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AE International – Asia
Atmospheric Environment 38 (2004) 6737–6750
www.elsevier.com/locate/atmosenv
Simulation of sea-land breezes and a discussion of their
implications on the transport of air pollution during a
multi-day ozone episode in the Pearl River Delta of China
Aijun Dinga,b, Tao Wangb,, Ming Zhaoa, Tijian Wanga, Zongkai Lia
a
Department of Atmospheric Sciences, Nanjing University, 210093, China
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China
b
Received 15 January 2004; received in revised form 16 August 2004; accepted 2 September 2004
Abstract
Sea-land breezes (SLBs) play an important role in transporting air pollution from urban areas on the coast. In this
study, a meso-scale model MM5 was used to simulate SLBs and to understand the transport of pollution during a
prolonged ozone episode observed in Hong Kong in September 2001. With the designed settings, the model performed
very well in the simulation of SLBs. The simulated surface winds and the planetary boundary layer (PBL) heights are
presented to contrast the characteristics of dispersion and transport on pre-episode and episode days. The diurnal
variations in horizontal and vertical winds on an episode day are then presented to illustrate the evolution of SLBs. The
results indicate that on episode days the onset of sea breezes (SBs) was delayed to noontime due to offshore synoptic
winds, while on pre-episode days the SBs had already penetrated deep inland by early afternoon. The simulation shows
that SBs propagated in both onshore and offshore directions in the afternoon, leading to the formation of nocturnal
regional-scale SBs. The maximum distance for the inland penetration of an SB front (SBF) was about 60–80 km, in
contrast to 120–150 km for offshore propagation. With the aid of high-resolution trajectories, the main meteorological
factors contributing to the occurrence of the observed ozone episode are discussed. It is believed that the offshore
synoptic wind, the delayed SBs, as well as the low mixing height contributed to the daytime transport of pollution and
high ozone on the coast. The trajectory analysis also indicates important contributions from regional sources of
emission.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: MM5; Sea-breeze front; Typhoon; Ozone episode; Back-trajectory
1. Introduction
The Pearl River Delta (PRD) is home to some 30
million inhabitants, and is the region where major
Chinese cities such as Hong Kong, Shenzhen, and
Corresponding author. Tel.: +852 2766 6059;
+852 2334 6389.
E-mail address: [email protected] (T. Wang).
fax:
Guangzhou are located. It is also the major manufacturing base of southern China. Like other urban and
industrialized areas, the PRD has experienced airpollution problems such as high concentrations of ozone
and particulate matter, and declining visibility.
Surrounded as it is by mountains to the north, east,
and west, the terrain of the PRD is complex. The
complexity of the topography is particularly evident in
Hong Kong, 70% of the land area of which is made up
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2004.09.017
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of mountains. Previous measurement studies have
shown that the western sector of Hong Kong over the
Pearl Estuary suffers from ozone pollution and from
poor visibility, which has been attributed to the unique
wind flows that allow for the accumulation of Hong
Kong emissions and for the transport of pollution
from inner PRD region (Kok et al., 1997; Wang et al.,
2001, 2003; Wang and Kwok, 2003; Wang, 2003).
These studies revealed the existence of complex boundary-layer wind such as sea-land breezes (SLBs) and
of topography-induced flows during high pollution
events. They also suggest that the sparse data on surface
winds over the Pearl Estuary are insufficient for
determination of the origin and transport patterns of
the air pollutants, and that there is a need for detailed
studies on meso-scale wind fields and the boundarylayer structure.
Some modeling studies have been conducted on wind
flow and other boundary-layer characteristics in the
PRD. Lin et al. (2001) simulated SLBs in Macau using
MM5; Liu and Chan (2002) examined the boundary
layer dynamics over Hong Kong with a 3-D hydrostatic
PBL model. These studies covered a relatively small
portion of the PRD region. Here, we use MM5 to
investigate the circulation of SLBs and the transport of
pollution over the PRD during a multi-day ozone
pollution episode reported by Wang and Kwok (2003).
We will first briefly describe the ozone episode and
the influencing Typhoon Nari, then proceed to describe
the details of the setup of the model and to validate
the modeled results with synoptic and local meteorological observations. We will then show the daytime
and nighttime surface wind patterns over the PRD for
the episode and pre-episode days, and will also
investigate the diurnal variations in the horizontal
and vertical structures of SLBs. Finally, we will discuss
the main meteorological mechanisms responsible for
the high ozone concentrations observed during the
episode.
2. Descriptions of Typhoon Nari and ozone episodes in
the PRD
It has been known that ozone episodes in Hong Kong
are often associated with the activities of tropical
cyclones in the western Pacific (e.g., Wang et al., 1998,
2001; Lee et al., 2002; So and Wang, 2003). Typhoon
Nari, however, is one of the most unusual tropical
cyclones of recent years. It emerged on 6 September,
2001 about 220 km east of Taiwan. After making three
anticlockwise looping motions, it gained in typhoon
intensity and moved towards Taiwan on 14 September.
On 16 September it reached its maximum intensity
(Sea level pressure: 954 hPa), but weakened rapidly
into a tropical storm after making landfall over northern
Taiwan. On 18 September, Nari entered the northern
part of the South China Sea, moved westward to
Hong Kong, weakened progressively, and eventually
dissipated over inland Guangdong (HKO, 2001).
The track of Nari during its entire lifetime is shown in
Fig. 1a.
During the 1 week before Nari made landfall (i.e.,
14–19 September), a severe and prolonged photochemical ozone episode occurred in the PRD. High ozone,
CO, and NOx were recorded at a number of air
monitoring stations in Hong Kong and in some
neighboring cities. Wang and Kwok (2003) presented
a detailed analysis of measurements of ozone and
other trace gases obtained at non-urban sites in Tai O
(TO), Hok Tsui (HT), and Taipa Grande (TG) and of
meteorological observations during this episode. Fig. 1b
shows a topographic map of the PRD region and the
locations of some cities, as well as the wind and airquality measuring stations used in the present study. The
ozone data will be presented in Section 4.3.
3. Model settings and experiment design
The numerical model used in this study is the FifthGeneration Pennsylvania State University-National
Center for Atmospheric Research Meso-scale Model
Version 3.6 (MM5 in brief, Dudhia et al., 2003).
The simulation was conducted from 12 September
(i.e., 2 days before this episode) to 20 September.
Considering that it is impossible to conduct a single
simulation for such a long period, we divided the whole
period into four stages (see Fig. 1a): Stage I–—1800
UTC 11–13 September (a 48-h run), during which Nari
moved in a northwesterly direction from Okinawa
(marked as squares); Stage II—1800 UTC 13–16
September (a 72-h run), during which Nari moved in a
southwesterly direction and landed over northern
Taiwan (as open cycles); Stage III—1800 UTC 16–18
September (a 48-h run), during which Nari swept across
Taiwan on a southwesterly course (as triangles); and
Stage IV—1800 UTC 18–20 September (a 48-h run),
during which Nari moved westwards and landed over
eastern Guangdong (as stars).
The four individual runs presented here all employ
multiple, two-way, interactive, nested grids. Five nested
domains were defined in Fig. 1a. The outermost domain
has an 81 km grid spacing (not shown here), which
covers most of East Asia. Domain 2 (DM2), with a
27 km grid spacing, covers South and Southeast China.
To resolve the central core and spiral rain bands of the
storm, a movable domain with a higher resolution of
9 km of grid spacing was pre-set to move along the Best
Track of Nari (data from the Joint Typhoon Warning
Center). The left two domains over South China, with 9
and 3 km grid spacing, were designed to capture the
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Fig. 1. (a) Domain settings of MM5 and the Best Track data of Typhoon Nari (data from the Joint Typhoon Warning Center) in
different stages of the typhoon (b) Topographic map of the Pearl River Delta in South China and the locations of ozone and weather
stations whose data are used in this study.
structure of the SLBs in the PRD. More detailed model
settings can be found in Table 1.
From the ground level to the top pressure of 50 hPa,
there are 32 vertical sigma layers (31 half-sigma levels)
to all grid meshes, with about 15 layers below 1.8 km
AGL. The outermost lateral boundary conditions
(i.e., for DM1) are specified with the 6-hourly NCAR/
NCEP global reanalysis data on 17 pressure levels,
which have a horizontal resolution of 2.51 in latitude
and longitude. The sea surface temperature (SST)
data used here is Reynolds SST, obtained from the
NOAA Climate Diagnostics Center, with a horizontal
resolution of 11 (Reynolds and Smith, 1994). The model
was also initialized with the same reanalysis data, but
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Table 1
The grid settings and physics options used in the simulation
Item
DM 1
DM 2
DM 3
DM4
DM 5
Dimensions (x,y)
Grid size (km)
Vertical layers
Time step (s)
60, 40
81
32
180
91, 64
27
32
60
58, 58
9
32
20
61, 61
9
32
20
91, 91
3
32
6.7
PBL scheme
Cumulus scheme
The high-resolution Blackdar Scheme
KF2
KF2
None
None None
Radiation scheme RRTM longwave scheme
Moisture scheme Reisner graupel (Reisner 2)
FDDA
Yes
No
No
No
No
has been enhanced by radiosonde and surface observations in Asia. Because the reanalysis data are too coarse
to represent the intensity of the vortex in the center of a
typhoon, bogus vortices were introduced during the
dynamic initializations of the outermost domains for all
of the four stages, using the NCAR-AFWA tropical
cyclone (TC) bogussing scheme. A more detailed
description of this scheme can be found in Davis and
Low-Nam (2001).
The physics parameterizations have significant effects
on simulations of both typhoons (the main synoptic
factor in this study case) and SLBs. Here, for all five
domains we used the high-resolution Balackadar scheme
in the PBL parameterizations, which has been successfully used in simulations of typhoons in other studies,
such as those by Liu et al. (1997); Wu et al. (2002).
Zhang and Zheng (2004) have suggested that this
scheme performs better than others in simulating the
diurnal cycles of surface wind under the condition of
summertime weak-gradient flows. The cumulus parameterizations used here are those of the modified
Kain–Fritsch scheme, i.e. KF2, for both the DM1 and
DM2. For the 9 and 3 km meshes no cumulus
parameterizations were used, as the grid sizes were
considered sufficient to resolve updrafts and downdrafts
in cumulus.
The Reisner graupel scheme (Reisner 2) was used as
explicit moisture scheme for all domains. The radiation
option was the rapid radiative transfer model (RRTM)
longwave scheme, which was combined with the cloudradiation shortwave scheme. The surface schemes
used were all five-layer soil models with the soil
temperature predicted. For the outermost domain, the
four-dimensional data assimilation (FDDA) method—
‘‘analysis nudging’’ was used with weak nudging
coefficients: 1.5E-4 for both winds and temperature
analyses but 1E-5 for the mixing ratio. In addition to the
experiment designed above, runs without a bogussing
vortex were also conducted for the four stages to make
comparisons.
4. Results and discussions
4.1. Validation of the model
4.1.1. Simulation of the main synoptic feature—Typhoon
Nari
Because a typhoon determines the large-scale background flow that affects meso-scale winds (Helmis et al.,
1995), an accurate simulation of the typhoon is thus
a prerequisite to the successful simulation of SLBs.
Figs. 2a and b show the simulated 6-hourly track and
intensity (represented by the minimum sea-level pressure) of Typhoon Nari in DM3, compared with the Best
Track data and that from no-bogussing runs. It can be
seen that the no-bogussing runs contained errors in the
locations of the center of the typhoon at the initial time
due to the interpolation from low-resolution analysis
data, which could be amplified especially in Stages II
and IV. The bogussing runs gave quite similar tracks
compared with the Best Tracks, with the largest errors of
about 150 km on 15 September. In addition, the
variations in the intensity of the typhoon were well
captured, especially the course of enhancement before its
landfall over Taiwan. However, the no-bogussing runs
failed to gain in typhoon intensity but only produced a
depression. We also compared the simulated radar
reflectivity with the satellite image and found that the
bogussing runs had a better performance (figures not
shown here). To summarize, with the designed grid
structure and selected parameterizations, the simulations with bogussing vortices in initialization successfully captured the main features of Typhoon Nari during
the whole period. This will help with the simulation of
SLBs in higher-resolution domains.
4.1.2. Simulation of meso-scale winds
To validate the simulation of small-scale winds in the
PRD, winds observed at three surface sites were used for
a comparison with DM5 simulations from bogussing
runs. The three wind stations were: Waglan (WL), which
is about 10 km southeast from Hong Kong Island; Tai
Mo Shan (TMS), which is the highest peak in Hong
Kong with a height of about 957 m ASL (see Fig. 1b);
and Taipa Grande (TG) in Macau. The mountain site
TMS was expected to represent high-level synoptic
winds; while the other two surface sites were used
to indicate wind flows on the two sides of the
Pearl Estuary.
Fig. 3 shows the simulated and observed time series of
wind vectors for the three stations. The simulated results
are shown as black arrows, and the observations are in
gray. At the mountain site, TMS, the model simulation
captured the day-to-day variations in wind speed and
direction very well. During the first 2 days, the winds
mainly came from the east; while during the succeeding
six episode days, they came predominantly from the
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Fig. 2. (a) Comparison of simulated typhoon tracks for bogussing and no-bogussing runs with the Best Track data; (b) Same as (a) but
with the intensity shown in minimum sea-level pressure.
Fig. 3. Time series of simulated and observed wind at three sites: Tai Mo Shan, Waglan Island, and Taipa Grande. (Black
arrows=observations; gray arrows=simulated results)
north or northwest direction, at a relatively high speed
of about 5 ms1, suggesting that the episode occurred
under the influence of offshore synoptic winds. At the
two surface sites, the observed and simulated winds also
agreed very well with each other, both exhibiting
obvious diurnal variations following a clockwise rotation, which agreed with other observations (Helmis
et al., 1995) and with the theory on SLBs (Simpson, 1996).
Some differences in the modeled and observed wind
speeds are noticeable, which could be due to factors such
as errors in the initial condition, numerical arithmetic,
and physical parameterizations. Nevertheless, for the
whole study period, the model reproduced diurnal
variations in SLBs with reasonable accuracy.
4.2. Structure and evolution of SLBs
We have shown that the MM5 has successfully
simulated the synoptic wind and the SLBs. In this
section, we examine the surface wind patterns, PBL
heights, and the diurnal variation in structure of SLBs
over the PRD.
4.2.1. Surface wind flow patterns and PBL heights over
the PRD
Figs. 4a–f show the daytime (1400 LT) and nighttime
(0200 LT) near-surface winds and the PBL heights
(PBLH, as shaded background) for 12 and 13 September, 15 and 16 September and 18 and 19 September. The
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black arrow in the bottom right-hand corner of each
figure shows the mean wind on the 925 hPa level (using
NCEP reanalysis data). Here, the PBLH was predicted
by MM5v3 with Blackadar PBL parameterization: in
the convective regime, it was based on the buoyancy
characteristics of the airmass; outside of the convective
regime, an empirical relationship that defines PBLH for
a stationary, neutral PBL was employed (Blackadar,
1979; Zhang and Anthes, 1982). Figs. 4a and b represent
the pre-episode day, while Figs. 4c–f represent the
episode days.
As discussed above, the synoptic winds were from the
east on 12 and 13 September, i.e., the winds blew
coastwise; while they were all from the north, i.e., they
were offshore winds, on the other days. On 14
September, the PRD began to be affected by the
anticlockwise circulation of Typhoon Nari.
From Fig. 4, it can be found that the SLBs were quite
different on pre-episode and episode days under
different synoptic conditions. At 1400 LT, the SBs
developed so well that the SB fronts (SBFs) penetrated
inland at quite a long distance on 12 September. In
comparison, on the two episode days, SBs only occurred
over the sea adjacent to the coast, with the SBFs
penetrating several kilometers inland. As a result of the
subsidence of up-level air, there was a calm area over
the sea at a distance of 10–20 km from the coast. These
results suggest that under the condition of offshore
synoptic winds, the onshore flow caused by the
differential heating of land and sea must first counteract
the offshore synoptic wind; therefore, the strong SBs
appeared at a later period. At 0200 LT, the land breezes
(LBs) developed on all three days with enhanced
offshore flow near the coastline, especially over the
Pearl Estuary. It is noticed that the nighttime surface
wind increased day by day, which may have been the
result of the increase in synoptic wind speed as Typhoon
Nari moved closer to the PRD.
The PBL height (see Fig. 4) during daytime was very
high over land, especially in some urban areas (up to
2000 m), but was only about 200–400 m over the sea.
However, the height over that part of the sea close to the
coastline was much lower than that over remote sea
areas on episode days. There might be two reasons
contributing to this phenomenon: one is that air with a
higher temperature transported or dispersed from inland
areas to the cold surface of the sea will cause an
inversion at a lower height; the other is that the
downdraft of air above the SB layer over this region
will depress the mixing height. By comparing Fig. 4a
with Figs. 4c and e, it can be found that at 1400 LT 12
September, due to the deep penetration inland of SBs,
the PBL height over the land was obviously lower than
that on the two other days, contrary to the situation
over the ocean. These results are consistent with the
observations of Wakimoto and Atkins (1994). At night
time, the PBL height was higher over the ocean, and
there were no obvious differences between pre-episode
and episode days.
4.2.2. Diurnal variations in the structure of SLBs on
episode days
In this section we will investigate the diurnal variation
in SLBs for one of the episode days to understand the
transport mechanism related to the high ozone observed
in Hong Kong. The simulation result for 14 September
is presented below.
Figs. 5a and b show the near-surface wind fields at
0800 LT 14 September taken from DM2 (27 km grid
spacing) and DM5 (3 km), respectively. The former
represents the synoptic flow and ‘‘regional-scale’’ SLBs,
and the latter shows the ‘‘local-scale’’ SLBs in the PRD.
The horizontal wind speed is shown as shaded background. It can be seen that at 0800 LT, LBs were well
developed on both the regional and local scales. Fig. 5a
shows an obvious offshore synoptic wind over South
China, but the wind speeds near the coastal areas (within
100 km) were obviously higher than those over the
inland areas and those farther over the ocean. The
strong winds near the coast are believed to be enhanced
by the regional-scale LB. In Fig. 5b, small-scale LBs can
be seen in the PRD, especially in Hong Kong and the
two sides of the Pearl Estuary.
Fig. 6a shows a height-latitude cross-section drawn on
a longitude of 113.41E (i.e., line DD0 in Fig. 5b), which
lies over relatively flat areas west of Macau (see Fig. 1b),
crossing the coastal line at 22.21N. The flat topography
is chosen to show the ‘‘pure’’ SLBs, which would be less
influenced by other types of circulation such as
mountain-valley breezes. Fig. 6a gives the v–w stream
lines and the contour line of vertical velocity (w). Here,
the north v-component with a speed greater than 3 ms1
was plotted as shaded background. It can be considered
to be an LB because the mean synoptic wind was about
3 ms1. It shows that at 0800 the LBs only occurred
below an altitude of 400 m, due to weak vertical
convection.
Figs. 5c,d, and 6b give the same picture for 1500 LT
on that day, but the shaded contour in Fig. 6b represents
onshore winds. At that time, the SBs had become
enhanced, and stronger onshore SBs could be found
along the coastal line in the PRD (see Fig. 5d). Over the
Pearl Estuary there was a clear area of divergence
with calm wind, indicative of the subsidence of the
re-circulated upper-level air of SB circulations. A largescale SBF was clearly indicated in Fig. 5c, but it was
close to the coastal line, suggesting that the SBF had not
penetrated deep inland. In contrast to the front line, the
divergence line was found over the ocean 70–80 km away
from the coastal line. The distance between them might
be considered to be the horizontal extent of SBs, also
indicated by the shaded area shown in Fig. 6b. But in the
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Fig. 4. Surface wind flow patterns and PBL heights (m) over the PRD (a) at 1400 LT 12 September, (b) at 0200 LT 13 September, (c) at
1400 LT 15 September, (d) at 0200 LT 16 September, (e) at 1400 LT 18 September and (f) at 0200 LT 19 September. The black arrows
at the bottom right-hand corner represents the synoptic wind on 925 hPa (from NCEP reanalysis data).
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Fig. 5. Diurnal variations in surface wind patterns (a) stream field in DM2 (27 km) with the shaded horizontal wind speed as the
background at 0800 LT 14 September, (b) wind vector in DM5 (3 km) with the shaded horizontal wind speed as the background at
0800 LT 14 September, (c) same as (a) but for 1500 LT, (d) same as (b) but for 1500 LT, (e) same as (a) but for 2300 LT, and (f) same as
(b) but for 2300 LT.
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Fig. 6. (a) Vertical cross-section taken over a longitude of 113.41E showing the v–w wind stream, the contour lines of vertical velocity,
and the shaded background for the v-component (northerly winds) larger than 3 ms1 at 0800 LT 14 September, (b) same as (a) but for
1500 LT with the shaded area representing the positive v-component, i.e., southerly winds, (c) same as (b) but for 2300 LT.
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vertical direction, the SBs showed an obviously different
extent over land and sea. The zero-v isotach showed a
clear ‘‘big head’’ (with a height of about 700 m) on the
top of the northward SBF, which was about twice the
height of the inflow layer behind. In front of the SBF,
there were strong updrafts. This result agrees very well
with the measurements made by Simpson et al. (1977). It
is believed that the Kelvin–Helmhotz (KH) instability
causes turbulent mixings in KH billows, contributing to
the elevated head (Simpson et al., 1977, 1994; Sha et al.,
1991). The offshore airflow creates a shear zone that lifts
the denser air, creating billows or horizontal vortices
along the interface at the head and the top of the front
(Sha et al., 1991; Simpson, 1994; Rao et al., 1999).
AT 2300LT, the regional-scale SBs with an SW
direction still existed (see Fig. 5e) but, at the same time,
LBs had already appeared along the coastline due to
faster radiation cooling over the land. Fig. 5f shows
some signals of the local LB, such as an enhancement of
the wind speed near the west bank of the Pearl Estuary
and a weakening in the opposite side. The vertical
structure shown in Fig. 6c indicates that the inflow layer
penetrated farther inland but then began to flatten,
probably because the gravity current SBs could only
advance horizontally due to reduced vertical mixing at
night. These results are similar to those presented by
Buckley and Kurzeja (1997).
To further examine the evolution of the structure
of SLBs, Fig. 7a presents a latitude-time cross-section
of the v-component and the height of the SB layer
(as zero v), which is also taken over line DD’ shown in
Fig. 5b. In Fig. 7b, the height-time cross-section of the
w-component and the u-v stream line are plotted at the
location of (113.41E, 22.21N); i.e., the cross of line DD’
with the coastline.
In Fig. 7a, the positive v-component represents the
evolution of the inland penetration and offshore
propagation of the SBs, and the contour lines reveal
the vertical expansion. It can be seen that the onset
of SBs did not occur until noon time due to the offshore
background wind. In the afternoon, the speed of
SBF moving inland was slower than that later in the
afternoon and at night, due to the KH instability.
The lowest speed at 1500–1600 LT corresponded with
the highest top of SBFs (see Figs. 7a and b). By contrast,
the speed of the offshore propagation of SBs over the
sea was faster and more uniform. These results are
consistent with the observations and simulations of
Finkele (1998). They show that the mean maximum
distance of inland penetration was about 60–80 km,
while that of offshore propagation was larger than
120 km and up to 150 km (in DM2; figures not shown).
Fig. 7a also shows that the higher v-component (larger
than 2ms1) only appeared near the coastal line with a
horizontal scale of 20–50 km later in the afternoon,
corresponding to the largest vertical expansion of SBs
near the coast. In addition, Fig. 7b shows that the winds
had clockwise rotation only in the SBs layer, but they
rarely changed above the SB layer.
For other days during the episode, the diurnal
variations in the v-components were quite similar to
those during 14 September, except that there were some
small differences in the inland penetration distance,
vertical extension height, the initial time of SBs, and
so forth. However, the u-component of the wind showed
obvious day-to-day differences because of the variation
in the synoptic winds. For example, on 17 and 18
September, the surface winds were more westerly than
on other days, and the winds were more northerly on
19 September. The day-to-day variations in the u- and
v-components could have a large impact on the patterns
of transport of air pollution, as discussed in the
following section.
4.3. Effects of typhoon and SLBs on the occurrence of the
observed ozone episode
As previously mentioned, ozone pollution episodes in
Hong Kong are often associated with SLBs under the
influence of a tropical cyclone/typhoon in the western
Pacific. Based on the above simulation of wind patterns
and on the diurnal variations in the structure of SLBs,
we can gain important insights into the meteorological
cause of this type of photochemical ozone episode.
First, when the western Pacific typhoon is located
near Taiwan, it generates an anti-clockwise circulation,
creating a continuous offshore synoptic wind in the
PRD region, which facilitates the transport of inland
pollution to coastal areas (see also Wang et al. (2001) for
a case study). Second, due to the large-scale subsidence
at the outskirts of a typhoon, the PRD region often has
fine weather and strong solar radiation (Wang et al.,
2001; Wang and Kwok, 2003), which are favorable for
the formation of SLBs and for the photochemical
production of ozone. The SBs that are delayed due to
the offshore synoptic flow are an important factor in the
high concentrations of ozone in coastal areas, allowing
polluted air masses to remain in coastal areas for a
longer period of time.
It is of interest to further study the relationship
between the complex winds indicated in the model
simulation and the variations in ozone concentration in
the coastal area of the PRD. Here we use HYSPLIT4
(HYbrid Single-Particle Lagrangian Integrated Trajectory model version 4.6) (Draxler and Rolph, 2003) to
calculate 12-h 3-D back-trajectories. The trajectories
were calculated every 4 h using hourly MM5 outputs
with a 3-km resolution. Fig. 8a shows the time series of
ozone measured at several rural/suburban sites in this
region (see Fig. 1b for locations). Fig. 8b exhibits the
horizontal patterns of trajectories arriving at the Tai O
site for all eight days.
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Fig. 7. (a) Latitude-time cross-section of the v-component and height of the SBs layer taken over a longitude of 113.41E; (b) Heighttime cross-section of the u–v stream and the w-component at the location of (113.41E, 22.21N).
It can be seen that the ozone levels at most sites on 12
and 13 September were much lower than those on
episode days, in part because there no significant
amounts of air pollutants were transported from the
inland cities in the morning. But during the episode
days, from midnight to noon of the following day, the
LBs and offshore synoptic winds brought the ozone
precursors from inland and coastal cities to areas over
the ocean, where the air pollutants could accumulate due
to a low mixing height and low wind speed. A high level
of ozone was then produced with strong solar radiation.
In the afternoon, the ozone-laden air masses were
transported or recycled to the coastal area by onshore
SBs, with most sites receiving the highest level of ozone
at 13:00–14:00 LT.
The ozone concentration and back-trajectories given
in Figs. 8a and b indicate different diurnal variations in
ozone associated with different transport patterns due
to day-to-day variations in the circulation of SLBs.
On 14–16 September, the trajectories exhibited a
more complete clockwise cycle compared to other days.
The higher level of ozone and the secondary peaks
observed at several sites in the later afternoon or at night
(see Fig. 8a) on 14–16 September could be attributed to
the re-circulation of ‘‘aged’’ daytime ozone plumes by
the regional-scale SBs (see trajectories at 2000 LT). In
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A. Ding et al. / Atmospheric Environment 38 (2004) 6737–6750
Fig. 8. (a) Time series of the ozone concentration measured at several coastal sites during mid-September, 2001; (b) 12-h backtrajectories arriving at Tai O at 4-h intervals. The dots on the trajectory lines indicate hourly locations.
comparison, almost all of the trajectories for 17 and 18
September came from the western area of the PRD
and manifested a fan shape due to the effects of SLBs.
On 19 September, the air mass at Tai O came from
the northwest or north, except during 1800–2200 LT,
when the air mass came from the urban area of
Hong Kong. The results from analyses of trajectories
are generally consistent with analyses of chemical tracers
such as CO/NOy and SO2/NOy, which were used
by Wang and Kwok (2003) to distinguish local
(Hong Kong) air masses from those from the PRD.
Both analyses highlight the contribution of inland PRD
emissions to the ozone level over the coastal area during
this episode.
It should be pointed out that the trajectories only
provide information about advective transport. Turbulence dispersion also plays an important role during the
transport of pollution. As discussed in Section 4.2, the
PBL height over the sea near the coast was remarkably
lower on episode days, which would diminish the
vertical dispersive ability of the atmosphere. In addition,
sea-breezes can re-circulate the pollutants by convective
transport. To evaluate the relative importance of these
physical mechanisms and chemical processes that occur
during transport, further studies using photochemical
models will be needed.
5. Summary and conclusions
Using the PSU-NCAR MM5v3, we successfully
simulated the main features of the SLBs associated with
a multi-day ozone episode observed in coastal southern
China in September 2001 under the influence of
Typhoon Nari. With Best Track data, the adoption of
the bogussing vortex in the dynamic initialization
significantly improved the simulation of the typhoon’s
track and intensity, even with the use of low-resolution
reanalysis data. The model reproduced the main features
of the synoptic flow over the PRD and the evolution of
the SLBs with reasonable accuracy.
Influenced by different synoptic winds, the SLBs and
PBL height over the PRD showed quite different
patterns in pre-episode and episode days. SBFs penetrated less inland and the PBL height was lower over the
coastal water on episode days with the offshore synoptic
winds. To counteract the synoptic wind, the onset of SBs
was delayed to the afternoon. The SBFs could penetrate
about 60–80 km inland, but their speed was affected by
the KH instability, while the offshore propagation was
faster and could reach up to 120–150 km away from the
coastline. The SBs became most intense in late afternoon
and formed a regional-scale SB, which contributed to
the nocturnal SBs. The SLBs also showed different
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vertical extensions at different stages. In the morning,
the LBs generally enhanced the offshore winds below the
level of 400 m. The SBFs only reached a very low
height of about 300 m at noontime, but could reach a
level of up to 700–800 m in the afternoon, which was
about twice the height of the inflow layer behind. The
main contributor to the ‘‘elevated’’ top of SBF is KH
instability.
The simulation and further trajectory analysis provided an improved understanding of the meteorological
mechanisms for the photochemical ozone episodes
associated with SLBs and tropical cyclones. The high
concentrations of ozone observed at the coastal sites in
the studied episode were mainly due to the transport of
pollution from inland areas of the PRD. The delayed
SBs and a lower mixing height could help pollution
plumes accumulate in low-level atmospheres over the
sea. This study highlights the important role of SLBs in
distributing air pollutants from coastal urban areas.
Acknowledgments
The authors would like to thank NCAR for releasing
and updating the MM5 model and the NOAA Air
Resources Laboratory (ARL) for providing the HYSPLIT model. We are grateful to the Hong Kong
Observatory for providing the radiosondes and the
surface meteorological data, and to the Hong Kong
Environment Protection Department and the Macau
Geophysical and Meteorological Bureau for providing
the O3 data from their monitoring network. The NCEP/
NCAR reanalysis data were obtained from NOAA
CDC. The Best Track data of Typhoon Nari was
obtained from the Joint Typhoon Warning Center. The
authors also thank Joey Kwok for his help in processing
the meteorological and air pollutant data and the two
anonymous referees for giving very helpful suggestions.
This work was funded by the Research Grants Council of
the Hong Kong Special Administrative Region (Project
No. PolyU 5059/00E and PolyU 5063/01E to Tao Wang).
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