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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D17112, doi:10.1029/2009JD011764, 2009
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Seasonal variation of the land-sea breeze circulation
in the Pearl River Delta region
Xi Lu,1 Kim-Chiu Chow,1,2 Teng Yao,1 Jimmy C. H. Fung,1,2 and Alexis K. H. Lau1,3
Received 18 January 2009; revised 15 May 2009; accepted 30 June 2009; published 12 September 2009.
[1] The data of a 1-year (2003–2004) simulation with a finest horizontal resolution of
1.5 km, using the Fifth-Generation Pennsylvania State University–National Center for
Atmospheric Research Mesoscale Model (MM5), were analyzed to investigate the
seasonal-mean features of the land-sea breeze (LSB) and regional circulation over the
Pearl River Delta (PRD) region in southern China. The seasonal-mean diurnal variations
reveal the general patterns of the LSB in the four seasons. These small-scale mean
flow fields in the region have not been revealed in any previous studies. The results reveal
a strong anomalous westerly sea breeze toward the eastern coast of the PRD in the
early afternoon that is present in all the four seasons but is particularly strong in autumn
and winter and may enhance the low-level convergence in Hong Kong. Furthermore,
the condition of the atmosphere in autumn and winter is much more stable when compared
with that in spring and summer, which is not favorable for the vertical dispersion of
pollutants. The overall effect of these mean meteorological conditions may be an
important factor for the generally higher air pollution index observed in Hong Kong
during autumn and winter.
Citation: Lu, X., K.-C. Chow, T. Yao, J. C. H. Fung, and A. K. H. Lau (2009), Seasonal variation of the land-sea breeze circulation in
the Pearl River Delta region, J. Geophys. Res., 114, D17112, doi:10.1029/2009JD011764.
1. Introduction
[2] Land-sea breeze (LSB) is a mesoscale phenomenon
caused by the difference in diurnal temperature variations
between land and sea due to the different heating and
cooling rates of the different surfaces. Sea breeze (SB)
circulation toward the land usually develops along the
coastline in the late morning of a fine sunny day as
the land surface heats up and this circulation may expand
in both the landward and seaward directions [Abbs and
Physick, 1992]. While at night, land breeze (LB) develops
because of the cooler land surface. The basic features and
structure of LSB have been discussed in numerous previous
studies [e.g., Arritt, 1993; Simpson, 1994; Buckley and
Kurzeja, 1997].
[3] The Pearl River Delta (PRD) region is located in
southern China with a population over 50 million, hosting a
number of the major cities in China including Guangzhou,
Shenzhen, Hong Kong, Macau, Dongguan, and Zhuhai
(Figure 1). The complex shore lines and terrain of the
PRD region makes LSB circulation particularly complicated
in this area. In particular, most of the urban areas in the PRD
region are located near the coastal regions and thus the LSB
1
Atmospheric Research Center, Hong Kong University of Science and
Technology Fok Ying Tung Graduate School, Guangzhou, China.
2
Department of Mathematics, Hong Kong University of Science and
Technology, Clear Water Bay, Hong Kong.
3
Environmental Central Facility, Institute for the Environment, Hong
Kong University of Science and Technology, Clear Water Bay, Hong Kong.
Copyright 2009 by the American Geophysical Union.
0148-0227/09/2009JD011764$09.00
circulation may have a significant effect on the pollutant
transport in these major cities. For example, Li et al. [1999]
indicated that the concentrations of air pollutants over the
PRD are not only correlated with the synoptic weather
patterns but also with the duration of these patterns. Wang
et al. [2001] analyzed the observational data from five sites
in Hong Kong, and their results show that the averaged
ozone levels at most sites generally have their maximum in
autumn and early winter and minimum in the summer. On
the other hand, it has also been pointed out by Zhang and
Zhang [1999] that in the PRD region, the frequency of LSB
in autumn and winter is generally higher than the other two
seasons. To our knowledge, most previous studies on air
pollution in the PRD region have focused on case studies
that occurred in autumn and winter. For example, Ding and
Wang [2004] simulated the LSB and investigated the
transport of pollutants during a prolonged ozone episode
observed in Hong Kong in September. Numerical and
analytical analyses were also made by Fung et al. [2005]
to understand the air pollution episode that occurred over
much of the western part of Hong Kong between 28 and
30 December 1999. Feng et al. [2007] used numerical
simulations and observational analyses to analyze the
aerosol episode that occurred over the PRD region during
1– 3 November 2003.
[4] Although some features of LSB and the particular
large-scale background flow in specific episodes have been
revealed in these case studies, the seasonal-mean features of
the LSB in different seasons as well as the mean atmospheric conditions responsible for the higher frequency of
air pollution episodes in autumn and winter have not been
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Figure 1. The MM5 domain discussed in this study
showing the terrain (m, contours) and locations of some
major cities in the Pearl River Delta region.
addressed in any previous works. Studying the seasonalmean characteristics of LSB is important for understanding
the basic LSB circulation and consequently the basic
atmospheric conditions associated with the air pollution
transport. Because of the relatively few and sparse distribution of weather stations in this region, observational data are
generally not sufficient to resolve a clear picture of the
seasonal-mean features of the small-scale LSB circulation in
the region. Therefore, it appears that numerical simulation is
the ideal tool for this kind of study. However, performing
the numerical simulation over such a small scale (highresolution simulation) for a long period of time (e.g., 1 year)
requires a large amount of computing resources, which is
still a difficult task in practice.
[5] In this study, we analyze the seasonal-mean LSB
circulation in the PRD region by using the simulation data
of Yim et al. [2007] (hereinafter referred to as YIM2007).
YIM2007 developed a high-resolution wind map for
complex terrain with the MM5 system to investigate the
potential of using wind energy in Hong Kong. In their study,
hourly wind fields were simulated for 1 year from 2003 to
2004 (more details will be discussed in section 2). On the
basis of this limited data, the year of 2003 – 2004 is chosen
in this study and three months averaged YIM2007 data in
winter (December 2003 to February 2004), spring (March
2004 to May 2004), summer (June 2004 to August 2004),
and autumn (September 2004 to November 2004) is used
to investigate the seasonal-mean features of the LSB, and
in particular the mean diurnal variations of the LSB in
different seasons.
[6] The details of the YIM2007 data used in this study
will be discussed in section 2. The results of the analyses
will be discussed in section 3. The main conclusions of this
study will be summarized in section 4.
2. Data Sources
[7] The meteorological model used by YIM2007 is the
Fifth-Generation Pennsylvania State University – National
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Center for Atmospheric Research Mesoscale Model
(MM5) version 3.6. In their study, four nested domains
with horizontal grid spacing of 40.5 km (D1), 13.5 km (D2),
4.5 km (D3), and 1.5 km (D4) were used. There were
25 sigma levels in the vertical direction with the first
10 layers being concentrated in the atmospheric boundary
layer (about 1 km above the ground level) to allow for finer
resolution of the planetary boundary layer.
[8] The PRD is a rapidly developing region, so the
original land use data used by MM5 from the U.S.
Geological Survey is not up to date (last updated for
1993). Therefore, YIM2007 used recently updated land
use data that were originally compiled by the Hong Kong
Planning Department (HKPD) [2003]. The HKPD land use
data were reformatted to supersede the original 30-arc
second in the MM5 data over the PRD region.
[9] The reanalysis data of the European Centre for
Medium-Range Weather Forecasts with 2.5° 2.5°
horizontal resolution at 6-h intervals were used for the
initial and boundary conditions. To enhance the quality of
the simulations, four-dimensional data assimilation (FDDA)
were applied to each MM5 run, which includes the grid
analysis nudging to relax the simulations with the global
telecommunication system observational data and the
reanalysis data for the outermost domain, D1, at 6-h intervals.
Observational nudging for the innermost domain D4 with the
surface wind observation from the stations of the Hong Kong
Observatory was also applied in the FDDA.
[10] The 1-year simulation data (December 2003 to
November 2004) are the integration of data from
121 separate MM5 simulations, each of which was a 4-day
run with the first day data scrapped for the spin-up period.
In the work by YIM2007, the 1-month period of March
2004 data was validated with the surface meteorological
station data operated by the Hong Kong Observatory in the
same period. Their results showed that the means of
the simulated wind speeds at most stations are fairly close
to the observed means. In particular, most indices of
agreement were close to 1 with an average value of 0.97.
The averaged value of the root-mean-square error of
the simulated wind speed is around 0.64 m s1. These
results suggest that their MM5 simulations of 2004 could
successfully reproduce the observed changes in wind
direction and that the YIM2007 data are reliable.
3. Seasonal-Mean Circulations in the PRD Region
3.1. Daily-Mean Fields in the Four Seasons
[11] The PRD region is located in the subtropical region
and is influenced by the Asian monsoon climate in addition
to the prevailing easterly trade wind from the northwestern
Pacific Ocean. In summer, the Asian Summer Monsoon
(ASM) brings warm and moist air from the Indian Ocean
and the South China Sea toward the region. Associated with
the large-scale southwesterly of the ASM, the prevailing
wind direction over the PRD is mainly southerly (Figure 2b).
This southerly wind brings clean marine air to the PRD
region, and the atmospheric conditions are generally humid
and conditionally unstable.
[12] Spring is the transition period when the large-scale
atmospheric circulation changes from a winter monsoon
state to a summer monsoon state and the corresponding
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Figure 2. Daily-mean surface wind and temperature fields averaged in (a) spring, (b) summer,
(c) autumn, and (d) winter of 2004.
monsoonal wind changes from northeasterly to southwesterly.
In this transition period, the wind field in the PRD region
(Figure 2a) basically changes from northeasterly in early
spring to southerly in late spring, although the seasonalaveraged wind field is southeasterly. On the other hand, the
corresponding seasonal-averaged wind field in the open
oceanic region southwest of the PRD is mainly easterly
(Figure 2a).
[13] In autumn, the ASM has already retreated and weak
high-pressure systems gradually develop over the Asian
continent. The northeasterly winter monsoon begins to
control the PRD region in late autumn, and the resulting
averaged wind field in the PRD region in autumn is thus
northeasterly (Figure 2c). The winter monsoon reaches a
mature stage in winter and the dominant surface wind in the
PRD region is again northeasterly but of a larger magnitude
(Figure 2d). The northeasterly wind from the China
mainland carries dry air to the PRD region in autumn and
winter. The prevailing northeasterly may also facilitate the
long-range transport of pollutants from the China mainland
to the PRD region, which is consistent with the generally
higher air pollution index in the PRD region in these two
seasons.
3.2. Seasonal-Mean Diurnal Cycles
[14] The diurnal variation of the surface wind and temperature fields averaged in the four seasons of 2004 can be
observed from the corresponding anomalous fields (local
times minus daily mean) shown in Figures 3 – 6. In spring,
the land surface is much cooler than the ocean and at 0800
and 0200 LT (Figures 3a and 3d) the LB is well developed
over the PRD estuary. At these night periods, a strong
anomalous northerly in the PRD estuary can be observed
(see Figures 3a and 3d). In the early afternoon period
(1400 LT), the increase in surface temperature over the land
results in significant southerly SB toward both the east and
west coasts of the PRD estuary (Figure 3b). From the
corresponding total wind field in this period (Figure 7a),
it is apparent that the SB is approximately symmetrical over
the estuary. However, it can be seen from the corresponding
anomalous field (Figure 3b) that the anomalous westerly
wind toward the eastern coast of the PRD is clearly
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Figure 3. Anomalous (LT minus daily mean) surface wind and temperature fields at (a) 0800, (b) 1400,
(c) 2000, and (d) 0200 LT in spring of 2004.
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Figure 4. Same as Figure 3 but for summer of 2004.
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Figure 5. Same as Figure 3 but for autumn of 2004.
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Figure 6. Same as Figure 3 but for winter of 2004.
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Figure 7. Surface wind and temperature fields at 1400 LT averaged in (a) spring, (b) summer,
(c) autumn, and (d) winter of 2004.
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dominant, possibly related to a larger temperature increase
over the land surface in the eastern part of the coastal PRD
region (Figure 3b). This significant variation of the SB in
the eastern and western coasts of the PRD is also consistent
with the findings of Arritt [1993]. From a series of numerical experiments, Arritt [1993] found that the direction and
magnitude of the large-scale background wind field may
significantly influence the organization of SB in the afternoon. If the background wind is onshore (same direction as
the SB), the afternoon SB is generally suppressed because
of the weaker convergence zone in the coastal region. On
the other hand, if the background wind is offshore (opposite
to the direction of the SB), the SB is stronger because of the
enhanced convergence zone in the coastal region. In the
case of the PRD estuary, the background wind is generally
easterly in spring (see Figures 2a and 7a). Therefore, for the
eastern coast of the PRD region the background wind is
basically onshore, while it is basically offshore for the
western coast. And hence, the stronger afternoon SB in
the western coast of the PRD estuary in this study generally
agrees with the results of Arritt [1993].
[15] The early evening period of 2000 LT is likely a
transition period when SB starts changing to LB and the
temperature contrast between the land and sea is small
(Figure 3c). During this period in spring slight SB can still
be recognized around the coastal region (Figure 3c).
[16] Similar to that in spring, LB can be observed at
0800 LT in the estuary in summer (northerly anomalous
wind in Figure 4a). Since the daily-mean prevailing wind is
southerly in the PRD region (Figure 2b), the actual wind
over the estuary (figure not shown) is generally weak at
0800 LT. In the early afternoon at 1400 LT, symmetrical SB
toward the eastern and western coasts can again be observed
in the PRD estuary (Figure 7b) as in spring. However,
unlike the case in spring, the corresponding anomalous
wind field (Figure 4b) also shows a high degree of
symmetry in the estuary, likely related to a similar surface
temperature increase in the eastern and western coastal
regions of the PRD estuary. Again, on the basis of the
finding of Arritt [1993], this result of symmetrical SB may
also be explained by the lack of a clear onshore or offshore
background wind to the eastern and western coasts of the
PRD estuary in summer (see Figures 2b and 4b).
[17] The surface wind and temperature fields in autumn
are significantly different from those in spring and summer
when the winter monsoon starts influencing the region. In
this season northeasterly wind is dominant in all time
periods (figures not shown). Nevertheless, LB over the
PRD estuary can also be identified at night to early morning
from the anomalous fields at 0800 and 0200 LT (Figures 5a
and 5d). As in spring and summer, early afternoon (1400 LT)
SB over the PRD estuary can also be observed (Figures 5b
and 7c). The actual SB is also rather symmetrical toward the
eastern and western coasts of the PRD estuary (Figure 7c)
but is from the north instead of from the south as in spring
and summer. It can be observed from the corresponding
anomalous fields (Figure 5b) that the SB is dominated by a
strong anomalous westerly wind toward the eastern coast of
the PRD estuary. This anomalous SB is similar to that in
spring (compare Figures 5b and 3b) but is larger in
magnitude, likely related to a significant imbalance in
temperature increase in the land regions in the western
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and eastern coasts of the estuary (Figure 5b). The stronger
SB in autumn may also be related to the larger magnitude
of the background wind in autumn (compare Figures 2a
and 2c) according to Arritt [1993]. It is worth mentioning
that this strong anomalous westerly SB together with the
anomalous southerly and easterly around the Hong Kong
region may increase the low-level convergence there,
which will be further discussed in section 3.3.
[18] The patterns of the anomalous surface wind and
temperature fields in winter are very similar to that in
autumn (compare Figures 5 and 6), although the actual
surface temperature is lower and the wind speed is slightly
stronger in winter (figures not shown). It is worth noting
that although the actual surface temperature in winter is
lower than that in autumn, the magnitudes of the diurnal
variation in surface temperature are similar in autumn and
winter. This and the similar background wind fields may
explain the similar magnitudes of the anomalous westerly
SB at 1400 LT in the PRD estuary.
3.3. Meteorological Conditions Pertinent to Air
Pollution
[19] It has been discussed in section 3.2 that anomalous
westerly winds over the PRD estuary in the early afternoon
period (1400 LT) can be observed in all the four seasons,
and the magnitude is particularly strong in autumn and
winter. This anomalous afternoon westerly SB may cause a
convergence zone in the eastern coastal region of the PRD
estuary, near Hong Kong (see Figure 1). The time variation
of the area-averaged low-level convergence (average of the
three vertical levels below 50 m) over the Hong Kong
region (113.8 – 114.4°E, 22.18 – 22.55°N, rectangle in
Figure 1) in Figure 8 shows that this convergent effect is
very significant in the afternoon period. The maximum
convergence occurs around 1400 LT, which is consistent
with the strongest anomalous westerly winds over the
eastern coast of PRD and is also consistent with the period
of maximum land surface temperature, around 1400 LT, for
the Hong Kong region (Figure 12). The convergence
generally starts to increase from about 0800 LT and attains
the maximum around 1400 LT although the time the
maximum magnitude occurs is slightly different in the four
seasons. The earliest time the maximum convergence occurs
is in summer, around 1100 LT, while the latest time is in
winter around 1500 LT. This seasonal variation of the
maximum convergence period is consistent with the annual
cycle of the solar declination angle. The seasonal change of
the time period of strongest solar radiation may change the
onset period of the SB. This trend is also noted by Zhang
and Zhang [1999]. The increase of the convergence is
particularly sharp in autumn and winter, which is consistent
with the larger surface temperature contrast between the
PRD estuary and the land surface in the eastern coastal land
region (compare Figures 3b, 4b, 5b, and 6b). It is worth
noting that the magnitude of the maximum convergence is
largest in autumn, while it is similar in the other three
seasons.
[ 20 ] The occurrence of the low-level convergence
discussed may have a trapping effect, impeding the locally
generated pollutants from being dispersed out of the region
by environmental winds. This may help explain the
observed generally higher air pollution levels in the region
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Figure 8. Diurnal variations of area-averaged divergence
over the Hong Kong region (113.8–114.4°E, 22.18–22.55°N)
at the lower troposphere (average of the three vertical levels
below 50 m) for the four seasons of 2004.
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in autumn. However, winter also has generally high air
pollution levels [Wang et al., 2001], but the low-level
convergence in winter shown in Figure 8 is not appreciably
larger than that in summer and spring. This discrepancy
may be explained by the seasonal variation of the atmospheric stability in the region, which is an important
meteorological factor when determining the vertical extent
to which the pollutants may disperse.
[21] The seasonal-mean vertical profiles of equivalent
potential temperature average over the land region of Hong
Kong (Figure 9) are typical and show that in summer the
equivalent potential temperature (EPT) decreases with
height up to the 600-hPa level (Figure 9b), denoting a
conditional unstable condition. The depth of this instability
zone in spring (Figure 9a) is similar to that in summer but is
weaker since the decrease in EPT is not as large as in
summer. The depth of the instability zone is significantly
lower in autumn and winter (Figures 9c and 9d) and this is
particularly apparent in winter, when the depth is at its
lowest (below the 975-hPa level) among the four seasons
because of the coolest ground temperature. The very stable
Figure 9. Vertical profiles of area-averaged equivalent potential temperature (K) over the land of Hong
Kong region (113.8 – 114.4°E, 22.18 – 22.55°N) at different periods (0200, 0800, 1400, and 2000 LT) in
(a) spring, (b) summer, (c) autumn, and (d) winter of 2004.
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Figure 10. As in Figure 9 but for observational data observed at the Kings Park Station in Hong Kong
(station 45004; 22.32°N, 114.17°E) in 2004 at 0800 and 2000 LT.
atmosphere in winter may also contribute to the generally
higher air pollution level in this season.
[22] The corresponding vertical EPT profiles from the
data observed in Hong Kong are depicted in Figure 10. The
similar magnitudes and trends in Figures 9 and 10 suggest
that the seasonal variation of atmospheric stability in the
four seasons of 2004 is well simulated by the model.
[23] It has been discussed that the westerly winds to the
east coast of the PRD in the early afternoon may increase
the convergence in the west of Hong Kong. The seasonal
features of LSB circulation can be further examined from
the anomalous (1400 LT) vertical circulation (Figure 11) in
the cross section through Hong Kong at 22.3°N (AA0 in
Figure 1). It can be observed that some anomalous circulations developed near the coastal region in autumn
(Figure 11c), the most prominent one being located over
the PRD estuary (113.6– 113.9°E). Near the west boundary
of Hong Kong (113.9°E), the increase of the upward flow
may attain up to approximately 1600 m and the maximum
anomalous vertical velocity can be over 0.04 m s1. On the
other hand, strong downward motion occurs over the PRD
estuary (113.9°E). These upward and downward flows are
clearly connected by the strong anomalous westerly
SB toward Hong Kong (as discussed in the sections 3.1
and 3.2) to form a semiclosed circulation. The anomalous
westerly SB is mainly below the height of 400 m. This
U-shaped SB circulation can also be observed in the other
three seasons (Figures 11a, 11c, and 11d) but is less well
defined and weaker compared with that in autumn. This
is consistent with the strongest low-level convergence in
Hong Kong occurring in autumn (Figure 8).
[24] From the magnitude of the anomalous vertical
circulations in these cross sections we can also see that
the altitude that the vertical flow may attain is clearly
different in the four seasons. The vertical level that the
vertical flow is confined below is lowest in winter and is
highest in summer, which is generally consistent with the
seasonal variation of atmospheric stability as discussed
earlier.
[25] The afternoon SB circulation in the PRD can be
further analyzed by examining the vertical circulation in the
cross section through the Pearl River estuary at 113.7°E
(BB0 in Figure 1). The anomalous vertical circulations at
1400 LT (Figure 12) show a prominent SB circulation in the
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Figure 11. Vertical cross sections at latitude 22.3°N (AA0 in Figure 1) showing the anomalous (1400 LT
minus daily mean) u-w streamlines, vertical velocities (m s1, shadings) in (a) spring, (b) summer,
(c) autumn, and (d) winter of 2004.
estuary between 22.4 and 23°N that can be observed in all
four seasons. The circulation is manifested by the anomalous upward motion over the land region near the northern
vertex of the PRD (around 22.9°N) and the anomalous
downward motion over the sea surface of the delta in the
south. This SB circulation is particularly obvious in autumn
and winter (Figures 12c and 12d). Its strength is strongest in
autumn (Figure 12c), which is consistent with the strongest
anomalous westerly SB toward the western coast of the
PRD (see Figures 5b and 10c) and the generally largest
land-sea surface temperature contrast in autumn (compare
Figures 3b, 4b, 5b, and 6b).
4. Concluding Remarks
[26] The seasonal-mean features of the LSB in the PRD
region in the four seasons of 2003 – 2004 have been investigated using the data from a high-resolution (horizontal
grid size at 1.5 km) numerical simulation by the mesoscale
meteorological model MM5 obtained from YIM2007. It has
been shown by YIM2007 that this 1-year data can
acceptably replicate the wind speed and wind direction in
the region in comparison to the observed data.
[27] The results of the analysis presented in this study
show that in autumn and winter, the prevailing wind in the
PRD region is northeasterly while it is mainly easterly and
southerly in spring and summer, respectively. The diurnal
variations of the wind and temperature fields indicate that in
early afternoon (around 1400 LT), the land-sea temperature
contrast in the PRD region is at its maximum and a strong
anomalous westerly sea breeze toward the eastern coast of
the PRD can be observed in all the four seasons. This
characteristic is particularly obvious in autumn and winter
and is likely related to the seasonal variation of the direction
and intensity of the prevailing large-scale background
surface wind as discussed in section 3.2. The enhanced
westerly sea breeze is also particularly significant to the
low-level convergence in Hong Kong around 1400 LT and
has a maximum impact in autumn and winter. This increase
in low-level convergence in Hong Kong may have a
trapping effect on the locally generated pollutants and
may worsen the local air pollution.
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Figure 12. Same as Figure 11 but for cross sections at longitude 113.7°E (BB0 in Figure 1).
[28] The presented results suggest that in the PRD region,
the higher air pollution levels in autumn and winter compared with that in spring and summer, as indicated in some
previous observational studies [e.g., Wang et al., 2001;
Lee and Hills, 2003], may be contributed by the seasonal
variation of the meteorological conditions in the region, as
summarized in the following three points. First, in autumn
and winter the prevailing large-scale wind is northeasterly in
the region, which may help the long-range transport of
pollutants from the Chinese mainland. While in spring and
summer, the prevailing large-scale wind is easterly and
southerly which is from the open ocean with relatively less
pollutants. The higher background pollutants transported to
the region in the cool seasons is consistent with the
generally higher pollution levels in these seasons. Second,
the stability of the atmosphere is generally stable in the cool
seasons, and so the level of the instability region is
generally much lower in autumn and winter compared with
that in spring and summer. The larger volume of the
instability region in the atmosphere in spring and summer
is favorable to the dispersion of locally generated pollutants.
Third, as mentioned in the last paragraph, stronger low-level
convergence associated with the stronger afternoon sea
breeze toward the eastern coast of the PRD in autumn and
winter may have a trapping effect on the locally generated
pollutants. All these seasonal variations in the LSB and
meteorological conditions in the PRD region may help
explain the generally higher air pollution levels in the region
during autumn and winter. Nevertheless, with regard to air
pollution it should be pointed out that the seasonal variations of other factors related to the pollutants are also
important, such as the seasonal variations of local pollutant
emissions and photochemistry in the region.
[29] Finally, it is worth mentioning that this study is based
on 1 year of simulation data in 2003 – 2004, and so the
generality of the results presented is rather limited.
Nevertheless, the presented results have revealed some
high-resolution seasonal-mean features of the LSB and
meteorological conditions in the region that may help elucidate
the basic air pollution meteorology in different seasons.
Further numerical simulation studies to be performed over a
longer time scale are of value so that a more general smallscale climatology for the PRD region can be obtained.
[30] Acknowledgments. This research was supported by the Atmospheric Research Centre of the HKUST Fok Ying Tung Graduate School
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and the Hong Kong RGC grant 612807. The authors would also like to
thank Zhang Sha for her help in the early data processing stage.
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K.-C. Chow and J. C. H. Fung, Department of Mathematics, Hong Kong
University of Science and Technology, Clear Water Bay, Hong Kong.
([email protected])
A. K. H. Lau, Environmental Central Facility, Institute for the
Environment, Hong Kong University of Science and Technology, Clear
Water Bay, Hong Kong.
X. Lu and T. Yao, Atmospheric Research Center, Hong Kong University
of Science and Technology Fok Ying Tung Graduate School, Guangzhou
511458, China.
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