind vectors of Caseat 1800 LST. The magnitude of the advection term decreases

1 at 1730 LST on the vertical cross section along the red line infrom the coast to the inland areas, but it hardly turns positiveFig. 1. At this time, the sea breeze penetrates about 35 km fromeven in the inland areas; therefore, it always contributes as a

the coast, and the potential temperature difference reaches overcooling term and does not cause temperature rise.

1000 m above the ground, which means that most of the heat

Potential temp. change (K/h)

Potential temp. change (K/h)

4.2 Thermal effect of each term from the coast to inland are-released in Osaka city is diluted vertically by the passage of a

54 1200 LST
3210-1-2-3-4-5-100102030
40
Distance from coast (km)54 1800 LST
32
10-1-2-3-4-5-10
0
10
2030
Distance from coast (km)Advection
Radiation

sea breeze front. Additionally, the increase in the heat released in
the city area causes a delay in sea breeze penetration to thedownwind areas (Fig.10); therefore, it leads to a delay in temperature decrease late in the afternoon in the inland areas. Differences in the daily minimum temperature were also investigated (not shown here). Similar to the result of the daily maximum temperature, the increase in the minimum temperature waslimited to the city and its vicinity.
5. Conclusion
This study confirms that sea breezes do not cause a rise in thedaily maximum temperature in inland areas. Sea breeze has acooling effect at all times. Even if anthropogenic heat released in
Osaka city is greatly increased, the effect on the inland areas isvery small because most of the heat is diluted vertically and thecooling effect of sea breezes is maintained. Therefore, promotion of ventilation in Osaka city to enable passage of sea breezes
will not lead to temperature rise in the inland areas.

40

Diffusion
TotalFig. 6 Variation of each term in the conservation equation ofpotential temperature along the distance from the coast in the lowest
layer, upper: 1200 LST, lower: 1800 LST on Aug 5, 2006.
4.3 Influence of heat transfer from the city area to the inland
areas by sea breeze
To investigate how the heat generated in the central area ofOsaka city affects the air temperature in the inland areas underEW-L conditions, a sensitivity analysis of hypothetical heat reFig. 7 Central area of Osaka city where anthropogenic heat ishypothetically increased.

lease was conducted. The amount of anthropogenic heat releasedin the entire Osaka city (see Fig.7) was increased by two times(Case 2) and three times (Case 3) that of the control case (Case-9-Difference of Tmax (℃)

1References

Case2-Case1
Case3-Case1

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KFig. 9 Distribution of potential temperature difference (Case2–Case1) and wind vectors on a vertical cross section in Case 1 at1730 LST on Aug 5, 2006. The cross section is along the redline shown in Fig.1.

(Received Dec. 12, 2014, Accepted Dec. 27, 2014)

Case1

km
Case2

km

Fig. 10 Distribution of potential temperature and wind vectors on avertical cross section at 1700 LST on Aug 5, 2006. Upper: Case1,
lower: Case2. The cross section is along the red line shown in Fig.1.
The white cross and a thick black arrow indicate the center of thesea breeze head and the sea breeze front near the ground surface,respectively.

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